Molluscs

Phylum Mollusca

Includes snails, clams, octopuses and others.

There are more species of molluscs in the ocean than any other animal group.

There are 200,000 species of molluscs.

Mollusk Structure

Have soft bodies enclosed in a calcium carbonate shell.

Mantle- thin layer of tissue that secretes the shell.

Bilaterally symmetrical.Ventral, muscular foot used for

locomotion.Head with eyes and other sensory organs.

Radula- ribbon of small teeth used to feed by rasping food from surfaces.

Radula made of chitin.Gills for gas exchange.

Types of Molluscs

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

Gastropods (Class Gastropoda)

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

Most use radula to scrape algae from rocks, like periwinkles, limpets, and abalones.

Some like mud snails are deposit feeders on the bottom.

Whelks, oyster drills, and cone shells are carnivores that prey on clams, oysters, worms and small fish.

Whelks

Oyster Drill

Cone Shell

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.,

Sea hares Aplysia have smal, thin shells buried in tissue that graze on seaweeds.

Janthina floating

Aplysia

Bivalves (Class Bivalva)

Clams, Mussels, Oysters Body is laterally compressed (flattened

sideways) and enclosed in a shell with two parts or valves.

No head and no radula.Folded and expanded gills used to obtain

oxygen and filter small food particles.Mantle lines the inner shell.Strong muscles close the valves.

Clams

Use shovel-foot to burrow in sand and mud.

Water is drawn in through a siphon (snorkel).

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

Many bivalves bore in coral, rock, or wood.Shipworms-fouling organisms that settle

on to bottom of boats, pilings and cause these to deteriorate.

Shipworms

Cephalopods (Class Cephalopoda)

Predators that include octopuses, squid, cuttlefish.

Nearly all are agile swimmers.Have complex nervous system and a

small or no shell.650 marine species.Cephalopod means “head-foot.”

The food is modified into arms and tentacles, usually with suckers used to capture prey.

Have large eyes on the side of head.Octopuses- round bodySquids- elongated bodyProtected by a thick and muscular mantle.

Squidward

Have two or four gills on head and water enters and leaves through a siphon (funnel).

Swim by forcing water out of mantle cavity through the siphon.

Use jet propulsion with siphon to move in any direction.

Octopuses

Have eight long arms and lack a shell.Common bottom dwellers.Size varies from 5cm (2in) to 9m (30ft).Efficient hunters of crabs, lobsters, and

shrimps.Bite prey with a beak-like jaw. Radulas

rasp away flesh.Secrete a paralyzing substance and some

have a toxic bite.

Make their homes in crevices, bottles,etc.They distract predators by emitting a cloud

of dark fluid produced in the ink sac.

Octopus

Squids

Better swimmers than octopuses.Elongated body covered with mantle has

two triangular fins.Use jet propulsion and can move forward

and backward.Eight arms and two tentacles with suckers

around a mouth.Shell reduced to a pen on upper mantle.

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

Cuttlefish

Have eight arms and two tentacles. Flat bodies and fin on the sides.Have calcified internal shell that aids in

buoyancy.Shell called cuttlebone.Cuttlebone sold as a source of calcium for

cage birds.

Cuttlefish

Chambered Nautilus

Coiled, external shell with gas-filled chambers that serve for buoyancy .

Body in the outer chamber has 60-90 short suckerless tentacles to capture prey.

Chambered Nautilus

Chitons (Class Polyplacophora)

800 speciesHave 8 overlapping shell plates on dorsal

surface.Internal organs not coiled.Live on shallow, hard bottoms using radula

to feed on algae.

Chiton

Scaphopds (Class Scaphopoda)

Tusk shells.Elongated shell open at top and tapered

elephant-like tuskLive on sand or muddy bottoms.Have thin tentacles with adhesive strips to

capture foraminiferans.

Tusk Shells

Feeding and Digestion

Use salivary and digestive glands that release enzymes to break down food.

Bivalves have crystalline style that secretes enzymes in the stomach.

Have open circulatory system where blood flows out of vessels and into the body cavity.

Cephalopods have closed circulation.

Nervous System and Behavior

Gastropods and bivalves have ganglia- clusters of nerve cells.

Octopuses have a brain that allow for learning .

When preyed upon some release ink and can change color.

Reproduction and Life History

Most have separate sexes.Some are hermaphrodites.Bivalves release sperm and eggs in the

water.Cephalopods mate and use a

spermatophore (modified arm) to transfer a packet of sperm.

Males have a long, flexible penis.