11 Molluscan Success This mai·ine nudibmnch (Chromodoris kuniei) is a member ofone of the most successfulanimal phyla.Members of the Mollusca have adapted to nearly every habitat on the earth. This cbapter describesthe remarkable diversity and adaptationsof members of tbis phylum. 11. 1 EVOLUTIONARY PERSPECTIVE Chapter Outline 11.1 Evolutionary Perspective LEARNING OUTCOMES Relationships to Other Animals 11.2 Molluscan Characteristics 1. Give examples of, and describe the prevalence of, members of the phylum 11.3 Class Gastropoda Mollusca. Torsion Shell Coiling 2. Discuss the relationship of the Mollusca to other animal phyla. Locomotion Feeding and Digestion Octopuses, squids, and cuttlefish (the cephalopods) are some of the invertebrate Other Maintenance Functions Reproduction and Development world's most adept predators. Predatory lifestyles have resulted in the evolu­ Gastropod Diversity tion of large brains (by invertebrate standards), complex sensory structures (by 11.4 Class Bivalvia any standards), rapid locomotion, grasping tentacles, and tearing mouthparts. In Shell and Associated Structures Gas Exchange, Filter Feeding, spite of these adaptations, cephalopods rarely make up a major component of and Digestion any community. Once numbering about 9,000 species, the class Cephalopoda Other Maintenance Functions now includes only about 550 species (figure 11.1). Reproduction and Development Bivalve Diversity Zoologists do not know why the cephalopods have declined so dramatically. 11.5 Class Cephalopoda Vertebrates may have outcompeted cephalopods because the vertebrates were also Shell making their appearance in prehistoric seas, and some ve1tebrates (e.g., bony fish) Locomotion Feeding and Digestion acquired active, predatory lifestyles. Other Maintenance Functions This evolutionary decline has not been the case for all molluscs. Overall, this Learning group has been very successful. If success is measured by numbers of species, Reproduction and Development 11.6 Class Polyplacophora the molluscs are twice as successful as ve1tebrates! The vast majority of the nearly 11.7 Class Scaphopoda 100,000 living species of molluscs belongs to two classes: Gastropoda, the snails 11.8 Class Monoplacophora and slugs; and Bivalvia, the clams and their close relatives. 11.9 Class Solenogastres 11.10 Class Caudofoveata Molluscs are triploblastic, as are all the remaining animals covered in this 11.11 Further Phylogenetic Considerations text. In addition, they are the first animals described in this text that possess a coelom, although the coelom of molluscs is only a small cavity (the pericardia! cav­ ity) surrounding the heart, nephridia, and gonads. A coelom is a body cavity that arises in mesoderm and is lined by a sheet of mesoderm called the peritoneum (see figure 7.11 c). Relationships to Other Animals Molluscs are members of the Lophotrochozoa. Their lophotrochozoan relatives include the annelids, along with other phyla discussed in chapter 10 (seepages xvi-xvii of this textbook). 198 CHAPTER ELEVEN ,'f ,. �, _..__:::i Prottsts Basal Lophotrochozoa Phyla FIGURE 11.1 Evolutionary Relationships of Molluscs to Other Animals. The figure shows one interpretation of the relationship of the Mollusca to other members of the animal kingdom. The relationships depicted here are based on evidence from developmental and molecular biology. Molluscs are placed within the Lophotrochozoa along with the Annelida, Plathyhelminthes, Rotifera, and others (see pages xvi-xvii). The phylum Mollusca includes nearly 100,000 living species, including members of the class Cephalopoda-some of the invertebrates' most adept predators. The blue-ringed octopus (Hapalochlaena) is shown here. Four species of Hapalochlaena are found in the Pacific Ocean from Japan to Australia. This ve1y small octopus has a ve1y powerful venom containing tetrodotoxin, the same found in cone snails and pufferfish. The venom causes respiratory paralysis and is powerful enough to kill a human. SECTION REVIEW 11.1 Characteristics of the phylum Mollusca include: Members of the phylum Mollusca include octopuses and their 1. Body of two parts: head-foot and visceral mass relatives, snails, bivalves, and others. They are members of 2. Mantle that secretes a calcareous shell and covers the the Lophmrochozoa with evoluliona1 y Lie:,; Lu Ll1e Aundida and other lophotrochozoans. 3. Mantle cavity functions in excretion, gas exchange, elimination of digestive wastes, and release of How do the molluscs illustrate the idea that complexity reproductive products does not always ensure evolutionary success? 4. Bilateral symmetry 5. Trochophore larvae, spiral cleavage, and schizocoelous coelom formation 11 .. 