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CHAPTER 16 Molluscs 16-1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fluted Giant Clam 40 cm 16-2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Characteristics Phylum Mollusca Over 90,000 living species and 70,000 fossil specie Include chitons, tusk shells, snails, slugs, nudibranchs, clams, mussels, oysters, squids, octopuses, and nautiluses QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. 16-3 QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. 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 Characteristics: Herbivorous grazers, predaceous carnivores, and filter feeders Most are marine, but some are terrestrial or freshwater aquatic Soft body Protostomes (mouth forms first) 16-5 Complete gut Triploblastic 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 Moved to brackish and freshwater Snails (gastropods) successfully invaded land 16-7 Indicates molluscs evolved in the sea Most have remained marine Limited to moist, sheltered habitats with calcium in the soil 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 locomotion organs Foot Functions in attachment or locomotion Modifications include 16-9 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. Visceral mass contains digestive, circulatory, and reproductive organs Mantle sheath of skin near body wall Mantle cavity houses the gills or a lung In most molluscs Mantle secretes a shell 16-10 Generalized Mollusk Anatomy Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Radula - in mouth 16-11 Unique to molluscs Found in all except bivalves Protruding, rasping, tongue-like organ Has rows of tiny teeth (up to 250,000) pointed backward Radula scrapes off 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 QuickTime™ and a decompressor are needed to see this picture. 16-12 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 16-13 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 16-14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Reproduction Most dioecious (separate sexes), some hermaphroditic Egg hatches and produces a freeswimming trochophore larva In many gastropods and bivalves Trochophore is followed by intermediate larval stage, the veliger. (includes viceral mass/foot) 16-15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Veliger QuickTime™ and a decompressor are needed to see this picture. Trochophore larva 16-16 QuickTime™ and a decompressor are needed to see this picture. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Polyplacophora - Chitons (7-8 dorsal plates) Class Gastropoda - Slugs, Snails, Nudibranch, Abalone 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 Gastropoda Most diverse class - over 70,000 living Forms range from marine to terrestrial snails and slugs Shells, if present, are chief defense Some produce distasteful or toxic secretions Species all have a “Foot” for locomotion 16-18 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Gastropod Shells 16-19 One-piece (univalve) Apex is smallest and oldest whorl Whorls become larger and spiral around central axis Terrestrial gastropods shells are thinner and smaller restricted by soil mineral content, temperature, dryness, and acidity Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-20 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Form and Function 16-21 Torsion Developmental process that changes the relative position of the shell, digestive tract and anus Digestive tract moves so that anus lies above head Ability to pull entire body mass into shell, prevents waste detection by prey 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 16-22 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Abalone Feed on kelp -herbivore Moon Snail feeds on clams and mussels Radula releases chemicals to soften shell, so they can get to their prey 16-23 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 16-24 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 16-25 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. 16-26 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Reproduction: Eggs emitted singly or in clusters Young may emerge as veliger larvae or pass this stage inside the egg Eggs - resemble grains of wheat 16-27 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 Prosobranchia, Opisthobranchia Pulmonata 16-28 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gastropods - Prosobranchia Includes most marine snails Have one pair of tentacles - eyes at base Diodora aspera Hole in Apex for water to leave 16-29 Flamingo Tongue Snails Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Opisthobranches: sea slugs, sea hares, sea butterflies, and nudibranch Marine, Shell is reduced or absent, 2 tentacles Sea Hare Rhinophore- Scent Receptors Oral Tentacle - Taste Receptors Eyes sit in head region - detect light 16-30 Sea Hare’s defense mechanism- a secretion from its purple gland Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-31 Nudibranch - calcareous spicules for protection Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pulmonates - Snail and Banana slug 2 sets of tentacles: posterior have eyes But snails can’t detect color or images Opening to Mantle Cavity 16-32 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 16-33 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mussels Scallops Escaping a Sea Star 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. Northwest Ugly Clams Siphons Incurrent brings in Food and Oxygen 16-36 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 16-37 In burrowing clams, mantle forms long siphons to reach the water above Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Locomotion 16-38 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. 16-39 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 16-40 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. 16-41 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-42 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clam Symbiotic relationship with Algae to gain most nutrients Siphonal Area 16-43 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, and veliger larval stages Freshwater clams have internal fertilization Larvae develop into a bivalved glochidia stage 16-44 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 “Hitchhiking” having helped distribute the species Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-45 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 16-46 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 16-47 Series of gas chambers in shell helps maintain neutral buoyancy - body mass does not fill shell Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Nautilus A. Feed on a Fish 16-48 B. Showing Gas filled chambers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-49 Cuttlefish Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Locomotion 16-50 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 16-51 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Respiration and Circulation 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!!) 16-52 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Nervous and Sensory Systems 16-53 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 16-54 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Communication 16-55 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 16-56 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. Eggs 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 16-57
