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Molluscs • PHYLUM MOLLUSCA

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Molluscs • PHYLUM MOLLUSCA CHAPTER 16 Molluscs • PHYLUM MOLLUSCA Mollusca Fluted giant clam, Tridacna maxima. A Signifi cant Space Long ago in the Precambrian era, the most complex animals popu- location for networks of blood vessels, and the suspended ali- lating the seas were acoelomate. The simplest, and probably the mentary canal could become more muscular, more highly spe- fi rst, mode of achieving a fl uid-fi lled space within the body was cialized, and more diversified without interfering with other retention of the embryonic blastocoel, as in pseudocoelomates. This organs. was not the best evolutionary solution because, for example, the Development of a coelom was a major step in the evolution organs lay loose in the body cavity. of larger and more complex forms. All major groups in chapters Some descendants of Precambrian acoelomate organisms to follow are coelomates. In some, a large fl uid-fi lled coelom sur- evolved a more elegant arrangement: a fluid-filled space within rounded by muscles becomes a hydrostatic skeleton that permits the mesoderm, the coelom. This meant that the space was lined rapid shape change and effi cient burrowing. However, the heavy with mesoderm and the organs were suspended by mesoder- molluscan shell makes shape change impossible, and the molluscan mal membranes, the mesenteries. Mesenteries provided an ideal coelom is relatively small. hic70049_ch16_331-361.indd 331 7/17/07 3:00:17 PM 332 PART THREE Diversity of Animal Life occurred along the shores, where food was abundant and habitats MOLLUSCS were varied. Only bivalves and gastropods moved into brackish Mollusca (mol-lusЈ ka) (L. molluscus, soft) is one of the largest ani- and freshwater habitats. As fi lter feeders, bivalves were unable to mal phyla after Arthropoda. There are over 90,000 living species leave aquatic surroundings. Only slugs and snails (gastropods) and some 70,000 fossil species. Molluscs are coelomate lophotro- actually invaded the land. Terrestrial snails are limited in their chozoan protostomes, and as such they develop via spiral mosaic range by their need for humidity, shelter, and presence of calcium cleavage and make a coelom by schizocoely. The ancestral larval in the soil. stage is a trochophore, but development is variously modifi ed Humans exploit molluscs in a variety of ways. Many kinds within the classes. of molluscs are used as food. Pearl buttons are obtained from The name Mollusca indicates one of their distinctive charac- shells of bivalves. The Mississippi and Missouri river basins and teristics, a soft body. This very diverse group ( Figure 16.1 ) includes artifi cial propagation furnish material for this industry in the chitons, tusk shells, snails, slugs, nudibranchs, sea butterfl ies, clams, United States. Pearls, both natural and cultured, are produced in mussels, oysters, squids, octopuses, and nautiluses. The group the shells of clams and oysters, most of them in a marine oyster, ranges from fairly simple organisms to some of the most complex Meleagrina, found around eastern Asia. of invertebrates; sizes range from almost microscopic to the giant Some molluscs are considered pests because of the damage squid Architeuthis. These huge molluscs may grow to nearly 20 m they cause. Burrowing shipworms, which are bivalves of several long, including their tentacles. They may weigh up to 900 kg (1980 species (see Figure 16.32 ), do great damage to wooden ships pounds). The shells of some giant clams, Tridacna gigas, which and wharves. To prevent the ravages of shipworms, wharves inhabit Indo-Pacifi c coral reefs, reach 1.5 m in length and weigh must be either creosoted or built of concrete (unfortunately, more than 250 kg. These are extremes, however, for probably 80% some shipworms ignore creosote, and some bivalves bore into of all molluscs are less than 10 cm in maximum shell size. The concrete). Snails and slugs frequently damage garden and other phylum includes some of the most sluggish and some of the swift- vegetation. In addition, snails often serve as intermediate hosts est and most active invertebrates. It includes herbivorous grazers, for serious parasites of humans and domestic animals. Boring predaceous carnivores, fi lter feeders, detritus feeders, and parasites. snails of genus Urosalpinx rival sea stars in destroying oysters. Molluscs are found in a great range of habitats, from the tropics In this chapter we explore the various major groups of mol- to polar seas, at altitudes exceeding 7000 m, in ponds, lakes, and luscs (Figure 16.2) including those groups having limited diver- streams, on mud fl ats, in pounding surf, and in open ocean from sity (classes Caudofoveata, Solenogastres, Monoplacophora, and the surface to abyssal depths. They represent a variety of lifestyles, Scaphopoda). Members of class Polyplacophora (chitons) are including bottom feeders, burrowers, borers, and pelagic forms. common to abundant marine animals, especially in the intertidal According to fossil evidence, molluscs originated in the sea, zone. Bivalves (class Bivalvia) have evolved many