Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CHAPTER 10 Molluscs 16-1

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Characteristics Phylum Mollusca Over 90,000 living species and 70,000 fossil species Soft body and belong to the lophotrochozoa protostomes Include chitons, tusk shells, snails, slugs, nudibranchs, sea butterflies, clams, mussels, oysters, squids, octopuses, and nautiluses 16-3

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Characteristics May weigh up to 900 kg and grow to nearly 20 m long, but 80% are under 10 cm in size Herbivorous grazers, predaceous carnivores, filter feeders, and parasites Most are marine, but some are terrestrial or freshwater aquatic 16-5

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Characteristics Economics Many are used as food Culturing of pearls and pearl buttons is an important industry Burrowing shipworms destroy wooden ships and wharfs Snails and slugs are garden pests Some snails are intermediate hosts for parasites 16-7

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Mollusc Body Plan: Head-Foot and Visceral Mass Portions Head-foot region contains feeding, cephalic sensory, and locomotor organs Visceral mass contains digestive, circulatory, respiratory, and reproductive organs Mantle Cavity – Two folds of skin form protective mantle – Mantle cavity houses the gills or a lung – In most molluscs Mantle secretes a shell over the visceral mass 16-8

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Radula – Unique to molluscs – Found in all except bivalves – Protruding, rasping, tongue-like organ – Ribbon-like membrane has rows of tiny teeth (up to 250,000) pointed backward – Radula rasps off particles of food from surfaces – Serves as a conveyor belt to move particles to digestive tract – New rows of teeth replace those that wear away 16-10

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Foot – Usually ventral – Functions in attachment to substratum or for locomotion – Modifications include Attachment disc of limpets Siphon jet of squids – Secreted mucus aids in adhesion or helps molluscs glide on cilia – Free-swimming forms have modified the foot into wing or fin-like swimming agents 16-12

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Shell – If present, secreted by the mantle and lined by it – Periostracum Outer horny layer Composed of conchiolin, a tanned protein – Middle prismatic layer Closely packed prisms of calcium carbonate – Inner nacreous layer Next to the mantle; the nacre is laid down in thin layers – Thick periostracum of freshwater molluscs protects against acid from leaf decay in streams 16-13

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Internal Structure and Function – Open circulatory system Pumping heart, blood vessels, and blood sinuses – Most cephalopods have a closed system with a heart, vessels, and capillaries – Most molluscs have a pair of kidneys – Kidney ducts also discharge sperm and eggs – Nervous system Pairs of ganglia but generally simpler than in annelids 16-15

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Reproduction and Life History – Most dioecious, some hermaphroditic – Egg hatches and produces a free-swimming trochophore larva – Larva undergoes direct metamorphosis into a small juvenile in chitons – In many gastropods and bivalves Intermediate larval stage, the veliger 16-16

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Monoplacophora Previously considered extinct Living specimen was discovered in 1952 About 25 extant species now known Small molluscs with a rounded shell, resemble limpets 16-19

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Polyplacophora: Chitons Chitons are somewhat flattened with 7or 8 dorsal plates Head and cephalic organs are reduced Most prefer rocky intertidal surfaces Chiton radula is reinforced with iron mineral – Scrapes algae from the rocks Mantle extends around margin 16-21

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Scaphopoda Tusk or tooth shells Live on the ocean bottom from subtidal zone to 6000 m depth Slender body covered with a mantle Tubular shell is open at both ends Unique body plan – Mantle is wrapped around the viscera and fused to form a tube – Foot protrudes from larger end to burrow into mud 16-23

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Gastropoda Most diverse class Over 70,000 living and more than 15,000 fossil species Snails, limpets, slugs, whelks, conches, periwinkles, sea slugs, sea hares, and sea butterflies Forms range from marine forms to air-breathing terrestrial snails and slugs Typically sluggish, sedentary animals Shells are chief defense – Some produce distasteful or toxic secretions 16-25

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs – Many snails have an operculum covering shell aperture – Snails serve as intermediate hosts to many parasites and are often harmed by larval stages – 3 gastropod subclasses Prosobranchia Opisthobranchia Pulmonata 16-27

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Form and Function – Torsion Developmental 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 – Contraction of a foot retractor muscle pulls shell and viscera 90  counterclockwise – Moves anus to the right side of the body 16-28

