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This document contains Chapters 24 to 25 CHAPTER 24 FISHES CHAPTER OUTLINE 24.1. Diversity A. Overview 1. The term “Fish” extends beyond what animals are actually considered fishes (e.g., starfish, etc.). 2. A modern fish is an aquatic vertebrate with gills, limbs (if present) in the form of fins, and usually with a skin covered in scales of dermal origin. 3. Fishes do not form a monophyletic group. In an evolutionary sense, fish can be defined as all vertebrates that are not tetrapods. 4. The common ancestor of fishes is also an ancestor of land vertebrates; therefore in pure cladistics, this would make land vertebrates “fish”—a nontraditional and awkward usage. 5. With over 28,000 living species, fish include more species than all other vertebrates combined. 6. They are adapted to live in a medium 800 times denser than air. 7. They can adjust to the salt and water balance of their environment. 8. Their gills are efficient at extracting oxygen from water that has 1/20 the oxygen of air. 9. A lateral line system detects water currents and vibrations, a sense of “distant touch.” 10. Evolution in an aquatic environment both shaped and constrained its evolution. 11. “Fish” refers to one or more individuals of one species; “fishes” refers to more than one species. 24.2. Ancestry and Relationships of Major Groups of Fishes A. History (Figures 24.1, 24.2) 1. Fishes descended from unknown free-swimming protochordate ancestor ~ 550 million years ago. 2. Earliest fish-like vertebrates were a group of agnathan fishes. a. Agnathans include extinct ostracoderms and living hagfishes and lampreys. b. Hagfishes lack vertebrae and lampreys have rudimentary vertebrae. c. Agnathans are included in subphylum Vertebrata because they have a cranium. d. Agnathans are unique enough to be assigned in separate classes. 3. Others have paired appendages and form a monophyletic lineage with tetrapods called gnathostomes. 4. They appear in the Silurian fossil record with fully formed jaws, and no intermediates are known. 5. The Devonian is called the Age of Fishes. 6. One group, the placoderms, became extinct in the Carboniferous and left no direct descendants. 7. Cartilaginous Fishes a. The cartilaginous fishes lost the heavy dermal armor and adopted cartilage as the skeleton. b. They flourished during the Devonian and Carboniferous. c. They nearly became extinct at the end of the Paleozoic. d. They increased in numbers in early Mesozoic, diversifying into a modern shark assemblage. 8. Acanthodians a. These fish were well represented in the Devonian but became extinct by the lower Permian. b. They resemble bony fish but have heavy spines on all fins except the caudal fin. c. They were probably the sister group of the bony fishes. 9. Bony Fishes a. These are the dominant fishes today. b. They have two distinct lineages: the ray-finned fishes and the lobe-finned fishes. c. The ray-finned fishes radiated to form modern bony fishes. d. Lobe-finned fishes are a relict group with few extant species. Include sister group of tetrapods. 24.3. Living Jawless Fishes A. Overview 1. Living jawless fishes include hagfishes and lampreys. (Figures 24.3, 24.4) 2. About 43 species of hagfishes are known and about 41 species of lamprey are described. 3. Members of both groups lack jaws, internal ossification, scales or paired fins. 4. Both groups share porelike gill openings and an eel-like body. 5. Recent molecular analysis shows lampreys and hagfishes forming a monophyletic unit. 6. Many zoologists do not support this grouping thus we take the view that hagfishes form the sister group of a clade that includes lampreys and gnathostomes. (Figure 24.2) B. Class Myxini: Hagfishes (Figure 24.3) 1. Hagfishes are entirely marine. 2. They are scavengers and predators of annelids, molluscs, dead or dying fishes, etc. 3. The hagfish enters a dead or dying animal through an orifice or by digging inside using keratinized plates on its tongue to rasp away bits of flesh. 4. It is nearly blind but can locate food by an acute sense of smell and touch. 5. To provide leverage, the hagfish ties a knot in its tail and passes it forward to press against prey. 6. Special glands along the body secrete fluid that becomes slimy in contact with seawater. 7. Body fluids of a hagfish are in osmotic equilibrium with seawater. 8. Circulatory system includes three accessory hearts in addition to a heart behind the gills. 9. Reproduction of Hagfishes a. The Copenhagen Academy of Science offered a prize over a century ago for information on hagfish breeding habits; it remains unclaimed. b. In some species, females outnumber males by 100 to 1. c. Females produce small numbers of surprisingly large, yolky eggs 2-7 centimeters in diameter. C. Class Petromyzontida: Lampreys (Figures 24.4--24.6) 1. Diversity a. All lampreys in the Northern Hemisphere belong to the family Petromyzontidae. b. The marine lamprey Petromyzon marinus occurs on both Atlantic coastlines and grows to a length of one meter. c. There are 20 species of lampreys in North America; half belong to nonparasitic brook-dwelling species. 2. Reproduction and Development (Figure 24.5) a. All lampreys ascend freshwater streams to breed. b. Marine forms are anadromous, leaving the sea where they were adults to spawn upstream. c. In North America, all lampreys spawn in winter or spring. d. Males build a nest by lifting stones with their oral discs and using body vibrations. e. The female anchors to a rock and the male attaches to her head. f. As eggs are shed into the nest, the male fertilizes them; adults die soon thereafter. g. Eggs hatch in two weeks into unique larvae (ammocoetes). h. The larva lives first on its yolk supply and then drifts downstream to burrow into sandy areas. i. The larva is a suspension-feeder until it metamorphoses into an adult. j. Change to an adult involves eruption of eyes, keratinized teeth replacing the hood, enlargement of fins, maturation of gonads and modification of gill openings. 3. Parasitic Lampreys (Figure 24.6) a. If marine, parasitic lampreys migrate to the sea; other species remain in freshwater. b. Attach to fish by a sucker-like mouth and sharp teeth rasp through flesh as they suck fluids. c. They inject anticoagulant into a wound to promote flow of blood. d. When engorged, the lamprey drops off but the wound may be fatal to the fish. e. Parasitic freshwater adults live 1–2 years before spawning and dying; anadromous forms live 2–3 years. f. Nonparasitic lampreys do not feed; their digestive tract degenerates as an adult, and they spawn and die. 4. Sea Lamprey Invasion of the Great Lakes Region a. No lampreys were in the U. S. Great Lakes west of Niagara Falls until the Welland Ship Canal was deepened between 1913 and 1918. b. Sea lampreys moved first through Lake Erie to Lakes Huron, Michigan, and Superior. c. Lampreys preferred lake trout and destroyed this commercial species along with overfishing. d. They then turned to rainbow trout, whitefish, lake herring, chubs and other species. e. The lamprey populations declined both from depletion of food and from control measures. f. Chemical larvicides are used in spawning streams; release of sterile males is also being used. 24.4. Class Chondrichthyes: Cartilaginous Fishes (Figure 24.7) A. Overview 1. About 970 living species are in the class Chondrichthyes. 2. Although a smaller and more ancient group, their well-developed sense organs, powerful jaws and predaceous habits helped them survive. 3. Although calcification may be extensive, true bone is completely absent throughout the class. 4. Phosphatized mineral tissues were retained in teeth, scales, and spines. 5. Nearly all are marine; only 28 species live primarily in freshwater. 6. After whales, sharks are the largest living vertebrates, reaching 12 meters in length. B. Subclass Elasmobranchii: Sharks, Skates and Rays 1. There are thirteen living orders of elasmobranchs with about 937 total species described. 2. Order Carcharhiniformes contains the coastal tiger and bull sharks and the hammerhead. 3. Order Lamniformes contains large, pelagic sharks such as the white and mako shark. 4. Dogfish sharks commonly studied in comparative anatomy classes are in the order Squaliformes. 5. Skates belong to the order Rajiformes and several groups of rays (stingrays, manta rays, etc.) belong to the order Myliobatiformes. 6. There are authenticated cases of attacks by the great white, mako, tiger, and hammerhead sharks, and casualties are more common in tropical and temperate waters of the Australian region. 