CH-16 Molluscs – Integrated Principles of Zoology : Chapter Notes

This document contains Chapters 16 to 17 CHAPTER 16 Molluscs CHAPTER OUTLINE 16.1. Mollusca (Figures 16.1, 16.2) A. Characteristics 1. They contain over 90,000 living species and 70,000 fossil species. 2. They have a soft body and belong to the lophotrochozoa protostomes. 3. They include chitons, tusk shells, snails, slugs, nudibranchs, sea butterflies, clams, mussels, oysters, squids, octopuses and nautiluses. 4. Some may weigh up to 900 kg and grow to nearly 20 m long, but 80% are under 10 cm in size. 5. Molluscs include herbivorous grazers, predaceous carnivores, filter feeders and parasites. 6. Most are marine, but some are terrestrial or freshwater aquatic. B. Evolution 1. Fossil evidence indicates molluscs evolved in the sea; most have remained marine. 2. Some bivalves and gastropods moved to brackish and freshwater. 3. Only snails (gastropods) have successfully invaded the land; they are limited to moist, sheltered habitats with calcium in the soil. 4. The cephalopods evolved to become relatively intelligent. 5. The coelom is limited to a chamber around the heart; some zoologists believe molluscs arose separately from annelids and their coeloms are not homologous. C. Economics 1. Many kinds of molluscs are used as food. 2. Culturing of pearls and pearl buttons is an important industry. 3. Burrowing shipworms destroy wooden ships and wharves. (Figure 16.36). 4. Snails and slugs are garden pests; some snails are intermediate hosts for parasites. 16.2. Form and Function A. Mollusc Body Plan: Head-Foot and Visceral Mass Portions (Figure 16.3) 1. The head-foot portion contains the feeding, cephalic sensory and locomotor organs. 2. The visceral mass portion contains digestive, circulatory, respiratory, and reproductive organs. B. Mantle Cavity 1. Two folds of skin form the protective mantle or pallium. 2. The space between mantle and body wall is the mantle cavity. 3. The mantle cavity houses the gills (ctenidia) or a lung. 4. In some molluscs, the mantle secretes a protective shell over the visceral mass. C. Head-Foot 1. Most molluscs have a well-developed head bearing the mouth and some sensory organs. 2. Photosensory receptors range from simple to complex eyes. 3. Tentacles may be present. 4. Posterior to the mouth is the chief locomotor organ, the foot. 5. Radula (Figure 16.4) a. The radula is unique to molluscs; it is found in all except bivalves and some solenogasters. b. The radula is a protruding, rasping, tongue-like organ. c. The ribbon-like membrane has rows of tiny teeth—up to 250,000—pointed backward. d. The radula rasps off fine particles of food material from surfaces. e. The radula serves as a conveyor belt to move particles to the digestive tract. f. New rows of teeth replace those that wear away. g. The pattern and number of teeth are used in classification of molluscs. h. Some are specialized to bore through hard material or harpoon prey. 6. Foot (Figure 16.3) a. The foot is usually ventral. b. It can function for attachment to the substratum or for locomotion. c. Modifications include the attachment disc of limpets, the hatchet foot of clams and the siphon jet of squids. d. Secreted mucus can aid in adhesion or help some molluscs glide on cilia. e. Snails and bivalves extend the foot hydraulically by engorgement with blood. f. Burrowers extend the foot into mud or sand, enlarge the tip as an anchor, and draw forward. g. Free-swimming forms have modified the foot into wing or fin-like swimming agents. 7. Visceral Mass a. Mantle and Mantle Cavity (Figure 16.3) 1) A mantle is a sheath of skin on each side of the body; it secretes the shell when present. 2) The mantle cavity houses the gills or lungs that develop from the mantle. 3) The exposed surface of the mantle also serves for gaseous exchange. 4) In aquatic molluscs, a continuous flow of water brings in oxygen and food, and flushes out wastes. 5) Products of digestive, excretory and reproductive systems empty into the mantle cavity. 6) Cephalopods use the head and mantle cavity to create jet propulsion. 7) A mollusc gill has leaf-like filaments; cilia propel water across the surface. (Figure 16.5) 8) Countercurrent blood movement in a gill absorbs oxygen efficiently. 9) In most molluscs, two ctenidia on opposite sides form an incurrent and an excurrent chamber. b. Shell (Figure 16.6) 1) When present, the shell is secreted by the mantle and lined by it. 2) The periostracum is the outer horny layer, composed of conchiolin, a tanned protein. 3) The middle prismatic layer has closely packed prisms of calcium carbonate. 4) The inner nacreous layer is next to the mantle; the nacre is laid down in thin layers. 5) The thick periostracum of freshwater molluscs protects against acid from leaf decay in streams. c. Internal Structure and Function 1) The open circulatory system includes a pumping heart, blood vessels and blood sinuses. 2) Most cephalopods have a closed system with a heart, vessels and capillaries. 3) Most molluscs have a pair of kidneys or metanephridia. 4) Kidney ducts also discharge sperm and eggs. 5) The nervous system has pairs of ganglia but is generally simpler than in annelids. 6) In air-breathing snails, the nervous system produces growth hormones. 7) Sense organs vary and may be highly specialized. 8. Reproduction and Life History (Figures 16.7, 16.8) a. Most are dioecious but some are hermaphroditic. b. The egg hatches and produces a free-swimming larva called a trochophore larva. c. This larva undergoes direct metamorphosis into a small juvenile in chitons. d. In many gastropods and bivalves, an intermediate larval stage—the veliger—is a derived state. e. The trochophore larvae are considered by some to unite molluscs with annelids, marine turbellarians, nemertines, phoronids, etc. in a taxon called Trochozoa within superphylum photrochozoa. 16.3. Classes of Molluscs A. Class Caudofoveata 1. Members are wormlike, marine organisms ranging from two to 140 millimeters in length. (Figure 16.2) 2. Most burrow; the terminal mantle cavity and gills are near the entrance. 3. They feed primarily on microorganisms and detritus. 4. They have no shell but their body is covered with calcareous scales. 5. A radula is present but may be reduced. 6. There are fewer than 120 species; they may resemble the common ancestor of living molluscs. B. Class Solenogasters 1. Formerly solenogasters were groups with caudofoveates in Aplacophora. 2. They resemble caudofoveates but have no radula or gills. 3. Their foot has a midventral, narrow furrow called the pedal groove. 4. They do not burrow but are bottom dwellers and feed on cnidarians. 5. There are about 250 species of solenogasters. 6. This class is sometimes called Neomeniomorpha. C. Class Polyplacophora: Chitons (Figures 16.9-16.11) 1. Chitons are somewhat flattened molluscs with seven or eight dorsal plates. 2. There are about 1000 currently described species. 3. The head and cephalic organs are reduced. 4. Photosensitive structures (esthetes) similar to eyes pierce the plates. 5. Most prefer rocky intertidal surfaces. 6. The chiton radula is reinforced with iron mineral; it scrapes algae from the rocks. 7. The mantle extends around the chiton margin. 8. Gills are suspended from the roof of the mantle cavity and grooves form a closed chamber so water flows from anterior to posterior. 9. A pair of osphradia serves as a sense organ to sample water in the mantle groove near the anus. 10. Blood pumped by a three-chambered heart travels through the aorta and sinuses to the gills. 11. A pair of metanephridia carries wastes from the pericardial cavity to the exterior. 12. Sexes are separate and trochophore larvae metamorphose into juveniles without a veliger stage. D. Class Monoplacophora (Figure 16.12) 1. Previously considered extinct, a living specimen was discovered in 1952; now about 25 extant species are known. 2. Small molluscs with a rounded shell, they resemble limpets. 3. However, some organs are repeated: 3–6 pairs of gills, two pairs of auricles, 3–7 pairs of metanephridia, one or two pairs of gonads and 10 pairs of pedal nerves. 4. They have a radula characteristic of many other molluscs. E. Class Gastropoda (Figure 16.13) 1. This class is the most diverse and contains over 70,000 living and more than 15,000 fossil species. 2. It contains snails, limpets, slugs, whelks, conches, periwinkles, sea slugs, sea hares and sea butterflies. 3. These forms range from marine forms to air-breathing terrestrial snails and slugs. 4. Gastropods are typically sluggish, sedentary animals. 5. Shells are their chief defense, although some produce distasteful or toxic secretions 6. Gastropod Shells a. It is a one-piece univalve, coiled or uncoiled. b. The apex is the smallest and oldest whorl. c. The whorls become larger and spiral around the central axis or columella. d. Many snails have an operculum covering the shell aperture e. Giant marine gastropods have a shell up to 60 cm long; some fossil forms are two meters long. 7. Terrestrial gastropods are restricted by soil mineral content, temperature, dryness and acidity. 8. Snails serve as intermediate hosts to many parasites and are often harmed by the larval stages. 9. There are three gastropod subclasses: Prosobranchia, Opisthobranchia, and Pulmonata 10. Form and Function a. Torsion (Figure 16.14) 1) Torsion is a development process that changes the relative position of the shell, digestive tract and anus, nerves that lie on both sides of the digestive tract, and the mantle cavity containing the gills. 