species, both and most of them have remained there. Much of their evolution marine and freshwater. Class Cephalopoda (squids, cuttlefi sh, ABC Figure 16.1 Molluscs: a diversity of life-forms. The basic body plan of this ancient group has become variously adapted for different habitats. A, A chiton (Tonicella lineata), class Polyplacophora. B, A marine snail (Calliostoma annulata), class Gastropoda. C, A nudibranch (Chromodoris sp.) class Gastropoda. D, Pacifi c giant clam (Panope abrupta), with siphons to the left, class Bivalvia. E, An octopus (Octopus briareus), class Cephalopoda, forages at night on a Caribbean coral reef. DE hic70049_ch16_331-361.indd 332 7/17/07 3:00:24 PM www.mhhe.com/hickmanipz14e CHAPTER 16 Molluscs 333 Mollusca Conchifera Caudofoveata Solenogastres Polyplacophora Monoplacophora Gastropoda Cephalopoda Bivalvia Scaphopoda Torsion Byssus Further concentration Loss of radula of visceral mass Bivalve shell Siphuncle Lateral compression Beaklike jaws of body Arms/tentacles and siphon Univalve, caplike shell Septate shell Captacula Loss of gills Serial repetition of soft parts Closed circulatory system Foot groove Loss of gills Copulatory spicules Tusk-shaped, open- at posterior Shell coiling ended shell Well-developed head Viscera concentrated dorsally Dorsoventrally elongated body Unique shell with 7–8 plates Mantle cavity extended Nervous system decentralized along sides of foot Head reduced Multiple gills Spatulate foot Expansion of mantle cavity Calcareous spicules to surround body form scales Single, well-defined shell gland Periostracum, prismatic, and nacreous layers Shell univalve Figure 16.2 Cladogram showing hypothetical relationships among Multiple foot retractor muscles classes of Mollusca. Synapomorphies that identify the Preoral tentacles various clades are shown, although a number of these have Large, muscular foot been modifi ed or lost in some descendants. For example, Concentration of shell gland to produce solid shell(s) the univalve shell (as well as shell coiling) has been reduced or lost in many gastropods and cephalopods, and many Posterior mantle cavity with 1 or more pairs of gills gastropods have undergone detorsion. The bivalve shell Radula of the Bivalvia was derived from an ancestral univalve Chambered heart with atria and ventricle shell. The byssus is not present in most adult bivalves but Muscular foot (or foot precursor) functions in larval attachment in many; therefore the Calcareous spicules produced by mantle shell gland byssus is considered a synapomorphy of Bivalvia. Mantle Source: Modifi ed from R. C. Brusca and G. J. Brusca, Reduction of coelom and development of hemocoel Invertebrates. Sinauer Associates, Inc., Sunderland, MA, 2003. octopuses, and their kin) contains the largest and most intelli- gent of all invertebrates. Most abundant and widespread of mol- FORM AND FUNCTION luscs, however, are snails and their relatives (class Gastropoda). The enormous variety, great beauty, and easy availability of shells Although enormously diverse, molluscs have in common a basic of molluscs have made shell collecting a popular pastime. How- body plan (pp. 334–336). The coelom in molluscs is limited to a ever, many amateur shell collectors, even though able to name space around the heart, and perhaps around the gonads and part hundreds of the shells that grace our beaches, know very little of the kidneys. Although it develops embryonically in a manner about the living animals that created those shells and once lived similar to the coelom of annelids (see ch. 17), the functional in them. Reduced to its simplest dimensions, the mollusc body consequences of this space are quite different because it is not plan may be said to consist of a head-foot portion and a visceral used in locomotion. mass portion ( Figure 16.3 ). The head-foot is the more active area, hic70049_ch16_331-361.indd 333 7/17/07 3:00:28 PM 334 PART THREE Diversity of Animal Life containing the feeding, cephalic sensory, and locomotor organs. It depends primarily on muscular action for its function. The visceral Characteristics of Phylum Mollusca mass is the portion containing digestive, circulatory, respiratory, and reproductive organs, and it depends primarily on ciliary tracts 1. Dorsal body wall forms pair of folds called the mantle, for its functioning. Two folds of skin, outgrowths of the dorsal body which encloses the mantle cavity, is modifi ed into gills or wall, form a protective mantle, which encloses a space between lungs, and secretes the shell (shell absent in some); ventral the mantle and body wall called the mantle cavity. The mantle body wall specialized as a muscular foot, variously modifi ed but used chiefl y for locomotion; radula in mouth cavity houses gills (ctenidia) or a lung, and in some molluscs the 2. Live in marine, freshwater, and terrestrial habitats mantle secretes a protective shell over the visceral mass. Modifi ca- 3. Free-living or occasionally parasitic tions of the structures that make up the head-foot and the visceral 4. Body bilaterally symmetrical (bilateral asymmetry in some); mass produce the great diversity of patterns observed in Mollusca.
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