7. Form and Function (Figure 24.8) a. Sharks are among the most gracefully streamlined of fishes; the body is fusiform. b. Thrust and lift is provided by an asymmetrical heterocercal tail in which the vertebral column turns upward and extends into the dorsal lobe of the caudal fin. c. Fins include paired pectoral and pelvic fins, one or two median dorsal fins, a median caudal fin, and sometimes a median anal fin. d. In the male, the medial part of the pelvic fin is modified to form a clasper used in copulation. e. Paired nostrils are anterior to the mouth. f. The lateral eyes are lidless; behind each eye is a spiracle, a remnant of the first gill slit. g. The tough, leathery skin has placoid scales that reduce water turbulence. h. Sharks track prey using an orderly sequence of sensitive senses. i. Sharks detect prey at a distance by large olfactory organs sensitive to one part per 10 billion. j. The nostrils to each side of hammerhead shark may improve stereo-olfaction. (Figure 24.9) k. Lateral line may assist in detecting/locating low frequency vibrations emitted by distant prey. l. Lateral line consists of neuromasts in interconnected tubes and pores on the side of the body. m. Most sharks have excellent vision even in dimly lit water. Use vision to detect close prey. n. Up close, sharks are guided by bioelectric fields that surround all animals. o. Electroreceptors, the ampullae of Lorenzini, are located on the shark’s head. (Figure 24.10) p. Upper and lower jaws are equipped with sharp, triangular teeth that are constantly replaced. q. The mouth opens into the large pharynx, which contains openings to gill slits and spiracles. r. A short esophagus runs to the stomach. s. A liver and pancreas open into the short, straight intestine. (Figure 24.11) t. The spiral valve in the intestine slows passage of food and increases absorptive area. u. The rectal gland secretes sodium chloride and in this regard assists the opisthonephric kidney. v. The heart chambers provide the standard circulatory flow through gills and body. w. Elasmobranchs retain nitrogenous compounds in the blood to raise blood solute concentrations and eliminate the osmotic inequality between blood and seawater. 8. Reproduction and Development a. All chondrichtheans have internal fertilization; maternal support of the embryo is variable. b. Those that lay large, yolky eggs immediately after fertilization are oviparous. c. Some sharks and all rays lay “mermaid’s purse” capsule that uses tendrils to latch onto kelp. d. The embryo is nourished from yolk for up to two years before hatching as a miniature adult. e. Sharks that retain embryos in the reproductive tract are ovoviviparous if the embryo is nourished by yolk. f. True viviparous reproduction occurs where embryos receive nourishment from the maternal bloodstream from nutritive secretions of the mother. g. Retention contributes to the success of this group but there is no further parental care. 9. Form and Function of Rays (Figures 24.12, 24.13) a. More than half of all elasmobranchs are rays; most are specialized for benthic life. b. The dorsoventrally flattened body and enlarged pectoral fins are used to propel themselves. c. Respiratory water enters through large spiracles on the top of the head. d. Teeth are adapted for crushing prey: molluscs, crustaceans and sometimes small fish. e. Stingrays have a whiplike tail with spines and venom glands. f. Electric rays have large electric organs on each side of the head. C. Subclass Holocephali: Chimeras (Figure 24.14) 1. Members of this small subclass are remnants of a line that diverged from the earliest shark lineage. 2. There are 33 extant species. 3. Fossil chimaeras first appeared in the Carboniferous and reached a zenith in the Cretaceous and early Tertiary, and then declined. 4. Mouth lacks teeth but has large flat plates for crushing food; the upper jaw is fused to the cranium. 5. The food includes a wide range of seaweed, molluscs, echinoderms, crustaceans and fish. 24.5. Osteichthyes: Bony Fishes (Figure 24.15) A. Origin, Evolution and Diversity 1. In the early to middle Silurian, a lineage of fishes with bony endoskeletons gave rise to a clade that contains 96% of living fishes and all living tetrapods. 2. Other early fishes are now known to also have had bone. 3. Three features unite bony fishes and tetrapod descendants. a. Endochondral bone is present that replaces cartilage developmentally. b. A lung or swim bladder is present that was evolved as an extension of the gut. c. They have several cranial and dental characters unique to this clade. 4. Osteichthyes does not define a valid taxon: is a term of convenience rather than a valid taxon. 5. A bony operculum and branchiostegal rays associate them with acanthodians. 6. By the middle Devonian, bony fishes developed into two major lineages. a. The ray-finned fishes, class Actinopterygii, radiated to form modern bony fishes. b. 7 species of lobe-finned fishes, class Sarcopterygii, include lungfishes and the coelacanth. 7. Operculum increases respiratory efficiency; outward rotation helps draw water across the gills. 8. The gas-filled structure off the esophagus helped in buoyancy and also in hypoxic waters. 9. In fishes that use these pouches for respiration, the pouches are called lungs; in fishes that use these pouches for buoyancy, the pouches are called swim bladders. 10. Specialization of jaw musculature improved feeding. 24.6. Class Actinopterygii: Ray-finned Fishes (Figures 24.16–24.21) A. Diversity 1. Over 27,000 species of ray-finned fishes constitute the most familiar bony fishes. 2. Palaeoniscids a. These were the earliest forms; they were small, had large eyes, a heterocercal caudal tail and interlocking scales with an outer layer of ganoin. b. They had a single dorsal fin and numerous bony rays derived from scales stacked end to end. c. Though late Silurian fossils exist, these fishes flourished throughout late Paleozoic. d. They were distinct from the lobe-finned fishes and saw the ostracoderms, etc. decline. e. Several clades arose from those earliest ray-finned fishes. 3. Bichirs a. This group is in the clade Cladista. b. They have lungs, heavy ganoid scales, and other characteristics similar to the palaeoniscids. c. There are 16 species of which all live in the freshwaters of Africa. 4. Chondrosteons a. This group contains 27 species of freshwater and anadromous sturgeons and paddlefishes. b. Dam construction, overfishing and pollution have led to their decline. 5. Neopterygians a. They appeared in the late Permian and radiated extensively during the Mesozoic. b. During the Mesozoic, one lineage gave rise to the modern bony fishes, the teleosts. c. Two surviving early neopterygians are the bowfin and the gars. d. Gars and bowfin gulp air and use the vascularized swim bladder to supplement the gills. 6. Teleosts a. Teleosts constitute 96% of all living fishes and half of all vertebrates. b. Perhaps 5,000–10,000 remain undescribed in remote areas, but also in North America. c. Teleosts range from 10 millimeters to 17 meters long, and up to 900 kilograms in weight. d. They survive from 5,200 meters altitude in Tibet to 8,000 meters below the ocean surface. e. Some can live in hot springs at 44o C while others survive under Antarctic ice at –2o C. f. Some live in salt concentrations three times seawater; others in swamps devoid of oxygen. B. Morphological Trends 1. Heavy dermal armor was replaced by light, thin, flexible cycloid and ctenoid scales. 2. Some eels, catfishes and others completely lost scales. 3. Increased mobility from shedding armor helps fish avoid predators and aided in food getting. 4. Fins changed to provide greater mobility and serve a variety of functions: braking, streamlining and social communication. 5. The homocercal tail allowed greater speed and buoyancy. 6. The jaw changed to increase suctioning and protrusion to secure food. 24.7. Class Sarcopterygii: Lobe-finned Fishes (Figures 24.22, 24.23) A. Diversity 1. Tetrapod ancestor is found within a group of extinct sarcopterygian fishes called rhipdistians. 2. Early sarcopterygians had lungs, gills, and a heterocercal-type tail. 3. During the Paleozoic, the tail became a symmetrical diphycercal tail. (Figure 24.16) 4. Sarcopterygians had powerful jaws, heavy, enameled scales with a dentine-like material called cosmine, and strong, fleshy, paired lobed fins. 5. Today, clade includes only 8 fish species: 6 species of lungfishes and 2 species of coelacanths. 6. Australia lungfishes, unlike close relatives, rely on gill respiration and cannot survive long out of water. 7. The South American and African lungfish can live out of water for long periods of time. 8. Rhipidistians and coelacanths were termed crossopterygians, a polyphyletic group no longer used. 9. Rhipidistians flourished in the late Paleozoic and then became extinct; they include the ancestors of the tetrapods. 10. The Coelacanth a. Coelacanths arose during the Devonian, radiated, reached a peak in the Mesozoic and dramatically declined. b. Thought to be extinct 70 million years, a specimen was dredged up in 1938. c. Eventually more were caught off the coast of the Comoro Islands, and in 1998, in Indonesia. d. The living coelacanth is a descendant of Devonian freshwater stock. e. The tail is diphycercal with a small lobe between the upper and lower caudal lobes. f. Young coelacanths are born fully formed after hatching from 0-9 cm diameter eggs. 24.8. Structural and Functional Adaptations of Fishes A. Locomotion in Water (Figures 24.24–24.26) 1. Speed a. Most fishes swim fastest at ten body lengths per second; larger fish therefore swim faster. b. Short bursts of speed are possible for a few seconds. 2. Mechanism a. The trunk and tail musculature propels a fish. b. Muscles are arranged in zigzag (W-shaped from the side) bands called myomeres.. c. Internally the bands are folded and nested; each myomere pulls on several vertebrae. d. Fish undulations move backward against the water, producing a reactive force with two parts. e. The thrust pushes the fish forward and overcomes drag. f. The lateral force makes the fish’s head “yaw”; a large and rigid head minimizes yaw. g. The swaying body generates too much drag for fast speed. h. Fast fish are less flexible and generate all thrust with their caudal fins. i. Fast oceanic fish have swept-back sickle-like tail fins like high-aspect ratio wings of birds. j. Swimming is the most economical form of motion because water buoys the animal. k. The energy cost per kilogram of body weight for traveling one kilometer is 0.39 Kcal for swimming, 1.45 Kcal for flying and 5.43 for walking. l. It is yet to be determined how aquatic animals can move through water with little turbulence. B. Neutral Buoyancy and the Swim Bladder (Figure 24.27) 1. Fish are slightly heavier than water. 