2) New studies show torsion should be described in a two-step process. 3) The first step involves contraction of a foot retractor muscle that pulls the shell and viscera 90 degrees counterclockwise. 4) This moves the anus to the right side of the body. 5) Recent studies have shown that shell movement is independent of visceral movement. 6) The first movements of the shell rotate it between 90 and 180 degrees into a permanent position. 7) New studies have shown that the mantle cavity develops on the right side of the body near the anus, but is initially separate from it. 8) The anus and mantle cavity usually move further to the right and the mantle cavity is remodeled to encompass the anus. 9) The digestive tract moves both laterally and dorsally so that the anus lies above the head within the mantle cavity. 10) After torsion, the anus and mantle cavity open above the mouth and head. 11) Certain viscera on the left are now on the right side, and certain viscera on the right are now on the left side; the nerve cords form a figure eight. 12) Varying degrees of detorsion in opisthobrachs and pulmonates have been observed (Figure 16.23). 13) This arrangement resulting from torsion creates fouling; fouling is the problem of wastes being washed back over the gills. 14) This developmental sequence is called ontogenetic torsion. 15) Torsion has been reinterpreted as a conserved anatomical stage, where the shell moves to the adult position and the anus and mantle cavity are on the right side of the body, rather than a conserved process of change. b. Coiling (Figure 16.15) 1) Coiling or spiral winding of the shell and visceral mass is not the same as torsion. 2) It occurs at the same larval stage as torsion but had a separate, earlier evolutionary origin. 3) All living gastropods descended from coiled, torted ancestors even if they now lack this trait. 4) A planospiral shell has all whorls in a single plane; it is the primitive state. 5) A conispiral shape provides more compactness; each whorl is to the side of the previous one. 6) Shifting the shell upward and back helped balance the uneven weight distribution. 7) However, the gill, auricle and kidney of the right side are lost in most species. 8) Loss of the right gill allows one solution to the problem of fouling; wastes expel to the right. c. Feeding Habits (Figures 16.16, 16.17) 1) Adaptation of the radula provides much variation in gastropod feeding habits. 2) Many are herbivorous and graze, browse or feed on plankton. 3) Some scavenge on decaying flesh; others are active carnivores that tear prey using their radula. 4) The oyster borer alternates rasping with chemical softening of the shell to bore a hole. 5) Species of Conus can deliver a lethal sting to secure prey; the venom is a conotoxin that is specific for the neuroreceptors of its preferred prey. 6) Some molluscs collect debris as a mucus ball to ingest it; sea butterflies secrete a mucus net. 7) Digestion is usually extracellular in the lumen of the stomach. 11. Internal Form and Function (Figures 16.18–16.20) a. Respiration in many molluscs is performed by ctenidia in the mantle cavity. b. Derived prosobranchs lost one gill and half of the remaining gill; the resulting attachment to the wall of the mantle cavity provided respiratory efficiency. c. Pulmonates lack gills but have a highly vascular area in the mantle that serves as a lung. d. The lung opens to the outside by a small opening, the pneumostome. e. Aquatic pulmonates surface to expel a gas bubble and inhale by curling, thus forming a siphon. f. Most have a single nephridium and well-developed circulatory and nervous systems. g. Sense organs include eyes, statocysts, tactile organs and chemoreceptors. h. Eyes vary from simple cups holding photoreceptors to a complex eye with a lens and cornea. i. A sensory osphradium at the base of the incurrent siphon may be chemosensory or mechanoreceptive. j. Gastropods include both monoecious and dioecious species. k. Copulation in monoecious species may involve exchange of spermatozoa or spermatophores. l. Many terrestrial species inject a dart to heighten arousal before copulation. m. Primitive gastropods discharge ova and sperm into water and fertilization is external. n. Eggs are emitted singly or in clusters, and may be transparent or in tough egg capsules. o. Young may emerge as veliger larvae or pass this stage inside the egg. p. Some species, including most freshwater snails, are ovoviviparous. 12. Major Groups of Gastropods a. Classification 1) Traditional classification has recognized three subclasses of Gastropoda. 2) Recent evidence suggests the Prosobranchia is paraphyletic. 3) Opisthobranchia may or may not be paraphyletic. 4) Opisthobranchia and Pulmonata together form a monophyletic grouping. b. Prosobranchs (Figures 16.21A) 1) This includes most marine snails and some freshwater and terrestrial gastropods. 2) The mantle cavity is anterior due to torsion, and gills are in front of the heart. 3) Water enters the left side and exits from the right side. 4) Long siphons may separate incurrent and excurrent flow. 5) Prosobranchs have one pair of tentacles, separate sexes, and usually have an operculum. c. Opisthobranchs (Figures 16.22, 16.23) 1) This group includes sea slugs, sea hares, sea butterflies and canoe shells. 2) Most are marine, shallow-water molluscs that often hide under stones and seaweed. 3) They show partial to complete detorsion; anus and gill(s) are displaced to the right side. 4) Two pairs of tentacles are found; one is further modified to increase chemo-absorption. 5) Their shell is reduced or absent and all are monoecious. 6) The sea hare Aplysia has large anterior tentacles and a vestigial shell. 7) The foot of pteropods is modified into fins for swimming and they are pelagic. d. Pulmonates (Figure 16.24) 1) This group includes all land and most freshwater snails and slugs. 2) The ancestral ctenidia have been lost and the vascularized mantle wall is now a lung. 3) Air fills this lung by contraction of the mantle floor. 4) The anus and nephridiopore open near the pneumostome; waste is forcibly expelled. 5) Pulmonates are monoecious. 6) Aquatic species have one pair of tentacles; landforms have two pair of tentacles and the posterior pair has eyes. F. Class Bivalvia (Pelecypoda) (Figures 16.25–16.36) 1. Bivalves include mussels, clams, scallops, oysters, and shipworms. 2. They range in size from 1–2 mm in length to the giant South Pacific clams. 3. Most are sedentary filter feeders dependent on ciliary currents to bring in food. 4. Bivalves lack a head, radula or other aspects of cephalization. 5. Most are marine; some live in freshwater streams, ponds and lakes. 6. Native freshwater clams in the U.S. are the most jeopardized animal group; of more than 300 species once present, nearly 24 are extinct, 60 are threatened or endangered and 100 more are of concern. 7. Freshwater clams are sensitive to water quality changes, including pollution and sedimentation. 8. Zebra mussels are a serious exotic invader into the Great Lakes Region. 9. Form and Function a. The two shells or valves are held together by a hinge ligament. b. The valves are drawn together by strong adductor muscles. c. The umbo is the bulge, the oldest part of the shell with growth occurring outward in rings. (Figure 16.26) d. Pearls are produced when an irritant is lodged between the shell and mantle (Figure 16.6); layers of nacre are secreted around the foreign material. e. Body and Mantle (Figures 16.27, 16.28) 1) A visceral mass is suspended from the dorsal midline; a foot is attached anteroventrally. 2) The ctenidia hang down on each side, each covered by a fold of the mantle. 3) Posterior edges of the mantle folds form excurrent and incurrent openings. 4) In burrowing clams, the mantle forms long siphons to reach the water above. f. Locomotion (Figures 16.28D, 16.29) 1) The slender foot is extended out from between the valves. 2) Blood is pumped into the foot; it swells and anchors the bivalve in the mud. 3) Shortening the foot then pulls the clam forward. 4) Scallops clap their valves to create a jet propulsion; the mantle edges direct the stream. (Figures 16.29, 16.33) g. Gills (Figures 16.27) 1) Both the mantle and gills perform gaseous exchange. 2) Gills are derived from primitive ctenidia by lengthening the filaments to each side. 3) The filaments fused to form plate-like lamellae with vertical water tubes inside. 4) Water enters the incurrent siphon, enters the water tubes through pores, and proceeds dorsally to the suprabranchial chamber and then out the excurrent siphon. h. Feeding (Figures 16.31 − 16.33) 1) Suspended organic matter enters the incurrent siphon. 2) Gland cells on the gills and labial palps secrete mucus to entangle particles. 3) Food in mucous masses slides to food grooves at the lower edge of the gills. 4) Cilia and grooves on the labial palps direct the mucous mass into the mouth. 5) Some bivalves feed on deposits in sand; shipworms excavate particles of wood. 6) Septibranchs draw in crustaceans by creating a sudden inflow of water. i. Internal Structure and Function (Figure 16.34) 1) The stomach is folded into ciliary tracts for sorting particles. 