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CHAPTER 16 Molluscs 16-1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fluted Giant Clam 16-2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Characteristics Phylum Mollusca 16-3 Over 90,000 living species and 70,000 fossil species Soft body and protostomes Include chitons, tusk shells, snails, slugs, nudibranchs, clams, mussels, oysters, squids, octopuses, and nautiluses Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Marine Snail Chiton Nudibranch Pacific Giant Clam Octopus 16-4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Characteristics 16-5 Herbivorous grazers, predaceous carnivores, filter feeders, and parasites Most are marine, but some are terrestrial or freshwater aquatic Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Characteristics Evolution Fossil evidence Some bivalves and gastropods Limited to moist, sheltered habitats with calcium in the soil Cephalopods 16-7 Moved to brackish and freshwater Snails (gastropods) successfully invaded land Indicates molluscs evolved in the sea Most have remained marine Evolved to become relatively intelligent Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Characteristics Economics 16-8 Many are used as food Culturing of pearls is an important industry Snails and slugs are garden pests 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, sensory, and locomotor organs (foot) Visceral mass contains digestive, circulatory, and reproductive organs Mantle Cavity 16-9 Space between mantle and body wall is the mantle cavity Mantle cavity houses the gills (ctenidia) or a lung In most molluscs Mantle secretes a shell over the visceral mass Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Generalized Mollusc Anatomy 16-10 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Radula 16-11 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 Pattern and number of teeth are used in classification of molluscs Some specialized to bore through hard material or harpoon prey Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Radula 16-12 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Foot Functions in attachment or locomotion Modifications include Hatchet foot of clams Siphon jet of squids Secreted mucus aids in adhesion or helps molluscs glide Snails and bivalves extend the foot by engorgement with blood Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Blue = Water Red = Blood Black = cleansing currents Mollusc Ctenidium: Gill System 16-14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Shell If present, secreted by the mantle Periostracum Middle prismatic layer Closely packed prisms of calcium carbonate Increases with animal growth Inner nacreous layer 16-15 Outer layer - wears away Composed of hardened protein Next to the mantle; the nacre is laid down in thin layers Aids in Pearl formation Shiny layer in abalone, nautilus, and bivalve Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A.Bivalve Shell B. Pearl Formation from a parasite or sand that enters shell into mantle, becomes covered with nacre 16-16 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 freeswimming trochophore larva In many gastropods and bivalves Trochophore is followed by intermediate larval stage, the veliger. 16-17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Trochophore larva 16-18 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Trochophore of an Annelida…similar to molluscs. Showing evolutionary connection. 16-19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Polyplacophora - Chitons Class Gastropoda - Slugs, Snails, Nudibranch Class Bivalvia - Clams, Mussels Class Cephalopoda - Squid, Octopus, Nautilus 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 16-21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Anatomy of Chiton 16-22 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-23 Mossy Chiton - hairs and bristles aid in defense Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Gastropoda Most diverse class Over 70,000 living Forms range from marine forms to airbreathing terrestrial snails and slugs Shells, if present, are chief defense Some produce distasteful or toxic secretions Use process of Torsion 16-24 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Gastropod Shells One-piece (univalve) Apex is smallest and oldest whorl Whorls become larger and spiral around central axis Giant marine gastropods have shell up to 60 cm long 16-25 Some fossil forms are 2 meters long Terrestrial gastropods shells are restricted by soil mineral content, temperature, dryness, and acidity Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-26 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Form and Function 16-27 Torsion Developmental process that changes the relative position of the shell, digestive tract and anus Digestive tract moves both laterally and dorsally so that anus lies above head within mantle cavity After torsion, anus and mantle cavity open above mouth and head Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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 16-29 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Feeding Habits 16-30 Use Radula Many are herbivorous and graze, browse or feed on plankton Some scavenge decaying flesh Others carnivores that tear prey (other molluscs) using radula Some contain radula with chemicals for softening the shell to bore a hole in prey and then eat flesh Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Abalone Feed on kelp Moon Snail feeds on clams and mussels 16-31 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Conus - Extends proboscis to capture prey. Then releases Conotoxins to paralyze (lethal to Humans) Hours later regurgitate scales and bones 16-32 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Internal Form and Function Respiration performed by vascular area in mantle cavity that serves as lung Most have a single nephridium (kidney) and welldeveloped open-circulatory and nervous systems 16-33 Sense organs include eyes, statocysts, tactile organs, and chemoreceptors Eyes vary from simple cups holding photoreceptors to a complex eye with a lens and cornea. (On tentacle of some) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Evolutionary development of gill 16-34 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-35 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Reproduction: 16-36 Many terrestrial species inject a dart to heighten arousal before copulation Eggs emitted singly or in clusters, and may be transparent or in tough egg capsules Young may emerge as veliger larvae or pass this stage inside the egg Some species, including most freshwater snails, are ovoviviparous Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Eggs of Mollusks Eggs - resemble grains of wheat 16-37 Egg ribbon of Nudibranch 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 16-38 Prosobranchia, Opisthobranchia, and Pulmonata Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Prosobranchs Includes most marine snails Have one pair of tentacles, separate sexes 16-39 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gastropods - Prosobranchs Includes most marine snails Have one pair of tentacles, separate sexes Diodora aspera Hole in Apex for water to leave 16-40 Flamingo Tongue Snails Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Opisthobranchs Includes sea slugs, sea hares, sea butterflies, and nudibranch Most are marine, shallow-water and often hide