2. To keep from sinking, a shark must continually move forward; fins keep it “angled up.” 3. Shark liver has a special fatty hydrocarbon, or squaline, that keeps the shark a little buoyant. 4. The swim bladder, as a gas-filled space, is the most efficient flotation device. 5. The swim bladder arose from the paired lungs of primitive Devonian bony fishes. 6. Swim bladders are absent in tunas, some abyssal fishes, and most bottom dwellers. 7. A fish can control depth by adjusting the volume of gas in the swim bladder. 8. As a fish descends, high pressure compresses bladder, making the total density of the fish greater. 9. As a fish ascends, the bladder expands making the fish lighter and it will rise ever faster. 10. Gas is removed in two ways. a. Primitive physostomous fishes have pneumatic duct connecting swim bladder and esophagus. b. Advanced teleosts are physoclistous; the pneumatic duct is lost and gas must be secreted into the blood from a vascularized area. 11. Both types require gas to be secreted into the bladder from the blood via the gas gland. 12. A network of capillaries called the rete mirabile is a countercurrent exchange system that is used to trap gases. 13. The gas gland secretes lactic acid that forces hemoglobin to release its load of oxygen. 14. The rete capillaries are short in surface-dwelling fish and very long in deep-sea fishes. 15. A few shallow-water-inhabiting physostomes gulp air to fill the swim bladder. 16. Even at 2400 meters, the bladder is inflated, mostly with oxygen, but also with variable amounts of nitrogen, carbon dioxide, argon, and even some carbon monoxide, and the pressure within the air bladder must exceed 240 atmospheres. C. Hearing and Weberian Ossicles (Figure 24.28) 1. Fish, like other vertebrates, detect sounds as vibrations in the inner ear. 2. A teleost group known as the ostariophysans possesses Weberian ossicles, which allow them to hear faint sounds over a much broader range than other teleosts. D. Respiration (Figure 24.29) 1. Fish gills are filaments with thin epidermal membranes folded into plate-like lamellae. 2. The gills are inside the pharyngeal cavity and covered with a movable flap, the operculum. 3. The operculum protects the delicate gill filaments and streamlines the body. 4. Pumping action by the operculum helps move water through the gills. 5. Although it appears pulsatile, water flow over gills is continuous. 6. Water flow opposes blood flow; countercurrent exchange maximizes gas exchange between them. 7. Some bony fishes remove 85% of the oxygen from water that passes over their gills. 8. Some fishes use ram ventilation; forward movement is sufficient to force water across gills. 9. Such fishes are asphyxiated in a restrictive aquarium even if the water is saturated with oxygen. 10. Fishes Out of Water a. Lungs of lungfishes allow them to respire from air. b. Eels can wriggle over land during rainy weather; use skin as their major respiratory surface. c. Bowfin uses gills at cooler temperatures and lung-like swim bladder at warmer temperatures. d. The electric eel has degenerate gills and gulps air through its vascular mouth cavity. e. The Indian climbing perch spends most of its time on land, breathing air in special chambers. E. Osmotic Regulation (Figure 24.30) 1. Freshwater has far less salt than is in fish blood; water tends to enter the body of the fish and salt is lost by diffusion. 2. The scaled and mucous-covered body is mostly impermeable, but gills allow water and salt fluxes. 3. Freshwater fishes are hyperosmotic regulators. a. The opisthonephric kidney pumps excess water out. b. Salt-absorbing cells within epithelium actively move salt ions from water to the fishes’ blood. c. Very efficient; a freshwater fish devotes little energy to keeping osmotic balance. 4. About 90% of bony fishes are restricted to either freshwater or seawater habitats. 5. Euryhaline fishes live in estuaries where salinity fluctuates throughout the day. 6. Marine bony fishes are hypoosmotic regulators. a. Marine fishes have a much lower blood salt concentration than in the seawater around them. b. Therefore they tend to lose water and gain salt; the marine fish risks “drying out.” c. To compensate for water loss, marine teleosts drink seawater to bring in more unneeded salt. d. Unneeded salt is carried by the blood to the gills and secreted by special salt-secretory cells. e. Divalent ions of magnesium, sulfate and calcium are left in intestine and leave with feces. f. Some divalent ions enter the bloodstream and are excreted by the kidney. g. Marine fish excrete divalent ions by tubular secretion; glomeruli are small or missing. F. Feeding Behavior (Figures 24.31, 24.32) 1. Fish devote most of their time searching for food to eat and eating. 2. With the evolution of jaws, fish left a passive filter-feeding life and entered a predator-prey battle. 3. Most fish are carnivores that feed on zooplankton, insect larvae and other aquatic animals. 4. Most fish do not chew food since it would block water flow across the gills. 5. A few can briefly crack prey items with their teeth, or have molar-like teeth in the throat. 6. Most swallow food whole as water pressure sweeps food into open mouth. 7. Some are herbivores and eat plants and algae; they are crucial intermediates in the food chain. 8. Suspension feeders are a third group, and crop the abundant microorganisms of the sea. 9. Many of the plankton feeders swim in large schools and use the gill rakers to strain food. 10. Omnivores can feed on both plant and animal food. 11. Scavengers feed on organic debris. 12. Detritovores consume fine particulate organic matter. 13. Parasitic fishes suck the body fluids of other fishes. 14. Digestion follows the vertebrate plan; a few lack stomachs entirely. 15. The intestine tends to be shorter in carnivores and long and coiled in herbivores. 16. The stomach primarily stores food; the intestine both digests and absorbs nutrients. 17. Only teleost fishes have pyloric ceca, apparently for fat absorption. G. Migration 1. Freshwater Eels (Figure 24.33) a. Eels have presented a life history puzzle for centuries. b. Eels are catadromous, developing to maturity in freshwater but migrating to the sea to spawn. c. Each fall large numbers of adults swim downriver to the sea to spawn, but none ever return. d. Each spring, many young eels or “elvers” appeared in coastal waters and swam upstream. e. Grassi and Calandruccio reported in 1896 that the elvers were advanced juveniles; the true larval eels were tiny leaf-shaped, transparent creatures. f. Johann Schmidt traced eel migrations by examining contents of commercial plankton nets. g. Adult eels were tracked to the Sargasso Sea southeast of Bermuda. h. At depths of 300 meters or more, eels spawn and die. i. Minute larvae journey back to the streams of Europe and North America. j. American eel larvae make the trip back in only eight months since the Sargasso Sea is much closer to the American coastline, whereas the European eel larvae take three years. 2. Homing Salmon (Figures 24.34, 24.35) a. Salmon are anadromous, growing up in the sea but returning to freshwater to spawn. b. There are six species of Pacific salmon and one Atlantic salmon that migrates. c. Atlantic salmon make repeated spawning runs, but the Pacific species spawn once and die. d. The Pacific species of sockeye salmon migrates downstream, roams the Pacific for four years, and then returns to spawn in the headwaters of its parent stream. e. Young fish are imprinted on the odor of their stream; they may navigate to the stream mouth by sensing Earth’s magnetic field or the angle of the sun, and then smelling their way home. f. Salmon are endangered from losses associated with stream degradation by logging, pollution and hydroelectric dams. H. Reproduction and Growth (Figures 24.35–24.38) 1. Variability in Mating Strategies a. Most fishes are dioecious with external fertilization and external development. b. Guppies and mollies represent ovoviviparous fish that develop in the ovarian cavity. c. Some sharks are viviparous with some kind of placental attachment to nourish young. d. Most oviparous pelagic fish lay many eggs; a female cod may release 4–6 million eggs. e. Near-shore and bottom-dwelling species lay larger, yolky, nonbuoyant and adhesive eggs. f. Some bury eggs. Many attach eggs to vegetation. Some incubate them in their mouths. g. Many benthic spawners guard their eggs; usually the male is the guard. h.. Freshwater fishes produce nonbuoyant eggs; when more care needed, clutch size is smaller. i. Freshwater fishes may have elaborate mating dances before spawning. j. Some fishes are sequential hermaphrodites, in which they exist as one sex, then become another, or synchronous hermaphrodites, in which they have both functional gonads. 2 Development and Growth a. An egg soon takes up water, the outer layer hardens and cleavage occurs. b. The blastoderm develops and the yolk is consumed. c. The fish hatches carrying a semitransparent yolk sac to supply food until it can forage. d. The change from larva to adult may be dramatic in body shape, fins, color patterns, etc. e. Growth is temperature dependent; warmer fish grow more rapidly. f. Annual rings on scales, otoliths, etc. reflect seasonal growth cycles. g. Most fish continue to grow throughout life and do not stop at maturity. h. Differences between morphologies of larvae and adults and males and females have made fish taxonomy difficult. 