2) The style sac secretes a crystalline style which is kept whirling by cilia in the style sac. 3) This rotating style helps free digestive enzymes and roll up a mucous food mass. 4) Dislodged particles are directed to a digestive gland or are engulfed by amebocytes. 5) The three-chambered heart has two atria and one ventricle. (Figure 16.31) 6) Some blood is oxygenated in the mantle; it returns to the ventricle through the auricles. 7) The rest circulates through sinuses, the kidneys, the gills, and then back to the auricles. 8) A pair of U-shaped kidneys is ventral and posterior to the heart. (Figure 16.31) 9) The nervous system has three pairs of widely separated ganglia connected together. 10) Sense organs are poorly developed: statocysts in the foot, osphradia in the mantle cavity, and some pigment cells on the mantle. 11) Some mantle eyes have a cornea, lens, retina and pigmented layer. (Figure 16.29) 12) Tentacles may have tactile and chemoreceptor cells. j. Reproduction and Development (Figures 16.31, 16.35) 1) Sexes are usually separate. 2) Gametes discharged in the suprabranchial chamber are carried out in the excurrent flow. 3) Fertilization is usually external. 4) Embryos develop as trochophore, the veliger, and lastly spat larval stages. 5) Freshwater clams have internal fertilization where sperm enter the incurrent siphon to fertilize eggs in the water tubes of the gills. (Figure 16.31) 6) Larvae develop into a bivalved glochidia stage that is discharged and attaches to the gills of passing fish where they live briefly as parasites. 7) They eventually sink to begin independent life on the streambed; the “hitchhiking” having helped distribute the species. k. Boring (Figure 16.32) 1) Burrowing has led some to evolve a mechanism for boring into harder surfaces. 2) Shipworms are destructive to ships and wharves; the radula functions as a wood rasp. 3) Symbiotic bacteria produce cellulase, which helps digest wood. 4) The bacteria also fix nitrogen; the diet is high in carbon but deficient in nitrogen. 5) Some clams can bore into rock and produce long burrows. G. Class Scaphopoda (Figure 16.37) 1. Tusk or tooth shells are found living on the ocean bottom from the subtidal zone to 6000 m depth. 2. There are about 900 living species of scaphopods. 3. The slender body is covered with a mantle; the tubular shell is open at both ends. 4. This is a unique body plan: the mantle is wrapped around the viscera and fused to form a tube. 5. The foot protrudes from the larger end to burrow into mud; the small end extends into water. 6. Foot and ciliary action moves respiratory water through the mantle cavity. 7. Gills are absent and gaseous exchange occurs via the mantle. 8. Detritus and protozoa are caught on cilia on the foot or the mucus-covered knobs of the tentacles. 9. The radula carries food to a crushing gizzard. 10. The head or captacula lacks eyes, tentacles or osphradia. H. Class Cephalopoda (Figure 16.38) 1. This class includes squids, octopuses, nautiluses, devilfish and cuttlefish. 2. All are marine predators. 3. The foot is in the head region and is modified for expelling water from the mantle cavity. 4. They range from 2 cm to the giant squid, which is the largest invertebrate known. 5. The cephalopod fossil record goes back to the Cambrian; earliest shells were straight. 6. Nautilus is the culmination of shell coiling; a remaining survivor of nautiloids in which a series of gas chambers in the shell helps it maintain neutral buoyancy. 7. Octopuses and squids apparently evolved from early straight-shelled ancestors. 8. Ammonoids are now extinct but had quite elaborate shells. 9. Cephalopods are mostly marine; octopuses are mostly intertidal and squids are deep-sea animals. 10. Form and Function a. Shell (Figures 16.38, 16.39) 1) Nautiloid and ammonoid shells had gas chambers allowing them to swim. 2) Unlike gastropod shells, the nautilus shell is divided into chambers. 3) The living animal only inhabits the last chamber. 4) A cord of living tissue, the siphuncle, connects the chambers to the visceral mass. 5) The cuttlefish shell is enclosed in the mantle. 6) The squid shell is a thin strip called the pen, enclosed in mantle. 7) The octopus has completely lost the shell. 8) Gas pressure in the nautilus chambers is only one atmosphere compared to the 41 atmospheres of pressure in the surrounding deep ocean. b. Locomotion (Figure 16.40) 1) Cephalopods swim by forcefully expelling water through a ventral funnel or siphon. 2) It can control the direction and the force of the water, thus determining its speed. 3) Lateral fins of squids and cuttlefishes are stabilizers. 4) The Nautilus swims mainly at night. 5) Octopuses mainly crawl on the bottom but can swim backward by spurting jets of water; some with webbing between their arms swim with a medusa-like action. c. The active life of cephalopods is reflected in their respiratory, circulatory and nervous systems. 11. Respiration and Circulation (Figure 16.40A) a. Except for nautiloids, cephalopods have one pair of gills. b. With higher oxygen demands, cephalopods have a muscular pumping system to keep water flowing through the mantle cavity. c. Their circulatory system has a network of vessels conducting blood through gill filaments. d. Blood goes to the systemic circulation before it goes to the gills; accessory or branchial hearts at the base of each gill increase pressure to blood going through gill capillaries. 12. Nervous and Sensory Systems (Figure 16.41) a. The cephalopod brain is the largest of any invertebrate. b. Squids have giant nerve fibers. c. Sense organs are well-developed; eyes are complex, complete with cornea, lens and retina. d. They can learn by reward and punishment, and by observation of others. e. Cephalopods lack a sense of hearing but have tactile and chemoreceptor cells in their arms. 13. Communication a. Cephalopods use chemical and visual signals to communicate. b. Chromatophores are cells in the skin that contain pigment granules. c. Contractions of the muscle fibers attached to the cell boundary causes the cell to expand and change the color pattern. d. Color patterns can be changed rapidly and target individuals in different directions. e. Deep-water cephalopods have elaborate luminescent organs. f. An ink sac empties into the rectum; it contains an ink gland that secretes sepia when the animal is alarmed. 14. Reproduction (Figure 16.42) a. Sexes are separate. b. In the male seminal vesicle, spermatozoa are packaged in spermatophores and stored. c. One arm of the male is modified as an intromittent organ, the hectocotylus. d. The hectocotylus plucks a spermatophore from the mantle cavity and inserts it into the female. e. Fertilized eggs leave the oviduct and are attached to stones, etc. f. The large, yolky eggs undergo meroblastic cleavage; they hatch into juveniles with no free-swimming larval stage. 15. Major Groups of Cephalopods (Figures 16.38-16.40) a. There are three subclasses of cephalopods. 1) Nautiloidea have two pairs of gills. 2) The Ammonoidea are entirely extinct. 3) Coleoidea have one pair of gills. b. Nautilus is the only surviving genus in Nautiloidea that populated the Paleozoic seas. 1) There are five or six living species. 2) The head of a nautilus has 60–90 tentacles that can extend from the opening of the shell. 3) The tentacles lack suckers but have adhesive secretions. c. The ammonoids became extinct at the end of the Cretaceous. 1) Chambers resembled those of the nautiloids but septa were more complex and frilled. 2) The reason for their extinction is unknown, but unrelated to the asteroid bombardment. d. Four orders of Coleoidea include all living cephalopods except Nautilus. 1) There are three subclasses of cephalopods: Nautiloidea, the extinct Ammonidea, and Coleoidea. 2) Only one genus of Nautiloidea remains. 3) Subclass Coleoidea includes all living cephalopods except Nautilus. 4) Order Sepioidea includes cuttlefishes with a round body, eight arms and two tentacles. (Figure 16.34) 5) Orders Myopsida and Oegopsida are squids with a more cylindrical body, eight arms and two tentacles. (Figure 16.39) 6) The single species of deepwater vampire squid is in the order Vampyromorpha. 7) The Octopoda have eight arms and no tentacles. (Figure 16.1E) 16.4. Phylogeny and Adaptive Diversification (Figure 16.43) A. Phylogeny 1. The first mollusc probably arose in Precambrian times. 2. Due to spiral cleavage, mesoderm from the 4d blastomere, and a trochophore larva, many zoologists consider the Mollusca as protostomes allied with annelids in Lophotrochozoa. 3. Opinions differ about the exact nature of the relationship among lophotrochozoans. 4. Until the lophotrochozoan phylogeny is better resolved, it is not possible to determine whether mollusks and annelids shared a coelomate ancestor. 5. A new cladogram places monoplacophorans as the sister taxa to chitons; it unites the two taxa with repeated body parts in a clade call Serialia. 6. The question remains, did segmentation originate independently within the three metameric taxa? 7. Ongoing studies suggest that differences in biochemical pathways and developmental steps that produce segmented bodies across taxa, support the hypothesis that segmentation arose several times independently. 6. The “Hypothetical Ancestral Mollusc” (Figure 16.2) a. It probably lacked a shell or crawling foot. b. It was probably small (about 1 mm). c. It likely was a worm-like organism with a ventral gliding surface. d. It probably possessed a dorsal mantle, a chitinous cuticle and calcareous scales. 