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Coiling – Coiling or spiral winding of the shell and visceral mass not the same as torsion – Occurs at same larval stage as torsion but had a separate, earlier evolutionary origin – All living gastropods descended from coiled, torted ancestors – Shifting the shell upward and back helped balance uneven weight distribution – Gill, auricle and kidney of right side are lost in most species – Loss of the right gill provides one solution to the problem of fouling Wastes expel to the right 16-29

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Feeding Habits – Adaptation of the radula provides much variation in gastropod feeding habits – Many are herbivorous and graze, browse or feed on plankton – Some scavenge decaying flesh – Others carnivores that tear prey using radula – Oyster borer alternates rasping with chemical softening of the shell to bore a hole 16-30

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs – Species of Conus deliver a lethal sting to secure prey Venom is a conotoxin Specific for the neuroreceptors of its preferred prey – Some collect debris as a mucus ball to ingest it 16-32

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Major Groups of Gastropods – Traditional classification has recognized three subclasses of Gastropoda Prosobranchia, Opisthobranchia, and Pulmonata 16-34

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs – Prosobranchs Includes most marine snails and some freshwater and terrestrial gastropods 16-35

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs – Opisthobranchs Includes sea slugs, sea hares, sea butterflies, and canoe shells Most are marine, shallow-water and often hide under stones and seaweed Shell is reduced or absent Monoecious Sea hare Aplysia – Large anterior tentacles and a vestigial shell 16-36

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs – Pulmonates Includes all land and most freshwater snails and slugs Ancestral gills have been lost and the vascularized mantle wall is now a lung Aquatic species have one pair of tentacles Landforms have two pair of tentacles and the posterior pair has eyes 16-38

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Bivalvia Mussels, clams, scallops, oysters, and shipworms Range in size from 1–2 mm in length to the giant South Pacific clams Most are sedentary filter feeders Bivalves lack a head, radula, or other aspects of cephalization Most are marine 16-40

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Form and Function – 2 shells or valves are held together by a hinge ligament – Valves are drawn together by strong adductor muscles – Umbo is the oldest part of the shell with growth occurring outward in rings – Pearls are produced when an irritant is lodged between the shell and mantle Layers of nacre are secreted around the foreign material – Visceral mass is suspended from the dorsal midline – Foot is attached anteroventrally 16-44

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Reproduction and Development – Sexes usually separate – Fertilization usually external – Freshwater clams have internal fertilization Sperm enter the incurrent siphon to fertilize eggs in water tubes of the gills – Larvae develop into a bivalved glochidia stage Attaches to gills of passing fish where they live briefly as parasites Eventually sink to begin independent life on the streambed “Hitchhiking” having helped distribute the species 16-46

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Cephalopoda Squids, octopuses, nautiluses, devilfish, and cuttlefish All marine predators Foot is in the head region – Modified for expelling water from mantle cavity Range from 2 cm to the giant squid – Largest invertebrate 16-48

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Form and Function – Shell Nautiloid and ammonoid shells had gas chambers allowing them to swim Cuttlefish shell is enclosed in mantle Squid shell is a thin strip called the pen, enclosed in mantle Octopus has completely lost the shell 16-50

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Locomotion – Cephalopods swim by forcefully expelling water through a ventral funnel or siphon – Nautilus swims mainly at night – 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 16-52

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Nervous and Sensory Systems – Cephalopod brain is the largest of any invertebrate – Squids have giant nerve fibers – Sense organs are well-developed Eyes are complex, complete with cornea, lens, and retina – Can learn by reward and punishment, and by observation of others 16-54

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Communication – Chromatophores are cells in the skin that contain pigment granules – Ink sac empties into rectum; Contains ink gland that secretes sepia when animal alarmed 16-56

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Reproduction – Sexes are separate – In male seminal vesicle, spermatozoa are packaged in spermatophores and stored – One arm of male is modified as an intromittent organ Removes a spermatophore from mantle cavity and inserts it into female – Fertilized eggs leave oviduct and are attached to stones, etc

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification Classification – Class Monoplacophora – Class Polyplacophora – Class Scaphopoda – Class Gastropoda – Class Bivalvia – Class Cephalopoda 16-59