24.9. Classification Phylum Chordata Subphylum Vertebrata (Craniata) Class Myxini Class Petromyzontida Superclass Gnathostomata Class Chondrichthyes Subclass Elasmobranchii Subclass Holocephali Class Actinopterygii Subclass Cladistia Subclass Chondrostei Subclass Neopterygii Class Sarcopterygii Lecture Enrichment 1. The prehistoric aquatic environment had ample invertebrates, but the fishes were immediately successful; this contrast can lead to discussion on the limitations of invertebrate body plans, advantages of size, and the potential for adaptations once a threshold is reached. Another case was the radiation that was possible when wings opened up the “aerial niche.” 2. The countercurrent concept introduced here has applications in other sections such as bird lungs, “warm-blooded” tunas, arctic fox legs, etc. Diagrams and a mathematical “walk-through” of gradients best illustrate countercurrent. 3. The ability to “sense” magnetic fields will be alien to students since humans do not consciously detect such fields. We can detect them if we use car radios or metal detectors that convert distortions in magnetic fields into audio signals; some students may be able to relate experiences with this device. Attenborough uses an aquatic version of a metal detector to detect mudfish in the fish section of the 13-hour version of Life on Earth. Commentary/Lesson Plan Background: Those students with fishing experience can relate their knowledge to classmates if the instructor directs questions concerning wetting hands before handling a fish that is to be released, etc. Misconceptions: Thanks to popular films, sharks will carry much baggage for unwarranted attacks and sinister behaviors well beyond their mental abilities. The ability of fish to detect changes in magnetic fields may appear mysterious, as will salmon navigation and other electric fish abilities because we do not detect these stimuli; this can be explained logically, and paranormal forces can be eliminated from consideration. Schedule: HOUR 1 24.1. Diversity A. Overview 24.2. Ancestry and Relationships of Major Groups of Fishes A. History 24.3. Superclass Agnatha: Jawless Fishes A. Overview B. Class Myxini: Hagfishes C. Class Petromyzontida: Lampreys 24.4. Class Chondrichthyes: Cartilaginous Fishes A. Overview B. Subclass Elasmobranchii: Sharks, Skates and Rays C. Subclass Holocephali: Chimeras 24.5. Osteichthyes: Bony Fishes A. Origin, Evolution and Diversity 24.6. Class Actinopterygii: Ray-finned Fishes A. Diversity B. Morphological Trends HOUR 2 24.7. Class Sarcopterygii: Lobe-finned Fishes A. Diversity 24.8. Structural and Functional Adaptations of Fishes A. Locomotion in Water B. Neutral Buoyancy and the Swim Bladder C. Respiration D. Osmotic Regulation E. Feeding Behavior F. Migration G. Reproduction and Growth 24.9. Classification ADVANCED CLASS QUESTIONS: 1. If you took a “time machine” back to the Devonian “Age of Fishes,” what groups would you see appearing? What groups would be ascending or descending from the scene? Which would most resemble our modern teleost fishes? Which would be closest to our ancestors? Answer: If you were to travel back to the Devonian period, often referred to as the "Age of Fishes," you would encounter a diverse array of aquatic vertebrates, including several groups that were emerging or diversifying during this time. Here are some of the key groups you might encounter: 1. Early Jawed Fishes (Placoderms and Acanthodians): The Devonian saw the rise of jawed fishes, with groups like placoderms and acanthodians being prominent. Placoderms were armored fishes with bony plates covering their heads and bodies, while acanthodians were small, spiny fishes with distinctive fin spines. 2. Early Cartilaginous Fishes (Chondrichthyans): Cartilaginous fishes, including early sharks and rays, were also present during the Devonian. These fishes had skeletons made of cartilage rather than bone and exhibited a wide range of body forms and lifestyles. 3. Early Bony Fishes (Osteichthyans): Osteichthyans, or bony fishes, were diversifying during the Devonian. Early groups like the lobe-finned fishes (sarcopterygians) and ray-finned fishes (actinopterygians) were present, with some species exhibiting primitive characteristics that would later give rise to modern bony fishes. 4. Tetrapodomorphs: Within the lobe-finned fishes, tetrapodomorphs were emerging, representing transitional forms between fish and tetrapods (four-limbed vertebrates). These included early tetrapods (amphibians) and their close relatives, such as Tiktaalik, which had fish-like features but also limb-like appendages capable of supporting its body on land. As for which group would most resemble modern teleost fishes, it's important to note that teleosts (modern ray-finned fishes) did not emerge until later in the Mesozoic era. During the Devonian, the ray-finned fishes were diversifying, but they had not yet evolved the characteristics that define modern teleosts. Instead, you would encounter early ray-finned fishes with features transitional between their lobe-finned ancestors and modern teleosts. Regarding which group would be closest to our ancestors, it depends on the specific lineage leading to tetrapods (four-limbed vertebrates), including mammals, reptiles, and birds. Tetrapodomorphs, particularly early tetrapods like Tiktaalik, would be among the closest relatives to our ancestors, as they represent the transition from aquatic to terrestrial life and possess key anatomical features shared with early tetrapods. Overall, the Devonian period was a time of significant evolutionary innovation and diversification among fishes, laying the foundation for the emergence of modern vertebrate groups, including tetrapods and teleost fishes, in subsequent geological periods. 2. Many features can be lost when a species takes up a simpler life as a parasite. Why are we certain that the hagfishes and lamprey are descendants of early primitive fishes instead of a modern fish that has simply lost many features due to its lifestyle? Answer:The classification of hagfishes and lampreys as descendants of early primitive fishes rather than modern fishes that have lost features due to their parasitic lifestyle is based on a combination of anatomical, developmental, genetic, and evolutionary evidence. Here are some key reasons why scientists are confident in this classification: 1. Anatomical and Developmental Features: Hagfishes and lampreys exhibit numerous anatomical and developmental characteristics that are primitive or ancestral traits shared with early vertebrates. These include a cartilaginous skeleton, lack of paired fins, and a notochord that persists throughout their lives, all of which are characteristic of early vertebrates like jawless fishes. 2. Evolutionary Relationships: Phylogenetic analyses, which compare anatomical, molecular, and developmental characteristics among different organisms to infer evolutionary relationships, consistently place hagfishes and lampreys as basal lineages within the vertebrate tree of life. These analyses suggest that hagfishes and lampreys diverged early from other vertebrates, indicating their primitive evolutionary status. 3. Fossil Record: Although hagfishes and lampreys have poor fossilization potential due to their soft-bodied nature, there is some fossil evidence suggesting that they have ancient origins dating back to the Paleozoic era. Fossilized remains of jawless fishes resembling hagfishes and lampreys have been found in deposits dating to the Devonian period, supporting their status as primitive vertebrates. 4. Genetic Evidence: Molecular studies comparing the genomes of hagfishes, lampreys, and other vertebrates provide further support for their primitive status. These studies have revealed shared genetic features and conserved gene regulatory networks between hagfishes, lampreys, and other jawless fishes, indicating their evolutionary continuity with early vertebrates. 5. Functional Constraints: While it is true that parasitic lifestyles can lead to the loss of certain features or adaptations, the specific anatomical and developmental characteristics of hagfishes and lampreys are not consistent with those expected from secondary simplification due to parasitism. Instead, their unique anatomical features are more likely indicative of their primitive evolutionary status as basal vertebrates. Overall, the cumulative evidence from comparative anatomy, developmental biology, genetics, evolutionary relationships, and the fossil record strongly supports the classification of hagfishes and lampreys as descendants of early primitive fishes rather than modern fishes that have lost features due to their parasitic lifestyle. 3. So many other species have been eliminated by the continuous changes of climate and environment on a geological time scale; speculate on how the coelacanth has managed to survive until modern times when so many of its cohort became extinct. Answer: The coelacanth's survival until modern times despite the extinction of many of its cohort can be attributed to several key factors: 1. Living Fossil Status: Coelacanths are often referred to as "living fossils" because they possess many primitive anatomical features that have remained relatively unchanged for millions of years. These features include lobed fins, a unique hinge-like structure in the skull, and a largely unchanged body plan. This evolutionary stasis may have helped coelacanths persist through environmental changes that caused the extinction of other species. 