2 6. Coelom reduced to cavities surrounding the heart, LLU nephridia, and gonads CHARACTE .s 7. Open circulatory system in all but one class ( Cephalopoda) LEARNING OUTCOME 8. Radula usually present and used in scraping food 1. Hypothesize about the structure of a hypothetical newly discovered class of molluscs. The body of a mollusc has two main regions-the head-foot and the visceral mass (figure 11.2). The head-footis elongate Molluscs range in size and body form from the largest of all with an anterior head, containing the mouth and certain ner­ invertebrates, the giant squid (.Architeuthis), measuring 18 m vous and senso1y structures, and an elongate foot, used for in length, to the smallest garden slug, less than 1 cm long. attachment and locomotion. The visceral mass contains the In spite of this diversity, the phylum Mollusca (mol-lus'kah) organs of digestion, circulation, reproduction, and excretion (L. molluscus, soft) is not difficult to characterize (table 11.1). and is positioned dorsal to the head-foot. Molluscan Success 199 T/\H!.F, i 1 � CLASSIFICATION OF THE MOLLUSCA Phylum Mollusca (mol-lus'kah) The coelomate animal phylum whose members possess a head-foot, visceral mass, mantle, and mantle cavity. Most molluscs also possess a radula and a calcareous she!L Nearly 100,000 species. Class Caudofoveata (kaw'do-fo"ve-a"ta) Wormlike molluscs with a cylindrical, shell-less body and scale-like, calcareous spicules; lack eyes, tentacles, statocysts, crystalline style, foot, and nephridia. Deep-water, marine burrowers. Chaetoderma. Approximately 150 species. Class Solenogastres (so-len"ah-gas'trez) Shell, mantle, and foot lacking; worm-like; ventral (pedal) groove; head poorly developed; surface dwellers on coral and other substrates. Marine. Neomenia. Approximately 250 species. Class Polyplacophora (pol"e-pla-kof' o-rah) Elongate, dorsoventrally flattened; head reduced in size; shell consisting of eight dorsal plates. Marine, on rocky intertidal substrates. Chiton. Class Monoplacophora (mon"o-pla-kof'o-rah) Molluscs with a single arched shell; foot broad and flat; certain structures serially repeated. Marine. Neopilina. Approximately 25 species. Class Gastropoda (gas-trop'o-dah) Shell, when present, usually coiled; body symmetty distorted by torsion; some monoecious species. Marine, freshwater, terrestrial. Nerita, Ot1haliculus, Helix. More than 35,000 species. Class Cephalopoda (sef'ah-lah"po'dah) Foot modified into a circle of tentacles and a siphon; shell reduced or absent; head in line with the elongate visceral mass. Marine. Octopus, Loligo, Sepia, Nautilus. Approximately 550 species. Class Bivalvia (bi"val've-ah) Body enclosed in a shell consisting of two valves, hinged dorsally; no head or radula; wedge-shaped foot. Marine and freshwater. Anodonta, Mytilus, Venus. Approximately 30,000 species. Class Scaphopoda (ska-fop'o-dah) Body enclosed in a tubular shell that is open at both ends; tentacles used for deposit feeding; no head. Marine. Dentalium. More than 300 species. Ibista.xonomfc listing reflects a phylogenetic sequence.1be discussionsthat follow, however, begin with molluscsthat areJamfliar to most students The mantle of a mollusc usually attaches to the vis­ ceral mass, enfolds most of the body, and may secrete a shell that overlies the mantle. The shell of a mollusc is secreted Digestive gland in three layers (figure 11.3). The outer layer of the shell is called the periostracum. Mantle cells at the mantle's outer margin secrete this protein layer. The middle layer of the shell, called the prismatic layer, is the thickest of the three layers and consists of calcium carbonate mixed with organic materials. Cells at the mantle's outer margin also secrete this layer. The inner layer of the shell, the nacreous layer, Foot retractor muscles forms from thin sheets of calcium carbonate alternating with FIGURE 11.2 organic matter. Cells along the entire epithelial border of the Molluscan Body Organization. This illustration of a hypothetical mantle secrete the nacreous layer. Nacre secretion thickens mollusc shows three features unique to the phylum. The head- the shell. foot is a muscular structure usually used for locomotion and Between the mantle and the foot is a space called the senso1y perception. The visceral mass contains organs of digestion, mantle cavity. The mantle cavity opens to the outside and circulation, reproduction, and excretion. The mantle is a sheet of functions in gas exchange, excretion, elimination of digestive tissue that enfolds the rest of the body and secretes the shell. Arrows indicate the flow of water through the mantle cavity. As will be wastes, and release of reproductive products. discussed later, the representation of the shell and muscular foot is The mouth of most molluscs possesses a rasping struc­ not accurate for the Cauclofoveata