7. Caudofoveates and solenogasters both probably branched off before development of a solid shell. 8. Polyplacophorans then branched off before the veliger was established as a larva. 9. The Gastropoda and Cephalopoda appear to form a sister group with Monoplacophora. B. Classification Class Caudofoveata Class Solenogastres Class Polyplacophora Class Monoplacophora Class Gastropoda Class Cephalopoda Class Bivalvia Class Scaphopoda Lecture Enrichment 1. A modern study found nocturnal snails in the Negev Desert scraped away limestone in order to feed upon algae that was in the pores of the calcium-based rocks. This scenario provides considerable classroom discussion material: how could we test the feces to demonstrate the rock was ingested; how would the snails adapt to desert conditions; and how could we measure the rate of erosion? See Herbivory in Rocks and the Weathering of a Desert in Science, 236: 1098–1099, May 29, 1987. 2. The giant squid is big and has undoubtedly led to some of the sea monster mythology. The suckers of known squid are barely half the size of sucker marks found on sperm whales that regularly eat giant squid, but such scars could also grow with the whale and not reflect the squid’s true size. 3. The critical role of certain animals in physiological research cannot be underestimated. The enormous nerve cells of the squid allowed breakthrough research on the physiology of nerve impulses that were not possible with humans. See “The Nerve Impulse and the Squid” in the December 1958 issue of Scientific American. 4. Nacreous shell of some U.S. freshwater mussels provides the best beads for “seeding” Asian pearls; this provides a local link to the exotic pearl market. 5. The zebra mussel is rather small and several can fit in a small vial for “pass-around” in even big lecture halls; this provides a good “hook” for the discussion of how small organisms can cause massive problems. Commentary/Lesson Plan Background: Freshwater clams, oyster beds, pearl-handled revolvers, and genuine pearls that were in the common experience base of many instructors who grew up in the 1950s to 1980s are no longer in the background of most students in this new century. Experience with clams as food will vary greatly between inland and coastal students, but many will not directly know the organism in “uncanned” form. Octopus and squid are still common in Asian coastal markets, and international students may be able to provide some observations about these organisms. Fewer students will have reared parakeets, etc. and understand the term “cuttlebone.” Since humans do not have chromatophores or luminescence, these concepts are particularly foreign and will require visuals. Misconceptions: The term “eye” is perceived by students as meaning something that forms a complex image, as in the human model; care should be taken to distinguish the gradations from mere sensing of light-and-dark to full imaging of shapes. Molluscs demonstrate this full range of ability. Students usually understand plants are limited by lack of certain soil nutrients, but rarely expect that animals may likewise have similar requirements. Schedule: HOUR 1 16.1. Characteristics A. Phylum Mollusca B. Evolution C. Economics 16.2. Form and Function A. Mollusc Body Plan B. Mantle Cavity C. Head-Foot 16.3. Classes of Molluscs A. Class Caudofoveata B. Class Solenogastres C. Class Polyplacophora: Chitons D. Class Monoplacophora HOUR 2 E. Class Gastropoda F. Class Cephalopoda G. Class Bivalvia (Pelecypoda) H. Class Scaphopoda 16.4. Phylogeny and Adaptive Diversification A. Phylogeny B. Classification ADVANCED CLASS QUESTIONS: 1. What physiological features make bivalves excellent indicator species for water pollution? Answer: 1. Filter Feeding: • Bivalves are filter feeders, constantly pumping water through their bodies to extract food particles and oxygen. • Accumulation of Pollutants: This feeding mechanism makes them efficient accumulators of pollutants present in water, such as heavy metals, pesticides, and organic contaminants. 2. Sediment Accumulation: • Bivalves often live in or on sediments, where pollutants tend to accumulate. • Bioaccumulation: Pollutants adsorbed to sediments can be ingested by bivalves, leading to bioaccumulation in their tissues. 3. Sensitivity to Environmental Changes: • Bivalves are sensitive to changes in water quality and can exhibit physiological and behavioral responses to pollution. • Indicator Species: Monitoring the health and abundance of bivalve populations can provide valuable information about the quality of aquatic environments. 2. Animals are strongly adapted to the manner they acquire food. How has this made Mollusca a classic case of adaptive radiation? Answer: Mollusca and Adaptive Radiation: 1. Diverse Feeding Strategies: • Mollusks exhibit a wide range of feeding strategies, including filter feeding, grazing, scavenging, predation, and parasitism. • Example: Bivalves are filter feeders, gastropods use a radula for grazing, cephalopods are active predators, and some mollusks are parasitic. 2. Exploitation of Ecological Niches: • The diverse feeding strategies of mollusks allow them to exploit a wide range of ecological niches in marine, freshwater, and terrestrial environments. • Adaptive Radiation: Mollusks have undergone adaptive radiation, diversifying into numerous species to occupy various habitats and ecological roles. 3. Examples of Adaptive Radiation: • Gastropods: The gastropods, the largest class of mollusks, have radiated into various habitats, including marine, freshwater, and terrestrial environments, utilizing different feeding strategies. • Cephalopods: Cephalopods, such as octopuses, squids, and cuttlefish, have evolved complex behaviors and specialized feeding structures, allowing them to exploit diverse marine environments. 3. You find land snails under a log and similar-shaped snails in a local pond. What physiological differences exist in these two gastropods that prevent the terrestrial snail from surviving in a freshwater aquarium and the aquatic snail from surviving in a terrarium? Answer: Physiological Differences Between Terrestrial and Aquatic Snails: 1. Respiration: • Terrestrial snails have a lung-like structure (pulmonate) for respiration, adapted to extract oxygen from air. • Aquatic snails have gills or modified mantle surfaces for respiration, adapted to extract oxygen from water. 2. Water Balance: • Terrestrial snails have a well-developed mantle cavity to prevent desiccation in air. • Aquatic snails have mechanisms to regulate osmotic balance in freshwater environments. 3. Habitat Adaptations: • Terrestrial snails have adapted to a terrestrial lifestyle, with adaptations for water conservation and gas exchange in air. • Aquatic snails have adapted to aquatic environments, with specialized structures for respiration and osmoregulation in water. 4. Many clams spend a few weeks in a glochidial stage as parasites on fish gills. Why do we not see some adapting completely to the parasitic way of life by just staying on the fish gills as they grow? Answer: Limitations of a Parasitic Lifestyle in Clams: 1. Feeding Limitations: • Glochidia are specialized larvae adapted for temporary parasitism on fish gills. • Staying permanently on fish gills would limit access to nutrients and hinder the clam's growth and development. 2. Metamorphosis Requirement: • Clams undergo metamorphosis after the glochidial stage to settle on suitable substrates and continue their development. • Remaining on fish gills would prevent this metamorphosis and limit the clam's ability to grow and reproduce. 3. Life Cycle Completeness: • Completing the life cycle, including settlement on appropriate substrates and reproduction, is essential for the long-term survival of clam populations. • Permanent attachment to fish gills would disrupt the completion of the life cycle and reduce the clam's reproductive success. CHAPTER 17 ANNELIDS AND ALLIED TAXA PHYLUM ANNELIDA, INCLUDING POGONOPHORANS (SIBOGLINIDS) AND ECHIURANS PHYLUM SIPUNCULA CHAPTER OUTLINE 17.1. Characteristics A. Diversity 1. Annelids illustrate segmentation or metamerism; their bodies are composed of serially repeated units. 2. Each unit contains components of most organ systems. 3. The evolution of metamerism allowed much greater complexity in structure and function. a. It increased burrowing efficiency by permitting independent movement of segments. b. It allowed the evolution of a more sophisticated nervous system. c. It provided a safety factor; if one segment failed, others could still function. 4. All animal phyla discussed in this chapter are wormlike coelomate protostomes belonging to subgroup Lophotrophozoa. 5. They develop by spiral mosaic cleavage, form mesoderm from derivatives of the 4d cell, make a coelom by schizocoely, and share a trochophore as the ancestral larval form. 6. Members of phyum Annelida are segmented worms living in marine, freshwater, and moist terrestrial habitats. a. Examples are marine bristle worms, leeches, and earthworms. b. Annelida now include pogonophorans and vestimentiferan worms. 7. Members of phylum Echiura and Sipuncula are benthic marine animals with unsegmented bodies. a. Molecular sequence data place echiurans within phylum Annelida. b. We depict echiurans as the sister taxon to Annelida and sipunculans as the sister taxon to a clade composed of Annelida andEchiura. (Figure 17.1) 17.2 Phylum Annelida, including Pogonophorans (Siboglinds) A. Characteristics 1. About 15,000 species of segmented worms are classified in the phylum Annelida. 