Marveling at Mollusks

Linnaen System of Classification

Phylum Mollusca• Molluscus means soft bodied.• Second largest phyllum aside from arthropoda • Mollusks include clams, snails, slugs, nautiluses,

cuttlefish, octopuses, and squid.• Most have an external or internal shell• Some are quite intelligent (some species of Octopus

actually play and learn!)• Many mollusks are characterized by a rasping feeding

organ called a radula, which has teeth for scraping food and drilling into shells of other organisms.

Where they live• Occupy a wide range of habitats including:

• Abyssal plains• Continental Shelves• Intertidal mudflats

Body Structure• Bilaterally symmetrical• Flow through digestive tracts• Well developed nervous systems• A few are segmented• Can secrete their own shells usually made of

calcite or aragonite

3 classes• Gastropoda• Bivalvia• Cephalopoda

Gastropods• Snails

• Gaster means stomach, pod means foot.

• Include abalone, conch, and common garden snail

• Some are grazers, some are suspension feeders, and some are predators.

• Pteropods and Heteropods are planktonic gastropods

• Benthic: Most marine gastropods are found on rocky bottoms or firm substrates

Gastropods• Gastropods build their own shells which it enlarges and

adds to as it grows. • Shell is often coiled for easier movement• Foot and head protrude from the shell• Shell has 3 layers

• A fibrous outer covering to distribute shock• A strong, crystalline layer of calcite to provide strength• An inner layer of smooth calcite to provide nonabrasive

surroundings for the snail. Wouldn’t want to get scratched.

• Sea slugs are gastropods without shells.

What’s that foot for?• Mostly for moving about slowly, but

sometimes used for burrowing.

Gastropod Morphology• Shells characterized by

torsion, where the respiratory organs are above the head.

• Allows for the withdrawal of head before the foot into the shell.

• Foot has a hard part called the operculum to protect the snail inside the shell.

• Shell made of aragonite or calcite. Usually conical with spirals called whorls. Has an opening called an aperture

Bivalves• Bi means two, valve means door. • Includes clams, oysters, mussels, and scallops• Surrendered mobility for protection, though some

scallops can swim by pushing their valves open and closed quickly.

• Eat by suspension feeding (filter feeding). • Benthic organisms• Infaunal and Epifaunal species

• Infaunal: Burrows into sediment• Epifaunal: Lives on top of sediment or attaches to rocks.

Infaunal BIvalves• Use their foot to burrow into soft sediment on

ocean floor.• Have a siphon that sticks up through

sediment to obtain water and eject wastes. • Many clams do this.