2. Habitat Stability: Coelacanths are deep-sea fish that inhabit relatively stable and secluded environments, such as underwater caves and rocky crevices. These habitats provide protection from environmental disturbances and fluctuations in temperature, salinity, and other factors that can threaten survival. By occupying stable habitats, coelacanths may have been less vulnerable to extinction events affecting other marine species. 3. Low Reproductive Rate: Coelacanths have relatively low reproductive rates compared to many other fish species. They produce few offspring per reproductive event, and females have long gestation periods. While low reproductive rates can limit population growth, they may also confer advantages in environments with low predation pressure and stable ecological conditions, allowing coelacanths to persist over geological time scales. 4. Behavioral Adaptations: Coelacanths exhibit unique behavioral adaptations that contribute to their survival in deep-sea environments. They are nocturnal, spending much of their time resting in caves during the day and actively foraging for prey at night. This behavior helps them avoid predators and conserve energy in the nutrient-poor deep-sea environment. 5. Evolutionary Flexibility: Despite their primitive features, coelacanths have demonstrated some degree of evolutionary flexibility and adaptability over time. While their overall body plan has remained relatively unchanged, they have likely undergone genetic and physiological adaptations to cope with changing environmental conditions and ecological pressures. Overall, the combination of primitive anatomical features, stable deep-sea habitats, low reproductive rates, unique behavioral adaptations, and some degree of evolutionary flexibility has likely contributed to the coelacanth's remarkable survival until modern times. Despite the extinction of many of its cohort, coelacanths have managed to persist as living relics of an ancient lineage, providing valuable insights into the evolutionary history of vertebrates. 4. If a person tires of keeping goldfish and decides to dump them overboard to fend for themselves in the ocean, what physiological problems will distress the goldfish? If a person decides to shut down their expensive marine aquarium and dump the fish in the nearest freshwater pond, what problems does this cause a marine fish? Answer: Dumping goldfish into the ocean or transferring marine fish to a freshwater pond can have serious physiological consequences for the fish involved: 1. Goldfish in the Ocean: - Temperature Shock: Goldfish are freshwater fish adapted to a specific range of temperature and salinity. Dumping them into the ocean exposes them to significantly different environmental conditions, including lower temperatures and higher salinity, which can lead to thermal shock and stress. - Osmotic Stress: Marine environments have higher salinity levels than freshwater habitats. When goldfish are introduced to saltwater, they experience osmotic stress as water moves out of their bodies to balance the higher salt concentration in the surrounding environment, leading to dehydration and electrolyte imbalances. - Predation Risk: Goldfish released into the ocean may become prey for marine predators, as they lack the adaptations and behaviors necessary to evade or defend against predators in marine ecosystems. This can result in increased mortality rates for the released goldfish. - Competition and Disease Transmission: Dumping goldfish into the ocean can introduce invasive species and disrupt native ecosystems. Goldfish may compete with native marine species for resources and habitat, potentially leading to ecological imbalances and declines in native biodiversity. Additionally, goldfish may carry diseases or parasites that could be transmitted to native marine populations, further impacting ecosystem health. 2. Marine Fish in Freshwater Ponds: - Osmotic Stress: Marine fish are adapted to living in saltwater environments and have physiological mechanisms to regulate their internal salt balance. When transferred to freshwater ponds with lower salinity levels, marine fish experience osmotic stress as water moves into their bodies, potentially causing cellular swelling, organ dysfunction, and electrolyte imbalances. - Respiratory Problems: Marine fish have specialized gills that are adapted to extract oxygen from saltwater. In freshwater environments, the osmotic gradient across their gills is disrupted, affecting gas exchange and potentially leading to respiratory distress. - pH Imbalance: The pH of freshwater ponds may differ significantly from that of marine environments. Sudden changes in pH can stress marine fish and disrupt their acid-base balance, affecting various physiological processes. - Predation and Competition: Marine fish introduced into freshwater ponds may face predation from native freshwater predators and competition from native freshwater fish species. Their lack of adaptations to freshwater environments puts them at a disadvantage, increasing their vulnerability to predation and competition. - Disease Susceptibility: Marine fish transferred to freshwater ponds may be more susceptible to diseases and infections due to stress and immune system suppression associated with environmental changes. They may also introduce novel pathogens to freshwater ecosystems, potentially impacting native fish populations. In summary, transferring goldfish to the ocean or marine fish to freshwater ponds can cause significant physiological stress, disrupt ecosystem dynamics, and pose risks to native biodiversity. It is essential to avoid releasing or transferring non-native fish species into natural environments and instead seek responsible alternatives such as rehoming with responsible pet owners or returning them to reputable aquarium stores. 5. Why would a darter, a bottom-dwelling fish in fast-moving rapids, evolve to lose its swim bladder? Answer: The loss of the swim bladder in darters, which are bottom-dwelling fish in fast-moving rapids, can be attributed to several selective pressures and ecological factors associated with their unique habitat and lifestyle: 1. Buoyancy Regulation: The swim bladder is an organ found in many fish species that helps control buoyancy by regulating the volume of gas within the bladder. However, in fast-flowing rivers and streams where darters typically inhabit, buoyancy control may be less critical compared to fish living in slower-moving or still waters. The strong currents in rapids provide natural buoyancy to fish, reducing the need for a swim bladder to maintain position in the water column. 2. Streamlined Body Shape: Darters have evolved streamlined body shapes and specialized fins to navigate fast-flowing currents and turbulent waters efficiently. The absence of a swim bladder may contribute to their streamlined morphology, reducing hydrodynamic drag and allowing them to maintain stability and maneuverability in turbulent environments. 3. Foraging Behavior: Darters are primarily benthic (bottom-dwelling) fish that feed on benthic invertebrates such as insect larvae, crustaceans, and small mollusks. The loss of a swim bladder may facilitate their bottom-dwelling foraging behavior by allowing them to maintain contact with the substrate and navigate rocky or uneven surfaces more effectively. 4. Resistance to Buoyancy Changes: In fast-flowing rivers and streams, water currents can change rapidly in speed and direction. Fish with swim bladders may experience challenges in adjusting their buoyancy quickly to changes in water flow, potentially affecting their ability to maintain position or stability. The absence of a swim bladder in darters may confer advantages in adapting to rapid changes in water flow and turbulence. 5. Energy Conservation: Maintaining a swim bladder requires metabolic energy for gas secretion and regulation. In fast-flowing habitats where energy resources may be limited, the loss of a swim bladder in darters may represent an energy-saving adaptation, allowing them to allocate resources more efficiently towards growth, reproduction, and survival. Overall, the loss of the swim bladder in darters is likely an adaptation to their specialized habitat and ecological niche in fast-moving rapids. While the swim bladder provides important functions in buoyancy control and gas exchange for many fish species, darters have evolved alternative morphological and behavioral adaptations that allow them to thrive in their unique environment without this organ. CHAPTER 25 EARLY TETRAPODS AND MODERN AMPHIBIANS CHAPTER OUTLINE 25.1. From Water to Land A. Adaptations 1. Animal composition is mostly water; land represents a relatively dangerous habitat. 2. Vascular plants, pulmonate snails and tracheate arthropods all made the transition earlier. 3. Amphibians most clearly represent this vertebrate transitional stage. 4. Accommodations address oxygen content, density, temperature regulation and habitat diversity. a. Oxygen is 20 times more abundant in air and diffuses much more rapidly through air. b. Air is 1000 times less dense and provides less buoyancy than water; limbs and the skeleton must therefore support more weight. c. Air rapidly fluctuates in temperature relative to water; animals must adjust to these extremes. d. The variety of terrestrial habitats allows dramatically greater opportunities for adaptation. 