2. About two-thirds are the more obscure marine worms. 3. Segmentation a. Body segments are marked by circular grooves called annuli. b. Metamerism is the repetition of organs in segments called metameres or somites. c. Walls, called septa, separate segments. d. Metamerism is found in arthropods, which is probably homologous with annelids, and in vertebrates, where it evolved independently. e. Metamerism is not limited to annelids, but is shared by arthropods and vertebrates. 4. Except for leeches, annelids have tiny chitinous bristles called setae. a. Short setae anchor a segment in an earthworm so it prevents slipping backward. b. Long setae help aquatic worms swim. 5. Polychaetes are mainly marine and usually benthic. 6. Oligochaetes and leeches are in freshwaters, or terrestrial soils; many leeches are predators. B. Body Plan 1. Body Wall a. A two part head is composed of the prostomium and a perstonium; the terminal portion bearing the anus is the pygidium. (Figure 17.2) b. The head and pygidium are not considered metameres. c. New metameres form in front of the pygidium; thus newest segments are at the posterior. d. The surface is covered with an epidermis and a thin outer layer of non-chitinous cuticle. e. Strong circular and longitudinal muscles underlie the body wall. 2. Coelom (Figure 17.3) a. Coelom develops embryonically as a split in mesoderm on each side of the gut (schizocoel). b. Peritoneum (mesodermal epithelium) lines body wall, forming dorsal and ventral mesenteries. c. Peritonea of adjacent segments meet to form the septa. d. The gut and longitudinal blood vessels extend through the septa. 3. Hydrostatic Skeleton a. Except in leeches, the coelom is filled with fluid and serves as a hydrostatic skeleton. b. The fluid volume remains constant. c. Therefore contraction of longitudinal muscles causes the segment to shorten and fatten. d. Contraction of circular muscles causes the body to narrow and lengthen. e. By separating this force into sections, widening and elongation move the whole animal. f. Alternate waves of contraction, or peristalsis, allow efficient burrowing. g. Swimming annelids use undulatory movements. C. Phylogeny 1. Traditionally, annelids are divided among three classes: Polychaeta, Oligochaeta, and Hirudinida. a. Polychaeta and Oligochaeta are paraphyletic groups. b. Leeches form a clade that arose within Oligochaeta and together they form the clade Clitellata; characterized by a reproductive structure called a clitellum. c. Oligochaetes arose within Polychaeta and so polychaete is a descriptive term rather than a taxonomic term. d. Polychaete denotes any of 80 morphologically distinct families of worm, typically those with many setae. e. Phylogenies based on molecular characteristics support two main groups of annelids: Errantia (freely moving) and Sedentaria (spending most of their lives in tubes or burrows). f. Sedentaria contains some polychaetes as well as the oligochaetes and leeches (Clitellata). Most other polychaetes are in Errantia. g. Errantia and Sedentaria together form the new group Pleistoannelida. h. There are a few polychaetes placed outside Pleistoannelida. One such annelid group is Chaetopteridae, which are tube-dwelling annelids with three distinct body regions. D. Chaetopterus 1. The parchment worm, Chaetopterus, lives in a buried U-shaped parchmentlike tube with ends extending from the sand or mud (Figure 17.5). The worm attaches inside the tube by ventral suckers. 2. Fans (modified parapodia on segments 14-16) pump water through the tube by rhythmical movements. 3. A pair of enlarged parapodia on segment 12 secretes a long mucous net that reaches back to a small food cup in front of the fans. Water passing through the tube is filtered through the mucous net. Food is rolled into a ball in the food cup and when the food cup reaches a size of about 3 mm diameter, the fans stop beating and the ball of food and mucus is rolled forward to the mouth and swallowed. 17.3. Pleistoannelida The Pleistoannelida comprise two large clades: Errantia and Sedentaria. A. Errantia 1. The Errantia comprise the motile polychaetes. 2. Most are marine. 2. They vary from 1 mm to 3 meters long and may be brightly colored. 3. They have differentiation of some somites. 4. There is more specialization of sensory organs than in clitellates. 5. Polychaetes can tolerate a wide range of salinity. 6. Warmer regions have more freshwater polychaetes. 7. Some live in crevices; others inhabit tubes or are pelagic. 8. Polychaetes are an important part of marine food chains. 9. Polychaetes have a well-differentiated head with sense organs. 10. Paired appendages called parapodia are on most segments. 11. They have no clitellum. 12. Many setae are arranged in bundles on the parapodia. B. Form and Function (Figures 17.2 and 17.7) 1. A prostomium may or may not be retractile; it often bears eyes, tentacles and sensory palps. 2. The peristomium surrounds the mouth and may have setae, palps or chitinous jaws. 3. Most segments of the trunk bear parapodia with lobes, cirri, setae and other parts. 4. Parapodia help crawl, swim, and anchor the worm in a tube. 5. Usually the parapodia are the chief respiratory organ although the worm may also possess gills. 6. Nutrition a. Errant polychaetes are predators or scavengers. b. A polychaete has a foregut, midgut and hindgut. c. The foregut has a stomodeum, pharynx and anterior esophagus lined with cuticle. d. The anterior midgut secretes enzymes and the posterior portion absorbs nutrients. e. The short hindgut leads to the anus on the pygidium. 7. Circulation and Respiration a. Most have parapodia and gills for gaseous exchange; some lack them and use the body surface. b. Circulation varies; Nereis has a dorsal vessel that carries blood forward and a ventral vessel that carries blood posteriorly. (Figure 17.2D) c. Blood flows across between these major vessels in networks around the parapodia and intestine. d. In some worms, the septa are incomplete and coelomic fluid serves circulatory function. e. Many polychaetes have respiratory pigments: hemoglobin, chlorocruorin or hemerythrin. 10. Excretion (Figures 17.2, 17.21) a. Excretory organs vary, from protonephridia to metanephridia, and mixed forms. b. There is one pair per metamere; the inner end (nephrostome) opens into the coelomic cavity. c. Coelomic fluid enters the nephrostome; selective resorption occurs along the nephridial duct. 11. Nervous System and Sense Organs (Figure 17.22) a. Dorsal cerebral ganglia connect to subpharyngeal ganglia by a circumpharyngeal commissure. b. A double ventral nerve cord runs the length of the worm with ganglia in each metamere. c. Sense organs include eyes, nuchal organs and statocysts. d. Eyes vary from simple eyespots to well-developed image-resolving eyes similar to mollusc eyes. e. Alciopid eyes have accessory retinas specialized for the wavelengths that penetrate deep seas. f. Nuchal organs are ciliated sensory pits that are probably chemoreceptive. g. Some burrowing and tube-building polychaetes use statocysts to orient their body. 12. Reproduction and Development (Figures 16.7, 17.6, and 17.7) a. Polychaetes have no permanent sex organs and usually have separate sexes. b. Gonads appear as simple temporary swellings of the peritoneum. c. Gametes are shed into coelom and exit by gonoducts, metanephridia or rupturing of the body. d. Fertilization is external and the early larva is a trochophore. C. Representative Members of Errantia 1. Clam Worms: Nereis (Figure 17.2) a. Sand or clam worms are errant polychaetes that live in mucus-lined burrows near the low tide level or mark. b. They wiggle out of hiding places at night to search for food. c. The prostomium bears a pair of palps sensitive to touch and taste, a pair of short sensory tentacles and two small dorsal eyes sensitive to light. d. The peristomium has a ventral mouth, a pair of jaws and four pairs of sensory tentacles. e. Parapodia 1) Each has two lobes, the dorsal notopodium and the ventral neuropodium. 2) One or more chitinous spines (acicula) support each lobe. 3) Abundant blood vessels assist respiration. 4) Parapodia function in creeping and swimming; oblique muscles in each somite power them. 5) Undulatory movements of the body provide free-swimming and burrow-pumping actions. f. Clam worms feed on small animals, other worms and larval forms. g. Food is moved through the alimentary canal by peristalsis. 2. Scale worms (Figures 17.8) a. These abundant worms belong to the family Polynoidae. b. Their flattened bodies are covered with broad scales. c. Some are large, all are carnivores and some are commensals in burrows of other organisms. 3. Fireworms (Figure 17.9) a. Fireworms have hollow, brittle setae that contain poisonous secretions; they feed on cnidarians. D. Sedentaria Sedentaria contains many polychaetes and oligochaetes that live in tubes or burrows. They include members of the former phyla Pogonophora and Echiura and also members of Clitellata. 