Molluscs Phylum Mollusca l Includes snails clams octopuses

Molluscs Phylum Mollusca

l Includes snails, clams, octopuses and others. l There are more species of molluscs in the ocean than any other animal group. l There are 200, 000 species of molluscs.

Mollusk Structure l Have soft bodies enclosed in a calcium carbonate shell. l Mantle- thin layer of tissue that secretes the shell. l Bilaterally symmetrical. l Ventral, muscular foot used for locomotion. l Head with eyes and other sensory organs.

l Radula- ribbon of small teeth used to feed by rasping food from surfaces. l Radula made of chitin. l Gills for gas exchange.

Types of Molluscs l They occupy all marine environments from the wave-splashed rocky shores to hydrothermal vents.

Gastropods (Class Gastropoda) l l l l Largest, most common, and most varied. Snails are most familiar. Also includes limpets, abalones, nudibranchs. 75, 000 species. Most are coiled mass of vital organs enclosed by a dorsal shell. Shell rests on a ventral creeping foot. Gastropod means “stomach foot. ”

Limpet

Abalones

Nudibranch

Nutrition l Most use radula to scrape algae from rocks, like periwinkles, limpets, and abalones. l Some like mud snails are deposit feeders on the bottom. l Whelks, oyster drills, and cone shells are carnivores that prey on clams, oysters, worms and small fish.

Whelks

Oyster Drill

Cone Shell

l The violet snail Janthina has a thin shell and produces a bubble raft out of mucous to float on surface and look for its prey. , l Sea hares Aplysia have smal, thin shells buried in tissue that graze on seaweeds.

Janthina floating

Aplysia

Bivalves (Class Bivalva) l Clams, Mussels, Oysters l Body is laterally compressed (flattened sideways) and enclosed in a shell with two parts or valves. l No head and no radula. l Folded and expanded gills used to obtain oxygen and filter small food particles. l Mantle lines the inner shell. l Strong muscles close the valves.

Clams l Use shovel-foot to burrow in sand mud. l Water is drawn in through a siphon (snorkel). l This allow them to get oxygen even when buried in the sand.

Geoduck Clam

Mussels-Secrete byssal threads that attach on rocks and other surfaces. Oysters-cement their shell to a hard surface. Pearl oysters- form pearls when the oyster secretes calcium carbonate to coat irritations in mantle

Green lipped mussel

Oyster with pearl

l Many bivalves bore in coral, rock, or wood. l Shipworms-fouling organisms that settle on to bottom of boats, pilings and cause these to deteriorate.

Shipworms

Cephalopods (Class Cephalopoda) l Predators that include octopuses, squid, cuttlefish. l Nearly all are agile swimmers. l Have complex nervous system and a small or no shell. l 650 marine species. l Cephalopod means “head-foot. ”

l The food is modified into arms and tentacles, usually with suckers used to capture prey. l Have large eyes on the side of head. l Octopuses- round body l Squids- elongated body l Protected by a thick and muscular mantle.

Squidward

l Have two or four gills on head and water enters and leaves through a siphon (funnel). l Swim by forcing water out of mantle cavity through the siphon. l Use jet propulsion with siphon to move in any direction.

Octopuses l Have eight long arms and lack a shell. l Common bottom dwellers. l Size varies from 5 cm (2 in) to 9 m (30 ft). l Efficient hunters of crabs, lobsters, and shrimps. l Bite prey with a beak-like jaw. Radulas rasp away flesh. l Secrete a paralyzing substance and some have a toxic bite.

l Make their homes in crevices, bottles, etc. l They distract predators by emitting a cloud of dark fluid produced in the ink sac.

Octopus

Squids l Better swimmers than octopuses. l Elongated body covered with mantle has two triangular fins. l Use jet propulsion and can move forward and backward. l Eight arms and two tentacles with suckers around a mouth. l Shell reduced to a pen on upper mantle.

l Size varies from few centimeters to largest living invertebrate 60 ft!

Cuttlefish l Have eight arms and two tentacles. l Flat bodies and fin on the sides. l Have calcified internal shell that aids in buoyancy. l Shell called cuttlebone. l Cuttlebone sold as a source of calcium for cage birds.

Cuttlefish

Chambered Nautilus l Coiled, external shell with gas-filled chambers that serve for buoyancy. l Body in the outer chamber has 60 -90 short suckerless tentacles to capture prey.

Chambered Nautilus

Chitons (Class Polyplacophora) l 800 species l Have 8 overlapping shell plates on dorsal surface. l Internal organs not coiled. l Live on shallow, hard bottoms using radula to feed on algae.

Chiton

Scaphopds (Class Scaphopoda) l Tusk shells. l Elongated shell open at top and tapered elephant-like tusk l Live on sand or muddy bottoms. l Have thin tentacles with adhesive strips to capture foraminiferans.

Tusk Shells

Feeding and Digestion l Use salivary and digestive glands that release enzymes to break down food. l Bivalves have crystalline style that secretes enzymes in the stomach. l Have open circulatory system where blood flows out of vessels and into the body cavity. l Cephalopods have closed circulation.

Nervous System and Behavior l Gastropods and bivalves have gangliaclusters of nerve cells. l Octopuses have a brain that allow for learning. l When preyed upon some release ink and can change color.

Reproduction and Life History l Most have separate sexes. l Some are hermaphrodites. l Bivalves release sperm and eggs in the water. l Cephalopods mate and use a spermatophore (modified arm) to transfer a packet of sperm. l Males have a long, flexible penis.