Marveling at Mollusks Linnaen System of Classification Phylum

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.