25.2. Devonian Origin of Tetrapods (Figures 25.1--25.3) A. Devonian Origin of Tetrapods 1. By Devonian period, bony fishes diversified to include many freshwater forms. B. Combinations of characteristics that originally evolved in aquatic habitats, gave its possessors the ability to explore terrestrial habitats. 1. Two structures that connected to the pharynx, an air-filled cavity functioned as a swim bladder and paired internal nares which functioned in chemoreception. a. On land, these structures would be used to draw in oxygen-rich air through the nares and into the air-filled cavity. 2. Bony elements of paired fins, modified for support and movement underwater, would provide the same function on land. C. Freshwater habitats are inherently unstable. D. Multiple fish groups, given a combination of structures that could be coopted for terrestrial breathing and locomotion, evolved some degree of terrestriality. 1. Mudskippers and lungfish are examples of evolution of terrestriality by fishes. 2. Only one transition during Devonian period, provided ancestral lineage of all tetrapod vertebrates. This lineage evolved the adaptations associated with air breathing, including increased vascularization of the air-filled cavity and a double circulation to direct deoxygenated blood into the lungs and oxygenated blood out of the lungs to other body tissues E. The bony elements of the fins of lobe-finned fishes resemble the homologous structures of amphibians. F. Eusthenopteron could paddle through the bottom mud; had both lungs and “walking” fins. G. The fossil genus Tiktaalik is an intermediate between lobe-finned fishes and tetrapods; it probably used its limbs to support its body while placing its snout above water to breathe air in shallow water. H. Acanthostega had clearly formed digits on both fore- and hindlimbs; its body still drug on the ground. I. Ichthyostega had bulkier limb muscles to walk onto land, although not with great efficiency. J. Fossils of Devonian tetrapods have > 5 fingers and toes, thus lobe-finned fishes are sister group to tetrapods. (Figure 25.3) K. Adaptations for life on land include skull, teeth, pectoral girdle and jointed limbs. L. Tetrapods also selected for stronger backbone, muscles to support the body in air, muscles to elevate the head, stronger shoulder and hip girdles, a more protective rib cage, ear structure and longer snout. M. Ichthyostega retained aquatic features including fin rays and opercular bones. N. Early Diversification of Tetrapods 1. Several extinct lineages, Lissamphibia, (including modern amphibians) formed the temnospondyls. 2. Temnospondyls generally had four rather than five digits on the forelimb. 3. During the Carboniferous, amphibians developed additional adaptations for living in water. 4. Lepospondyls and anthracosaurs are, based on skull structure, closer to amniotes than temnospondyls. 25.3. Modern Amphibians A. Diversity 1. Over 6770 living species are known in the three amphibian orders. 2. Metamorphosed adults use redesigned olfactory epithelium to sense airborne odors. 3. They remain tied to water; eggs are aquatic, and the larvae depend on gills for respiration. 4. Some salamanders retain aquatic morphology throughout life; others lack the larval phase. 5. The thin skin loses water rapidly; this restricts even terrestrial forms to moist habitats. 6. Being ectothermic, their body temperature depends on the environment and restricts their range. 7. Eggs easily desiccate and must be shed into water or kept moist; a few brood their young. B. Caecilians: Order Gymnophiona (Apoda) (Figure 25.4) 1. About 190 living species of elongate, limbless, burrowing caecilians are known. 2. They live in tropical forests in South America, Africa, India, and Southeast Asia. 3. Their long bodies have many vertebrae, long ribs, no limbs, and a terminal anus. 4. They eat primarily worms and small underground invertebrates. 5. Internal fertilization; male has protrusible copulatory organ. 6. Eggs are deposited in moist ground near water. 7. Some species have aquatic larvae; for others, larval development occurs within the egg. 8. In some species, eggs are guarded and develop in folds of the body. 9. In other species, viviparity allows embryos to obtain nourishment by eating the wall of the oviduct. C. Salamanders: Order Caudata (Urodela) 1. About 620 species of living salamanders are found mostly in northern temperate regions. 2. Most are small, under 15 centimeters long, but the Japanese giant salamander is 1.5 meters long. 3. Usually, limbs are at right angles to the trunk; forelimbs and hindlimbs are about equal in length. 4. Burrowing species and some aquatic forms may have lost their limbs. 5. Are carnivorous as both larvae and adults, eating worms, small arthropods and molluscs. 6. Food is relatively rich in proteins; therefore they do not store much fat or glycogen. 7. They are ectotherms with a low metabolic rate. 8. Life Cycles (Figures 25.5 − 25.7) a. Some are aquatic throughout their life cycle; most have aquatic larvae and terrestrial adults. b. Most salamanders fertilize eggs internally. c. The female picks up a spermatophore that has been deposited on a leaf or stick. d. Aquatic species lay eggs in clusters or stringy masses. e. Completely terrestrial species lay eggs in small, grape-like clusters under logs or in soft earth. f. Terrestrial species undergo direct development, hatching as miniature adults. g. Some North American newts have aquatic larvae that metamorphose into terrestrial juveniles that again metamorphose into secondarily aquatic, breeding adults. h. Some newt populations skip the terrestrial “red eft” stage and remain entirely aquatic. 9. Respiration (Figure 25.8) a. Salamanders have a wide array of respiratory mechanisms. b. They have extensive vascular nets in skin that exchange oxygen and carbon dioxide. c. At various stages, they may also have external gills, lungs, both gills and lungs, or neither. d. Salamanders with an aquatic stage hatch with gills and lose them at metamorphosis. e. Several diverse lineages fail to undergo metamorphosis and retain gills and a fin-like tail. f. Where present, lungs are present from birth and become functional following metamorphosis. g. Aquatic amphiumas lose gills and respire by lungs, holding nostrils above the water surface. h. Many species in the terrestrial family Plethodontidae are lungless and strictly terrestrial. i. Respiratory gases may also be exchanged across the vascularized lining of the mouth cavity. j. Lungless salamanders likely evolved in cold streams where lungs would have been too buoyant. 10. Paedomorphosis (Figures 25.9, 25.10) a. Paedomorphosis is the preservation of pre-adult features into adulthood. b. Eliminating ancestral adult morphology is a trend found in salamander evolution. c. Non-metamorphic species that retain gills, etc. are perennibranchiate. d. Obligate perennibranchiates, like the mudpuppy, have never been observed to metamorphose. e. Others have larval morphology when sexually mature.. f. In Mexico and the U.S., Ambystoma tigrinum may stay in a gilled stage as an axolotl. g. When ponds dry up, they may metamorphose into a terrestrial form and migrate to a new pond. h. Axolotls treated with thyroid hormone will metamorphose artificially; the pituitary gland appears to be the controlling factor in natural populations. i. Paedomorphosis may be partial; the amphioxus shifts to lungs but otherwise remains larval. j. In another species, the larval appendages have been maintained to preserve a climbing ability. D. Frogs and Toads: Order Anura (Salientia) (Figures 25.11–25.13) 1. Includes ~ 5970 species of frogs and toads. 2. This group is known from the Triassic period, 250 million years ago. 3. Aquatic reproduction and a water-permeable skin, they must be near water. 4. Ectothermy keeps anurans from inhabiting polar and subarctic habitats. 5. All have a tailed larval stage to become tailless, jumping adults. 6. Eggs hatch into tadpoles with a long, finned tail, no legs, internal and external gills and specialized mouthparts for (usually) herbivorous feeding. 7. Their internal anatomy is different and they look and act different from adult frogs. 8. The perennibranchiate condition never occurs in frogs and toads. 9. There are 49 families of frogs and toads. a. Family Ranidae contains the common larger frogs in North America. b. Family Hylidae includes the tree frogs. c. Family Bufonidae contains toads with thicker skins and prominent warts. 10. Habitats and Distribution a. Members of the family Ranidae are common in temperate and tropical regions. b. Lithobates sylvatica, the wood frog, found on damp forest floors, breed in pools. c. Bullfrogs and green frogs occur in or near permanent water and swamps. d. The leopard frog is widespread and commonly studied in laboratories. e. Populations declining worldwide and becoming geographically fragmented; epidemic fungal infections and habitat loss are partly responsible. f. Malformed limbs are often associated with trematode infections. g. Most larger frogs are solitary until breeding season. h. During the breeding season, males are especially noisy when trying to attract a female. i. They hold forelimbs near the body when swimming with their powerful hindlimbs. j. When they surface to breathe, only the head and foreparts are exposed. k. During winter in temperate climates, they hibernate in soft mud in the bottom of pools. l. During hibernation period, the little energy they use is provided from stored glycogen and fat. m. Frost-tolerant frogs prepare for freezing by accumulating glucose and glycerol in body fluids; this protects them from the otherwise damaging effects of ice-crystal formation. n. Many are easy prey; they defend themselves by aggression, concealment, and poison glands. o. Many species of amphibians have suffered from changes in the environment and climate brought about by human impact. (Figure 25.14) p. Declines in population survival may be accompanied by an increased incidence of malformed individuals such as frogs with extra limbs. q. Climatic changes that reduce water depth at oviposition sites increases ultraviolet exposure of embryos and make them more susceptible to fungal infection. r. Decline of some amphibians may be caused by other amphibians such as Bufo marinus. 