under stones and seaweed Two pairs of tentacles, one pair modified to increase chemo-absorption Shell is reduced or absent Monoecious 16-41 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sea Hare 16-42 Sea Hare’s defense mechanism- a secretion from its purple gland Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-43 Nudibranch - calcareous spicules for protection 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 Aquatic species have one pair of tentacles Landforms have two pair of tentacles and the posterior pair has eyes 16-44 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pulmonates - Snail and Banana slug Opening to Mantle Cavity 16-45 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Bivalvia Mussels, clams, scallops, oysters Range in size from 1–2 mm in length to the giant South Pacific clams (1m) Most are sedentary filter feeders Bivalves lack a head, radula, or other aspects of cephalization Contain Siphons 16-46 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mussels Scallops Escaping a Sea Star 16-47 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-48 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Northwest Ugly Clams Siphons Incurrent brings in Food and Oxygen 16-49 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Environmental Issues of Bivalves Native freshwater clams in the U.S. are the most jeopardized animal group Of more than 300 species once present, 12 are extinct, 42 are threatened or endangered and 88 more are of concern Sensitive to water quality changes, including pollution and sedimentation Zebra mussels are a serious exotic invader into the Great Lakes Region 16-50 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 Posterior edges of the mantle folds form excurrent and incurrent openings 16-51 In burrowing clams, mantle forms long siphons to reach the water above Pearls are produced when an irritant (parasite, sand) is lodged between the shell and mantle Layers of nacre are secreted around the foreign material Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Locomotion 16-52 Foot is extended out from between the valves Blood is pumped into the foot Foot swells and anchors the bivalve in the mud Shortening of the foot pulls the clam forward Scallops clap valves to create a jet propulsion Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-53 Scallop - developed sensory organs along mantle edges (tentacles and blue eyes) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Gills Both mantle and gills perform gaseous exchange Siphon used in respiratory Water enters incurrent siphon Gas diffused out Exits through the excurrent siphon Circulatory - Open circulatory system 16-54 3 chambered heart has two atria and one ventricle Blood vessels line gills to receive oxygen Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-55 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Feeding 16-56 Suspended organic matter enters incurrent siphon Gland cells on gills secrete mucus to entangle particles Cilia direct the mucous mass into mouth Feed on deposits in sand, particles of wood, small crustaceans Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-57 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Shipworms BivalvesBurrows into wood on docks and piers 16-58 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clam Symbiotic relationship with Algae to gain most nutrients Siphonal Area 16-59 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Reproduction and Development Sexes usually separate Gametes discharged in excurrent flow Fertilization usually external Embryos develop as trochophore, veliger, and spat larval stages Freshwater clams have internal fertilization Larvae develop into a bivalved glochidia stage 16-60 Sperm enter the incurrent siphon to fertilize eggs in water tubes of the gills 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-61 Life Cycle of an Oyster Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Glochidium - freshwater clam larva Attach to fish’s gills by clamping their valve closed. Stay for several weeks. Pocketbook Mussel mimics a small minnow, when a Smallmouth Bass comes to dine, it releases its glochidia 16-62 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Cephalopoda Squids, octopuses, nautiluses, and cuttlefish All marine predators Foot is in the head region Range from 2 cm to the giant squid (60 ft) Modified for expelling water from mantle cavity Largest invertebrate Nautilus - only one with external shell 16-63 Series of gas chambers in shell helps maintain neutral buoyancy Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Nautilus A. Feed on a Fish 16-64 B. Showing Gas filled chambers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Form and Function Shell Nautiloid had gas chambers allowing them to swim Nautilus shell is divided into chambers Living animal only inhabits last chamber Cord of living tissue, the siphuncle, connects chambers to visceral mass Cuttlefish shell is enclosed in mantle Squid shell is a thin strip called the pen, enclosed in mantle Octopus has completely lost the shell 16-65 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-66 Cuttlefish Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Locomotion 16-67 Cephalopods swim by forcefully expelling water through a ventral funnel or siphon Control direction and force of the water, thus determining its speed Lateral fins of squids and cuttlefishes are stabilizers Nautilus swims mainly at night Octopuses mainly crawl on the bottom but can swim Some with webbing between their arms swim with a medusa-like action Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Squid - Pen is only remains of shell 16-68 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Respiration and Circulation Except for nautiloids, cephalopods have one pair of gills With higher oxygen demands, cephalopods have a muscular pumping system to keep water flowing through the mantle cavity Circulatory system has a network of vessels conducting blood through gill filaments (Closed Circulatory System) 16-69 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Nervous and Sensory Systems 16-70 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 Cephalopods lack a sense of hearing but have tactile and chemoreceptor cells in their arms Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cuttlefish Eye 16-71 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Communication 16-72 Use chemical and visual signals to communicate Chromatophores are cells in the skin that contain pigment granules Contractions of the muscle fibers attached to the cell causes the cell to expand and change the color pattern Color patterns can be changed rapidly Deep-water cephalopods have elaborate luminescent organs Ink sac empties into rectum; (Not in Nautiloids) Contains ink gland that secretes sepia (dark fluid) when animal is alarmed Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Reproduction 16-73 Sexes are separate In male seminal vesicle, spermatozoa are packaged in spermatophores and stored One arm of male is modified as an intromittent organ, the hectocotylus Removes a spermatophore from mantle cavity and inserts it into female Fertilized eggs leave oviduct and are attached to stones, etc. Large, yolky eggs undergo meroblastic cleavage (not full cleavage) Hatch into juveniles with no free-swimming larval stage Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copulation in Cephalopods Male Octopus uses modified arm 16-74
