Phylum Mollusca

• Head, visceral mass, muscular foot, and thin mantle that covers the body and secretes the shell

• Class Gastropoda: snails and limpets

• Class Bivalvia (Pelycypoda): mussels and clams

Gastropoda

Aquatic Gastropoda• In N.S. 15 families, ~500 species• Two suborders

– Prosobranchia-gilled snails– Pulmonata-lunged (pouch) snails

• Mouth has a radula, a ribbon of rasping teeth• Gilled snails have operculum (trap door cover)

Sinistral Dextral

Natural History• Most are scrapers• Gilled snails respire by an internal gill• Pulmonate snails have a pouched gill; many come to surface to

breathe surface air, hence can tolerate low oxygen conditions• Reproduction: most gilled snails have separate sexes; most

pulmonates are hermaphroditic• Egg masses often resemble blobs of mucous• Development is within the egg; hatch as small snails with only 1-2

whorls (most adults have3-4)• Some are intermediate hosts for human parasites• Are food for many aquatic vertebrates (crush) and invertebrates

(invade)• Bioindicator status:

– Gilled snails-sensitive to facultative– Pulmonate snails-most are tolerant

Bivalvia

• ~270 freshwater species in N.A.• Two shells connected by strong hinge

ligament

Anatomy

Bivalve Natural History

• Most abundant and diverse in moderate current in medium to large rivers;

• US is center of origin• Intolerant of siltation and low oxygen• All are filter feeders of suspended algae,

bacteria, and detritus• Filtering mechanism doubles as a gill

Reproduction

Glochidium-for dispersal, not nutrition

Species of Note• Fingernail clams release large,

developed young; this smallest of clams produces the largest of eggs and juveniles

•Asian Clam-introduced;

•1st observed in 1938

•Cannot tolerate cold

Zebra Mussels, Dreissena polymorpha

Veliger

Endangered Unionids

Northern Clubshell Cracking Pearlymussel

Fat Pocketbook

Rough Pigtoe

White Wartyback