1. Body Plan a. The body plan of sedentary polychaetes is similar to that of the errant polychaetes, except that the head is often modified by the addition of tentacles for food capture. b. Parapodia are often small and sometimes modified for anchoring the worm in the tube. c. Setae may be hooklike to attach to the tube wall d. Parapodia may function in respiration but many also have gills (i.e. Amphitrite in Figure 17.10 and Arenicola in Figure 17.11). e. The expanded surface area of the branching within the tentacles is useful for both functions. 2. Representative Members of Sedentaria 3. Tubeworms (Figures 17.4, 17.10, and 17.11) a. Polychaete tube-dwellers secrete many types of tubes. Some are parchmentlike or leathery and others are firm, calcareous tubes and some are simply grains of sand, shell, or seaweed cemented together with mucous secretions. b. Tube-dwellers may line their burrows with mucus and use cilia or mucus to obtain food. c. Some deposit feeders protrude their heads above the mud and extend long tentacles to find food (Figure 17.10). d. Lugworms, Arenicola, use a combination of suspension and deposit feeding from its U-shaped burrow (Figure 17.11). 4. Fanworms (Figures 17.4 and 17.12) a. Fanworms unfurl tentacular crowns to feed; food is moved from radioles to the mouth by ciliary action. 17.4 Clade Siboglinidae (Pogonophorans) (Figures 17.13, 17.15) A. Members were formerly of the phylum Pogonophora or beardworms. B. Beardworms weren’t discovered until 1900. C. Characteristics 1. Some 150 species described thus far. 2. Most species are small, less than 1 mm in diameter, some 10 to 75 cm in diameter, but some giant beardworms that live in deepwater hydrothermal vents are 3 m long and 5 cm in diameter 3. Most live in mud on the ocean floor at depths of 100 to 10,000 m. 4. They are sessile animals that secrete and live in long chitinous tubes. 5. Tubes have general upright orientation in bottom sediments. 6. Tubes are generally three or four times the length of the animal. D. Body 1. The apostrophe long cylindrical body is covered with cuticle. 2. The body is divided into a short anterior forepart; a long slender trunk; and a small, segmented opisthosoma. (Figure 17.13) 3. Tentacles are located on a cephalic lobe. a. Tentacles are hollow extensions of the coelom and bear minute pinnules. b. Tentacles lie parallel to one another, enclosing a cylindrical intertentacular space. E. Internal body (Figure 17.14) 1. Siboglinids have no mouth or digestive tract; mode of digestion is puzzling. a. Nutrients such as glucose and amino acids are absorbed from seawater through the pinnules and microvilli of their tentacles. b. Most energy is derived from a mutualistic relationship with chemoautrophic bacteria that oxidizes hydrogen sulfide. c. An organ called a trophosome, derived embryonically from the midgut, houses the bacteria. F. Reproduction and Development (Figure 17.15) a. Sexes are separate; pair of gonads and gonoducts in the truck section. b. Little developmental work has been done on the deep sea worms, but research suggests that cleavage is unequal and atypical; seems spiral. c. Development of the coelom is schizocoelic. d. The embryo is worm-shaped and ciliated, but a poor swimmer; probably carried by water currents until it settles. 17.5 Family Echiuridae (Figures 17.16 − 17.18) A. Diversity 1. This family consists of about 140 species of marine worms that burrow into mud or sand. 2. They live in empty snail shells or sand-dollar tests, or rocky crevices. 3. They are found in all oceans. 4. Their length varies from a few millimeters to 40 or 50 cm. B. Form and Function 1. Echiurans are sausage-shaped and have an inextensible proboscis anterior to the mouth. 2. They are often called “spoon worms.” 3. They have a simple nervous system with a ventral nerve that runs the length of the body. 4. A ciliated groove on the proboscis allows them to gather detritus over the mud while lying buried. 5. The muscular body wall is covered by a cuticle and epidermis which may be smooth or covered by papillae. 6. They have a large coelom. 7. The digestive tract is long and coiled. 8. A pair of anal sacs may have an excretory and osmoregulatory function. 9. Most have a closed circulatory system with colorless blood. a. Hemoglobin is found in certain cells and in coelomic corpuscles. 10. Respiration probably occurs in the hindgut which is continually filled and emptied by cloacal irrigation. 11. Sexes are separate. a. Gonads are produced by special regions in the peritoneum in each sex. b. Fertilization is usually external. c. Early cleavage and trochophore stages are similar to annelids. 17.6 Clade Clitellata A. This clade contains earthworms and their relatives and leeches in class Hirudinida. B. Members share a reproductive structure called a clitellum. 1. The clitellum is a ring of secretory cells found in a fat band around the body. 2. It is visible always in oligochaetes but visible only during the reproductive season in leeches. C. Members lack parapodia. D. Clitellates are hermaphroditic (monoecious) animals that exhibit direct development. 1. Young develop inside a cocoon secreted by the clitellum, and emerge as small worms. E. Oligochaeta 1. Diversity a. The Oligochaetes do not form a monophyletic group but include over 3000 species with an oligochaete body plan. They occur in a variety of habitats and sizes. b. They include the familiar earthworms and other terrestrial and freshwater forms. Some are parasitic and a few are marine or live in brackish water. b. Nearly all bear setae; although highly varied, setae are fewer than in polychaetes. 2. Form and Function (Figures 17.19, 17.20) a. Earthworms are sometimes called “night crawlers”. b. The burrow in moist rich soil and usually live in branched interconnected tunnels. c. The species Lumbricus terrestris is commonly studied in laboratories. d. Its size ranges from 12 to 30 cm; giant tropical forms range from 4m and have some 150 to 250 segments. e. In damp rainy weather, earthworms stay near the surface but in dry weather, they burrow deep underground and go dormant coiled in a slime chamber. f. Earthworms use peristaltic movements: Contractions of circular muscles in the anterior end lengthen the body, pushing the anterior end forward where it anchors. 1) Anchoring is accomplished by contraction of the longitudinal muscles in forward segments. 2) This causes the segments to become short and wide, pushing against the burrow. 3) Seta is a bristlelike rod set in a sac and moved by tiny muscles 4) Setae project outward through small pores in the cuticle; setae aid in anchoring by digging into the walls of the burrow. 3. Nutrition a. Most oligochaetes are scavengers, feeding on decayed organic matter, leaves, refuse, etc. b. Food is moistened by the mouth and drawn in by a sucking action of the muscular pharynx. c. Soil calcium produces a high blood calcium level; calciferous glands along the esophagus keep down the calcium ion concentration in the blood and are ion-regulatory rather than digestive in function. d. Food passes the esophagus to be stored in a thin-walled crop. e. The muscular gizzard grinds food into small pieces. f. Digestion and absorption occur in the intestine; an infolded typhlosole increases surface area. g. Chloragogen tissue surrounds the intestine and synthesizes glycogen and fat; cells full of fat float free in the coelom as eleocytes. h. Chloragogen cells also function in excretion. 4. Circulation and Respiration a. Both coelomic fluid and blood carry food, wastes and respiratory gases. b. Blood circulates in a closed system with five main trunks running lengthwise in the body. c. The dorsal vessel above the alimentary canal has valves and functions as a true heart. d. The dorsal vessel pumps blood anteriorly into five pairs of aortic arches. e. The aortic arches maintain steady pressure into the ventral vessels. f. A ventral vessel serves as an aorta, delivering blood to body walls, nephridia and digestive tract. g. Blood contains colorless ameboid cells and dissolved hemoglobin. h. Earthworms have no special gaseous exchange organs; the moist skin handles all exchanges. 5. Excretion (Figure 17.21) a. Each somite, except the first three and last one, have a pair of metanephridia. b. Each unit occupies parts of two adjacent somites. c. A ciliated funnel, the nephrostome, draws in wastes and leads through the septum. d. These coil until the nephridial duct ends at a bladder that empties outside at the nephridiopore. e. Wastes from both the coelom and the blood capillary beds are discharged. f. Aquatic oligochaetes excrete toxic ammonia; terrestrial worms excrete the less toxic urea. g. Chloragogen cells that break and enter the nephridia produce both urea and ammonia. h. Salts pass across the integument, apparently by active transport. 6. Nervous System and Sense Organs (Figures 17.22, 17.23) a. Earthworms have both a central nervous system and peripheral nerves. b. A pair of cerebral ganglia connects around the pharynx to the ganglia of the ventral nerve cord. c. Fused ganglia in each somite contain both sensory and motor fibers. d. Neurosecretory cells in the brain and ganglia secrete neurohormones to regulate reproduction, secondary sex characteristics and regeneration. e. One or more giant axons are located in the ventral nerve cord to increase the rate of conduction and stimulate contractions of muscles in many segments. f. Earthworms lack eyes but have many photoreceptors in the epidermis. g. Free nerve endings in the tegument are probably tactile. 