11. Integument and Coloration (Figures 25.15–25.17) a. Frog skin is thin, moist and attached loosely to the body at a few points. b. The skin is composed of an outer stratified epidermis and an inner spongy dermis. c. The epidermal layer is shed periodically; it contains deposits of keratin. d. More terrestrial amphibians have heavier deposits of keratin; it remains soft. e. The epidermis has two types of integumentary glands: mucous glands secrete protective waterproofing and large serous glands produce a whitish, watery poison. f. Dendrobatid frogs of South America secrete highly toxic skin poisons. g. Specialized pigment cells, chromatophores, produce skin color in frogs. h. Uppermost are xanthophores with yellow, orange or red pigments. i. Middle layer made of iridophores with a silvery light-reflecting pigment that acts like a mirror. j. Deeper are melanophores containing black or brown melanin. k. The green hue is an interaction of xanthophores containing yellow and underlying iridophores. l. Many frogs can adjust their color to camouflage themselves. 12. Skeletal and Muscular Systems (Figure 25.18) a. Endoskeleton of bone and cartilage supports body and its muscular movements. b. Movement to land provided new mechanical stress problems. c. Anurans show dramatic changes in the musculoskeletal system for jumping and swimming. d. Vertebral column lost much flexibility to transmit force from limbs to the body. e. Anurans have an extremely shortened body; they have only nine trunk vertebrae and a urostyle. f. Caecilians have not moved toward tetrapod locomotion and have as many as 285 vertebrae. g. The front of the frog skull, containing the brain, eyes, and nose is lightweight and flattened; the back of the skull, which contained the gill apparatus in fishes, is reduced. h. The posterior limbs have three main joints: hip, knee and ankle. i. The foot generally has five rays and the hand is four-rayed; both have several joints in the digits. j. This system is derived from the pattern in rhipidistian lobe-finned fish. k. Limb musculature is in two groups: an anterior, ventral group pulls the limb forward and toward the midline, and the posterior, dorsal group draws the limb backward and away from the body. l. The myomeres have been highly modified to support the head and brace the vertebral column. 13. Respiration and Vocalization (Figure 25.19) a. Amphibians use three respiratory surfaces for gas exchange in air. 1) The skin provides cutaneous breathing. 2) The mouth provides buccal breathing. 3) Lungs are usually present in adults. b. Frogs and toads depend on lung breathing more than salamanders. c. The skin is critical during winter hibernation. d. Carbon dioxide is mostly lost across the skin while oxygen is absorbed across the lungs. e. Lungs are supplied by pulmonary arteries and these return directly to the left atrium. f. Frog lungs are ovoid, elastic sacs; the inner surfaces divide into a network of smaller chambers. g. The absorptive surface in a frog lung is 20 cm2 per cc of air compared to 300 cm2 for humans. h. Positive Pressure Breathing 1) A frog moves air into the lung by force. 2) Rhythmic throat movements gulp air and force it backward. 3) The rib cage does not expand to draw air into the lung, as is the case with amniotes. i. Vocal cords are located in the larynx and are much more developed in males than females. 1) Air passed back and forth over the vocal cords between the lungs to large pair of vocal sacs. 2) Most species have unique sound patterns. 14. Circulation (Figure 25.20) a. Circulation is closed with a single pressure pump moving blood through the peripheral network. b. The main difference in circuitry is the shift from gill to lung breathing. c. The elimination of gills reduced one obstacle to blood flow in the arterial circuit. d. Conversion of the sixth aortic arch into a pulmonary artery provided a blood circuit to the lungs. e. Separating the oxygenated blood from the deoxygenated blood circuit is only partial. f. Frog Heart 1) The frog heart has a single undivided ventricle and two separate atria. 2) Blood from the body enters through the sinus venosus and right atrium. 3) Blood from the lung enters the left atrium. 4) Both atria contract at the same time, driving blood into the ventricle. 5) When the ventricle contracts, blood moves to the lungs or body. 6) Although there is no septum, deoxygenated blood goes primarily to lungs and oxygenated blood goes mostly to the body due to separation by a spiral valve in the conus arteriosus. 7) Right and left atria contract asynchronously so that although the ventricle is undivided, blood remains mostly separated when it enters this chamber. 8) Blood separation is aided by the spiral valve, which divides the systemic and pulmonary flows in the conus arteriosus, and by different blood pressure in the pulmonary and systemic blood vessels leaving the conus arteriosus. 9) The exact mechanism and precision of separation of oxygenated and deoxygenated blood in the conus arteriosus remain unclear. 15. Feeding and Digestion a. Most adult amphibians are carnivorous, feeding on insects, spiders, worms, slugs, etc. b. They catch prey with a tongue that is attached at the front of the mouth. c. The free end of the tongue is glandular; a sticky secretion adheres to prey. d. Any teeth that are present function to hold prey; they do not bite or chew. e. The short digestive tract produces enzymes for digesting fats, carbohydrates and proteins. f. Larval stages of tadpoles are usually herbivorous; their digestive tract is relatively long. 16. Nervous System and Special Senses (Figures 25.21–25.23) a. The brain has three fundamental parts. 1) The forebrain or telencephalon interprets the sense of smell. 2) The midbrain or mesencephalon perceives vision. 3) The hindbrain or rhombencephalon perceives hearing and balance. b. The brain is gradually assuming more information processing ability independent of the spine. c. However, a headless frog still has highly coordinated behavior based on spinal cord alone. d. The forebrain contains the olfactory center but the rest, the cerebrum, is of little function. e. Complex integrative activities occur in the midbrain optic lobes. f. The hindbrain is divided into an anterior cerebellum and a posterior medulla. g. A cerebellum critical in movement coordination in other vertebrates is minor in frogs. h. All sensory neurons except vision and olfaction pass through the medulla which is on the anterior end of the spinal cord. i. The medulla has centers for auditory reflexes, respiration, swallowing and vasomotor control. j. The pressure-sensitive lateral line is only found in amphibian larvae and aquatic adults. k. The ear becomes specialized for detecting airborne sounds. 1) A large tympanic membrane or eardrum passes vibrations to the inner ear via the columella. 2) The inner ear has a utricle with three semicircular canals and a saccule with a lagena. 3) A lagena is covered with a tectorial membrane that is similar to the mammalian cochlea. l. Frogs are sensitive to low-frequency sound energy under 4000 Hz (cycles per second). m. Except for blind caecilians, vision is the dominant sense in many amphibians. n. Lachrymal glands and eyelids evolved to keep the eye moist, free of dust, and protected. o. The cornea and the lens bend light rays to focus an image on the retina. p. At rest, the fish eye focuses on near objects and the frog eye focuses on distant objects. q. The amphibian retina contains both rods and cones; the cones provide frogs with color vision. r. The iris can rapidly change aperture to adjust to light levels. s. The upper eyelid is fixed; the lower is folded into a transparent nictitating membrane. t. Other sensory receptors include chemical receptors in skin, taste buds on the tongue and olfactory epithelium in the nasal cavity. 17. Reproduction and Development (Figures 25.24, 25.25) a. Frogs and toads are ectothermic; therefore they breed, feed and grow during the warm seasons. b. In the spring, males call to attract females. c. When the eggs are mature, females enter the water and the males clasp them in amplexus. d. As the female lays eggs, the male discharges sperm over them. e. The jelly layers absorb water and swell; the eggs are usually laid in large masses. f. Development begins immediately; a tadpole may hatch in 6–9 days. g. The tadpole head has horny jaws for feeding and a ventral adhesive disc for clinging to objects. h. Three pairs of external gills soon develop into internal gills covered with a flap of skin. i. On the right side of a tadpole, the operculum fuses with the body wall. j. On left side, a spiracle remains; water enters mouth to flow past gills, then out the same spiracle. k. Metamorphosis 1) Hindlegs are first to appear; the forelegs are temporarily hidden in folds of the operculum. 