Phylum Mollusca Characteristics of Phylum Mollusca Bilaterally symmetrical

Phylum Mollusca

Characteristics of Phylum Mollusca Bilaterally symmetrical (Bilaterally asymmetrical in some) Unsegmented Usually has a definite head (cephalized) Protostomes/Spiral Cleavage Coelom mainly a heart cavity

Characteristics of Phylum Mollusca Muscular foot Visceral mass Mantle formed in dorsal body wall Shell. (shell is absent in some mollusks) a rasping tongue called a radula. Most with open circulatory system (except cephalopods).

Characteristics of Phylum Mollusca Respiration by gills, through the mantle cavity, or the mantle. Excretion by one or two metanephridia (simple kidneys) Nervous system consists of cerebral ganglia. Organ systems are well-developed. Two larval stages called a trochophore and a veliger (in some).

General Mollusc Anatomy

Trochophore Larva

Veliger Larva

Ecology of Phylum Mollusca live in a wide variety of habitats from tropical regions, polar regions, altitudes of 7000 meters (about as tall as Denali). every water habitat from high mountain lakes to the ocean bottom. Ecological niches include bottom feeders, grazers, carnivores, and filter feeders. A variety of mollusks are used as food by humans and cultivated for their pearls. Some can be destructive like zebra mussels and shipworms that destroy wooden ships.

Shipworms

Zebra Mussels

Mollusc Anatomy The body of a mollusk consists of a head-foot portion, and a visceral mass portion. The head-foot portion of the body consists of a head with a mouth and radula, eyes, tentacles, and the muscular foot (used for locomotion). The visceral mass portion of the body consists of digestive, circulatory, respiratory, and reproductive organs.

Mollusc Anatomy The visceral mass is contained in the Mantle. The outer surface of the mantle secretes a shell made of calcium carbonate and protein. The Mantle Cavity houses respiratory organs: gills, or is a respiratory organ itself: some terrestrial snails

Mantle Cavity

Mollusc Anatomy Most mollusks have an open circulatory system with a heart, blood vessels, and blood sinuses Cephalopods (octopus, squid etc. ) have a closed circulatory system with a heart, blood vessels, and capillaries.

Open Circulatory System

Mollusc Taxonomy Gastropoda-snails, slugs Bivalves (Pelecypoda)clams, oysters, mussels Cephalopoda-octopus, squid, nautilus, cuttlefish Monoplacophora-singleshelled mollusks Polyplacophora-chitons Scaphopoda-tusk shells

Class Gastropoda Snails and relatives Bilaterally asymmetrical in a coiled shell (shell absent in some) Well developed head Radula present Dioecious and monoecious; some with trochophore and a veliger larva

Class Gastropoda Most diverse group; includes the snails, limpets, slugs, whelks, conchs, periwinkles, sea slugs, sea hares, sea butterflies and others. Some snails have a hard protein plate called the operculum, that protects the body and prevents water loss.

Class Bivalvia (Pelecypoda) Body enclosed in a twolobed mantle and shell with a hinge Head reduced No radula or eyes (except in some like the bay scallop) Plate-like gills (and siphons) Wedge-shaped foot Sexes separate; trochophore and veliger larva

Blue Eyes on a Bay Scallop

Bivalves (Pelecypods) Two-shelled mollusks: includes mussels, clams, scallops, oysters, shipworms. Most are suspension feeders: depend on their gills (and siphons) to bring in food. No head or radula; very little or no cephalization. Two shells (valves) held together by a hinge ligament: keeps the two valves open. Adductor muscles work antagonistically to close the valves shut.

Hinge Ligament/Adductor Muscles

Bivalve Reproduction and Mimicry Some bivalves produce special veliger larva called glochidia. The pocketbook mussel (Lampsilis ovata) mimics a small minnow. When a bass comes to eat the minnow decoy, glochidia rapidly shoot out and attach to the gills of the bass.

Class Cephalopoda Octopus, Squid, Cuttlefish, Nautilus Shell reduced or completely absent Head and eyes are welldeveloped with radula Head with arms and tentacles Well-developed brain for an invertebrate Separate sexes Marine predators

Cephalopods The name cephalopod means head-foot Unusual body shape: head and muscular foot are indistinguishable. The foot is the arms, tentacles, and the funnel for expelling water from the mantle cavity; move quickly by jet propulsion. Strong beak-like jaws: grasps prey, Radula tears off pieces of flesh. Octopus and cuttlefish have salivary glands: secrete venom to immobilize prey.

Anatomy of a Squid

Squid Anatomy

Giant Squid (Architeuthis sp. ) Very little is known about the life of a giant squid because no one has ever been able to study a living specimen. The anatomy has only been studied from stranded specimens on beaches, those caught in fishermen's nets, and those taken from the stomachs of sperm whales. Total length up to 13 meters (43 feet). Eyes are up to 25 cm wide (10 inches). Live at depths of 1000 meters (1 km).

Giant Squid (Architeuthis sp. )

Colossal Squid (Mesonychoteuthis hamiltoni) Maximum size 15 meters (49 feet). Colossal Squid's suckers are armed with sharp hooks. Largest eyes in the animal kingdom; up to 27 cm (11 inches). Found in Antarctic waters. Can reach depths of 2000 meters (2 km)

Colossal Squid (Mesonychoteuthis hamiltoni)

Color Changes in Cephalopods There are special pigment cells called chromatophores in the skin of most cephalopods. When these pigment cells expand contract they can produce color changes controlled by the nervous system. Cephalopods can change color for camouflage, and to communicate with other cephalopods. Many deep sea squid are also bioluminescent.

Chromatophores

Class Monoplacophora Single cap-like shell Mantle cavity with five or six pairs of gills Radula present Separate sexes Found at extreme depths (abyssal zone) Only about 10 species known (possibly 11 -20) Thought to be extinct until around 1952 Ex. Neopilina galatheae

Class Polyplacophora Chitons Flattened body with reduced head Found mostly close to shore in the littoral zone Radula present Shell of seven or eight dorsal plates (usually 8) Separate sexes Trochophore, but no veliger larva

Polyplacophora (Chitons) Chitons are somewhat flattened and have a convex surface that bears eight (sometimes seven) calcareous plates or valves

Class Scaphopoda Tusk shells Tube-shaped shell Cone-shaped foot Mouth with radula and tentacles Found mostly in the benthic zone Head absent Respiration occurs through the mantle Sexes separate; Both trochophore and veliger larva