7. General Behavior a. Although they lack specialized sense organs, they are sensitive to many stimuli. b. They avoid light unless it is very dim. c. Chemical stimuli are important to find food. d. Earthworms have limited learning ability; it is mostly trial-and-error learning. 8. Reproduction and Development (Figures 17.19B, 17.24) a. Earthworms are monoecious. b. In Lumbricus, reproductive systems are in somites 9 through 15. c. Immature sperm from testes mature in seminal vesicles and then pass into sperm ducts. d. Eggs are discharged by ovaries into the coelomic cavity; ciliated funnels carry them outside. e. Two pairs of seminal receptacles receive and store sperm during copulation. f. Earthworms mate at night during warm, moist weather. g. They mate by aligning in different directions with ventral surfaces together. h. Mucus secreted by the clitellum holds them together. i. Sperm travel to the seminal receptacles of the other worm along seminal grooves. j. After mutual copulation, each worm secretes a mucus tube and chitinous band to form a cocoon. k. As the cocoon passes forward, eggs, albumin, and sperm pour into it. l. Fertilization and embryogenesis takes place in the cocoon; young worms emerge. F. Representative Oligochaetes (Figure 17.25) 1. Freshwater oligochaetes are generally smaller with longer setae than on earthworms. 2. Aquatic oligochaetes are important food for fishes; a few are ectoparasites. 3. A wide variety occurs in water, mud, and moist soil. G. Class Hirudinida: Leeches 1. This class is divided into three orders: Hirudinea (‘true” leeches), Acanthobdellidae, and Branchiobdellidae. a. True leeches have 34 segements, lack setae, and possess anterior and posterior suckers. b. Members of Acanthobdellidae have 27 segments, bear setae only on the first five segments, and have a posterior sucker. c. Members of Branchiobdellidae have 14 or 15 segments, no setae, and an anterior sucker. d. Branchiobdellids are commensal or parasitic on crayfish. 2. Diversity (Figure 17.26) a. Most leeches live in freshwater but a few are marine or in moist terrestrial environments. b. They are more common in the tropics than temperate zones. c. Leeches vary in color: black, brown, red and olive green. d. Most are flattened. e. Some are carnivores on small invertebrates; others are temporary or permanent parasites. f. Leeches are hermaphroditic and have a clitellum during the breeding season. g. The clitellum secretes a cocoon for reception of eggs. 3. Form and Function (Figure 17.27) a. Leeches usually have a fixed number of segments, but appear to have more due to superficial annuli. b. Leeches lack distinct coelomic compartments and septa have disappeared. c. In most, the coelomic cavity is filled with connective tissue and spaces called lacunae. d. The lacunae channels may serve as an auxiliary circulatory system. e. They have lost setae and developed suckers for attachment while sucking blood. f. The gut is specialized for storage of large quantities of blood. g. Most leeches use suckers to attach so they can “inchworm” along the surface. 4. Nutrition (Figure 17.28) a. Although popularly considered parasites, many are predaceous. b. Freshwater leeches have a proboscis for ingesting small invertebrates as well as to suck blood. c. Some terrestrial leeches feed on insect larvae, earthworms and slugs. d. Other terrestrial leeches climb trees or bushes to reach warm-blooded vertebrates such as baby birds. e. Most are fluid feeders that prefer tissue fluids and blood pumped from open wounds. f. Some parasitic leeches leave a host during breeding season; some fish leeches remain on a host. g. Medicinal leeches were used when it was wrongly believed disorders were caused by excess blood. 5. Respiration and Excretion a. Some fish leeches have gills; all other leeches exchange gases across the skin. b. There are 10 to 17 pairs of nephridia; coelomocytes and other special cells may assist in excretion. 6. Nervous and Sensory Systems a. Leeches have two “brains”; the anterior fused ganglia form a ring around the pharynx. b. Seven pairs of fused ganglia are at the posterior. c. 21 pairs of segmental ganglia are in between along a double nerve cord. d. The epidermis contains free sensory nerve endings and photoreceptor cells. e. A row of sensillae is in the central annulus of each segment. f. Pigment-cup ocelli are present. 7. Reproduction a. Leeches are hermaphroditic and cross-fertilize during copulation. b. Sperm are transferred by hypodermic impregnation. c. The clitellum secretes a cocoon to receive the sperm and egg. d. The cocoons are buried in mud or damp soil, and development is similar to that of oligochaetes. 8. Circulation a. The coelom has been reduced by invasion of connective tissue and chloragogen tissue. b. This forms a system of coelomic sinuses and channels. c. Some leeches have a typical oligochaete circulatory system; the coelomic system is auxiliary. d. Some leeches lack blood vessels and the coelomic sinuses are the only vascular system. H. Classification of Phylum Annelida Class Polychaeta Class Oligochaeta Class Hirudinida 17.7 Phylum Sipuncula A. Diversity (Figures 17.29, 17.30) 1. This phylum consists of about 250 species of benthic marine worms. 2. They are sedentary, living in burrows of mud or sand, snail shells, coral crevices, or among vegetation. 3. More than half are restricted to tropical zones. 4. Some are tiny, slender worms, but most range from 3 to 10 cm in length. 5. Some are known as “peanut worms” because when disturbed, they contract to a peanut shape. B. Form and Function 1. Sipunculans have no segmentation or setae. 2. They have a slender, retractable introvert or proboscis at their anterior end. 3. Walls of the trunk are muscular. C. Nutrition 1. Little is known about their feeding habits. 2. Some appear to be detritivores and others suspension feeders. 3. Some nutrition may come from dissolved organic matter in the surrounding water. 4. From their burrow or hiding place, they extend their tentacles to explore and feed. 5. Collected organic matter is moved from the mucus on the tentacles to the mouth by ciliary action. 6. A retractor muscle retracts the introvert to allow tubular compensation sacs that lie along the esophagus to accept fluid from the tentacles. 7. They have a large fluid-filled coelom. 8. The digestive tract is U-shaped. D. Respiration 1. They lack a circulatory and respiratory system. 2. Gas exchange appears to occur across the introvert and tentacles. E. Nervous and Sensory Systems 1. They have a bilobed cerebral ganglion behind the tentacles. 2. A ventral cord extends the length of the body. F. Reproduction 1. Sexes are separate. 2. Sex organs develop seasonally within the connective tissue covering the origins of the retractor muscles. 3. Sex cells are released through the nephridia. 4. Asexual reproduction occurs through transverse fission. 17.8 Evolutionary Significance of a Coelom and Metamerism A. Origins of Metamerism and the Coelom 1. No satisfactory explanation for the origins of metamerism and the coelom has gained acceptance. The coelom appears to have evolved independently in protostomes and deuterostomes. Metamerism has apparently arisen separately at least three times: once in deuterostomes, once in Ecdysozoan protostomes, and once in lophotrochozoan protostomes. 2. The coelom may have been very advantageous as a hydrostatic skeleton. 4. Coelomic fluid would have acted as a circulatory fluid and reduce the need for flame cells everywhere. 5. A coelom could store gametes for timed release; this would require nervous and endocrine control. 6. It is unlikely that segmentation is homologous among annelids, arthropods, and chordates. 7. Current evidence supports the hypothesis that segmentation arose independently multiple times. 8. A selective advantage of a segmented body for annelids appears to lie in the efficiency of burrowing. 17.9 Phylogeny and Adaptive Diversification A. Phylogeny 1. Annelids and molluscs share many developmental features so they were presumed by biologists to be closely related. 2. However, these shared features are likely to be a retained ancestral feature for lophotrochozoan protostomes. 3. Pogonophorans and vestimentiferans were once placed outside phylum Annelida but are now in clade Siboglinidae within this group. 4. Molecular analyses place sipunculids and echiurans as members of Sedentaria. 5. Echiurans have setae are present but segmentation has been lost. There are serially repeated structures (nerve-cord ganglia and mucous glands in echiurans larvae and serially repeated nephridial in adults. 6. The presence of paired epidermal setae in some echiurans species provides strong support for placing them with Annelida. 7. Placement of Sipuncula is contentious. a. Members are not metameric, and do not have setae. b. Larval development is similar to annelids, molluscs, and echiurans. c. Presently, they are depicted as the sister taxon to a clade of annelids but a recent phylogeny using mitochondrial genomes place Sipuncula within Annelida. B. Adaptive Diversification 1. Annelids are an ancient group that has undergone extensive adaptive diversification. 2. The basic body structure lends itself to almost endless modification. 3. Polychaetes inhabit a wide range of habitats. 4. Septal arrangement with fluid-filled compartments has been varied for precise movements. 5. Feeding adaptations vary widely, from chitinous jaws to specialized tentacles. 6. In polychaetes the parapodia have been adapted in many ways, chiefly for locomotion and repiration. 