2) The tail is resorbed. 3) The intestine becomes shorter. 4) The mouth transforms to the adult condition. 5) Lungs develop and the gills are resorbed. l. Males migrate back to breeding ponds or streams. m. Tropical anurans have different reproductive strategies. (Figure 25.26) 1) Some lay eggs in foam masses that float on the surface of water. 2) Some deposit eggs on leaves over-hanging ponds and streams into which tadpoles drop. 3) Others lay eggs in water trapped in tree cavities or water-filled chambers of bromeliads. 4) Poison-dart frogs tend their eggs; tadpoles hatch on their back and can be carried. 5) Marsupial frogs carry their eggs in a pouch on the back. 6) Eleutherodactylus mate on land and eggs hatch directly into froglets. n. Some salamanders have a strong homing instinct, returning to the same pool to breed. o. Stimulation to migrate depends on hormone changes and sensitivity to temperature and humidity. 25.4. Classification Class Amphibia Order Gymnophiona Order Urodela Order Anura Lecture Enrichment 1. A toad is relatively slow moving and easy to catch and has some food value. Nevertheless, its glands under the skin are so effective in secreting emetic fluids that there is no way for shipwrecked survivors to prepare them; survival guides simply indicate “avoid them.” The effectiveness of these glands is easily seen in the reaction of a naive puppy that playfully “mouths” a toad—once! 2. Draw the conversion from fish-to-amphibian-to-reptilian circulation, by adding shunts of arteries and veins, and extending the heart septum. If this is done on the chalkboard or marker board, students are more likely to draw it and learn it, than if it is present already illustrated on an overhead. 3. Attenborough’s full version of the Life on Earth series presents a parallel illustration of Eusthenopteron as well as the first live footage of a coelacanth. 4. The world’s foremost expert on caecilians was Dr. Edward Taylor, a naturalist-scientist who also served as a spy during World War II and always carried a bag for herptiles with him while behind enemy lines. 5. What prevents a silent male from joining a chorus of male frogs? This has made for recent debate on the advantages of gathering to sing in a group to attract females from a farther distance. 6. The evolution of caecilians, amphisbaenids and early snakes all reflect selection for burrowing forms. Commentary/Lesson Plan Background: The use of frog dissection in high school biology labwork as well as in introductory college biology laboratory courses is declining and an instructor can no longer assume that this experience will be in students’ backgrounds. Few are likely to have seen salamanders in the wild, but hopefully many will have experienced catching frogs and toads. Misconceptions: The toads-cause-warts belief is still common among many younger students. As we approach vertebrate groups closer to humans, there will be a tendency for some students to “read” human-modes-of-thought into animal behaviors. The discussion of a headless amphibian still being able to make “decisions” on food choice is one of many examples that may need to be made to distinguish some animal behaviors from what appear to be equivalent human behaviors. Schedule: HOUR 1 25.1. Movement Onto Land A. Adaptations 25.2. Early Evolution of Terrestrial Vertebrates A. Devonian Origin of Tetrapods B. Carboniferous Radiation of Tetrapods 25.3. Modern Amphibians A. Diversity B. Caecilians: Order Gymnophiona (Apoda) C. Salamanders: Order Caudata (Urodela) D. Frogs and Toads: Order Anura (Salientia) 25.4. Classification ADVANCED CLASS QUESTIONS: 1. If amphibians are intermediate in evolution between ancestral fish and reptiles, how do zoologists explain the development of more aquatic adaptations in amphibians during the Carboniferous? Answer:The development of more aquatic adaptations in amphibians during the Carboniferous period can be understood through several key factors related to evolutionary pressures, environmental changes, and ecological opportunities: 1. Habitat Availability: During the Carboniferous period, which spanned from approximately 359 to 299 million years ago, vast swampy environments covered much of the Earth's surface. These wetland habitats provided abundant opportunities for amphibians to exploit aquatic niches and evolve specialized adaptations for aquatic life. 2. Competition and Predation: In these swampy environments, competition for resources and predation pressure may have influenced the evolution of amphibians towards more aquatic lifestyles. By inhabiting aquatic habitats, amphibians could exploit new food sources, avoid terrestrial predators, and reduce competition with other terrestrial vertebrates. 3. Respiratory Adaptations: The transition to aquatic habitats may have been facilitated by the development of respiratory adaptations that allowed amphibians to breathe efficiently in water. While most amphibians rely on cutaneous respiration (breathing through their skin) and lung respiration (breathing air), some species have evolved specialized respiratory structures, such as gills or lung modifications, to enhance oxygen uptake in aquatic environments. 4. Reproductive Strategies: Aquatic habitats offer unique opportunities for amphibian reproduction, including access to water for egg deposition and larval development. Many amphibians exhibit aquatic breeding behaviors, with eggs laid in water and larvae undergoing aquatic development before transitioning to terrestrial life as adults. These reproductive strategies may have favored adaptations for aquatic life in certain amphibian lineages. 5. Ecological Diversity: The Carboniferous period was characterized by high levels of biodiversity, including diverse aquatic ecosystems. Amphibians evolved to occupy various ecological niches within these aquatic environments, leading to the diversification of aquatic adaptations among different amphibian species. 6. Evolutionary Innovation: Evolutionary innovation and adaptation play a crucial role in the development of aquatic adaptations in amphibians. Over time, natural selection acts on heritable traits that confer advantages in aquatic environments, leading to the evolution of specialized anatomical, physiological, and behavioral adaptations for life in water. In summary, the development of more aquatic adaptations in amphibians during the Carboniferous period can be attributed to a combination of environmental factors, ecological opportunities, reproductive strategies, and evolutionary processes. These adaptations allowed amphibians to exploit aquatic habitats and diversify into a wide range of aquatic niches, contributing to their ecological success and evolutionary diversification during this time period. 2. Why do most U.S. frogs appear green, if they do not have green skin pigment? Answer:The green coloration observed in many U.S. frogs, despite lacking green skin pigment, is primarily due to structural coloration and camouflage adaptations. Here's how it works: 1. Structural Coloration: The green appearance of frogs is often the result of structural coloration rather than pigmentation. Structural coloration occurs when microscopic structures within the frog's skin interact with light, causing certain wavelengths to be reflected and others to be absorbed. In the case of green frogs, specialized skin cells called iridophores or chromatophores contain layers of microscopic structures that selectively reflect blue and yellow wavelengths of light, giving the frog a green appearance to human observers. 2. Camouflage Adaptations: The green coloration of frogs serves as camouflage, helping them blend into their natural environments, such as vegetation, moss, or algae-covered surfaces. This camouflage is particularly advantageous for ambush predators like frogs, which rely on stealth and concealment to avoid detection by prey and predators alike. By matching the coloration of their surroundings, green frogs can effectively hide from potential threats and enhance their chances of hunting success or avoiding predation. 3. Environmental Factors: The intensity and hue of green coloration in frogs can vary depending on environmental factors such as lighting conditions, background coloration, and the frog's own physiological state. Frogs may adjust their coloration to better match their surroundings or regulate their body temperature, contributing to variations in observed green coloration among individuals or populations. 4. Genetic Variation: While structural coloration is the primary mechanism for producing green coloration in frogs, genetic factors may also play a role in determining the specific shade or intensity of green exhibited by individuals within a species. Genetic variation within populations can influence the development and expression of structural coloration, leading to differences in appearance among individuals. In summary, the green coloration observed in many U.S. frogs is predominantly a result of structural coloration and camouflage adaptations rather than green pigmentation. This coloration helps frogs blend into their natural environments and enhances their survival by providing effective camouflage from predators and prey. Instructor Manual for Integrated Principles of Zoology Cleveland Hickman, Jr., Susan Keen, Allan Larson, David Eisenhour, Helen I'Anson, Larry Roberts 9780073524214