7. Leeches have developed both parasitic and predatory adaptations. Lecture Enrichment 1. Annelids occasionally are central to native life and customs. The epitokes of the marine palolo worms rise to the surface in huge numbers during their reproductive cycle and natives wade out to collect them in tubs and hold feasts on this egg-like food. 2. The problem of the evolutionary origin and function of segmentation provides an excellent illustration of how a concept that may seem to have several very “obvious” advantages remains a “just so” story until researchers can find fossil evidence or devise an experimental regime to actually demonstrate its advantages. “Proving” an evolutionary advantage requires perceptive research. 3. Ask students to consider why a segmented animal might not bud segments from the tail end, rather than somewhere between the first and last segments. Commentary/Lesson Plan Background: Coastal students may have encountered clam worms, etc. in beachcombing, but the wide array of polychaetes will require visual illustrations or specimens. Only the avid fishermen among students may have directly experienced leeches; their testimony can generate student interest. Others may remember leech scenes from movies such as “Stand By Me” or “African Queen.” Students who put earthworms on hooks when fishing may have made one insight into earthworm anatomy; they should be aware of the fact that earthworms have some blood vessels and blood. Misconceptions: The urban legend that various hamburger chains use earthworms instead of beef is still extant; note that it is both impractical and highly uneconomical to substitute this puny animal for such use. Some have been led to believe that the “comeback” of leeches in medicine in some way admits that the earlier abandonment of leeching was premature and a medical mistake. Note that the original theory (Galen’s “body humors”) remains completely disproved and that modern use centers around a very narrow application well within the modern medical paradigm: reattaching appendages. Even biology majors often mentally envision all oligochaetes as terrestrial earthworms and may even “rescue” a poor “earthworm” found drowning in a freshwater stream; use of Tubifex and other aquatic oligochaetes will help rectify this misconception. Again, students often assume that evolution only adds complexity; leeches are an example where quite a few features are reduced or lost from ancestors. The use of the term “brain” for fused ganglia and “heart” for pumping tubes with valves may lead students to assume more complexity and efficiency to these structures than is justified. Schedule: HOUR 1 17.1. Characteristics A. Diversity 17.2. Phylum Annelida A. Characteristics B. Body Plan C. Phylogeny 17.3. Pleistoannelida A. Errantia B. Sedentaria HOUR 2 17.4. Family Siboglindae A. Characteristics 17.5. Phylum Echiura A. Diversity 17.6. Clade Clitellata A. Characteristics B. Oligochaeta C. Representative Oligochaetes A. Class Hirudinea: Leeches 17.7. Phylum Sipuncula A. Diversity 17.8. Evolutionary Significance of a Coelom and Metamerism A. Origins of the Coelom and Metamerism 17.9. Phylogeny and Adaptive Diversification A. Phylogeny B. Adaptive Diversification ADVANCED CLASS QUESTIONS: 1. How does the development of segmentation or metamerism lead to specialization of parts? Answer: Specialization through Segmentation: 1. Repetitive Body Segments: • Segmentation or metamerism involves the division of the body into a series of repetitive segments or metameres. • Each segment can develop specialized structures, leading to functional specialization within the organism. 2. Specialization of Parts: • Differentiation: Segments can differentiate into specialized structures such as appendages, sensory organs, reproductive organs, and locomotor structures. • Functional Diversity: Specialization of segments allows different parts of the body to perform specific functions, increasing the efficiency of various physiological processes. 3. Advantages of Specialization: • Adaptation: Specialized segments enhance the organism's ability to perform specific tasks related to locomotion, feeding, reproduction, and sensory perception. • Efficient Functioning: By dividing the body into specialized segments, organisms can efficiently perform complex activities required for survival and reproduction. 2. Animals are adapted to the manner they acquire food. How does this explain so much of polychaete, oligochaete, and leech diversity? What keeps members of these groups from evolving back into the other’s adaptive “territory”? Answer: Adaptation to Feeding Strategies in Annelids: 1. Diverse Feeding Strategies: • Annelids, including polychaetes, oligochaetes, and leeches, exhibit a wide range of feeding strategies adapted to their ecological niches. • Polychaetes: Many polychaetes are active predators or suspension feeders, using specialized feeding appendages such as parapodia. • Oligochaetes: Oligochaetes are primarily detritivores, feeding on decaying organic matter in sediments. • Leeches: Leeches are often ectoparasites or predators, using specialized structures such as suckers to feed on the blood or body fluids of other organisms. 2. Adaptive Radiation: • The diversity of feeding strategies in polychaetes, oligochaetes, and leeches is a result of adaptive radiation, where ancestral forms diversified into various ecological niches. • Ecological Niches: Different feeding strategies allow annelids to exploit diverse habitats, including marine, freshwater, and terrestrial environments. 3. Barriers to Reversal: • Specialized Structures: Each group of annelids has evolved specialized feeding structures and physiological adaptations suited to their specific feeding strategies. • Ecological Specialization: Members of each group have become highly adapted to their specific ecological niches, making it unlikely for them to evolve back into the adaptive territory of another group. • Ecological Competition: The presence of other organisms occupying similar ecological niches creates competition, further limiting the possibility of reversal to another feeding strategy. 3. Peripatus is a living organism that resembles an earthworm with stubby legs, and it has been considered a “link” between annelids and arthropods. Peripatus uses hypodermic insemination. Does this method of reproduction logically fit with its classical position as a surviving member of the common ancestor? Answer: Hypodermic Insemination in Peripatus: 1. Logical Fit with Common Ancestor: • Hypodermic insemination, where the male injects sperm directly into the female's body wall, is a characteristic feature of many annelids, including leeches. • Peripatus, resembling both annelids and arthropods, exhibits this primitive form of reproduction. • This method of reproduction logically fits with Peripatus's classical position as a surviving member of the common ancestor between annelids and arthropods. 2. Annelid-Like Traits: • Peripatus shares several anatomical and physiological features with annelids, such as segmented body, stubby legs, and hypodermic insemination. • These characteristics suggest a close evolutionary relationship with annelids, supporting its position as a link between annelids and arthropods. 4. Describe a scenario for the evolution of leeches from ancestral oligochaetes. Would the ancestor be terrestrial or aquatic? What features would gradually be gained or lost? What chemosensors would be needed, and what digestive enzymes would be selected? How is this scenario supported or not supported by modern leech anatomy, physiology, behavior, and environmental occurrence? Answer: Evolution of Leeches from Ancestral Oligochaetes: 1. Scenario for Evolution: • Ancestral Environment: The ancestor of leeches would likely be aquatic, similar to most oligochaetes, which inhabit freshwater environments. • Gradual Changes: • Adaptation to Blood Feeding: Some oligochaetes may have adapted to feeding on blood, leading to the evolution of blood-feeding behavior in leeches. • Loss of Chaetae: Leeches would gradually lose their chaetae (bristles) to adapt to a parasitic lifestyle, as these structures are unnecessary for blood feeding. • Development of Suckers: Suckers would evolve to aid in attachment to host organisms and facilitate blood feeding. • Chemosensors: Leeches would develop specialized chemosensory organs to detect hosts and locate blood sources. • Digestive Enzymes: Leeches would evolve digestive enzymes capable of breaking down blood components, such as proteases and anticoagulants. 2. Support from Modern Leech Anatomy, Physiology, and Behavior: • Anatomy: Modern leeches lack chaetae and possess suckers for attachment to hosts, supporting the scenario of gradual adaptation from ancestral oligochaetes. • Physiology: Leeches produce anticoagulants and proteolytic enzymes to facilitate blood feeding, indicating a specialization for this mode of nutrition. • Behavior: Leeches exhibit host-seeking behavior, relying on chemosensory cues to locate hosts and blood sources. • Environmental Occurrence: Most leech species are found in freshwater environments, similar to their ancestral oligochaete relatives, supporting their evolutionary origin from aquatic ancestors. Instructor Manual for Integrated Principles of Zoology Cleveland Hickman, Jr., Susan Keen, Allan Larson, David Eisenhour, Helen I'Anson, Larry Roberts 9780073524214