Exercise 38

Phyla Mollusca and Annelida

Mollusca

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

Class Polyplacophora: Chitons

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

Geoduck:

The Legend

Geoduck-the reality

Anatomy

Freshwater 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

Class Cephalopoda: Squid, Octupus, Nautilus

Phylum Annelida

Segmented Worms

World’s largest earthworm:Megascolides australis

Class: Polychaeta (‘few bristles’)

Nereis virens, common ‘sandworm’ of east coast fishermen

Class Oligochaeta (‘few bristles’)

How Earthworms hook up

• About 170 N.A. species; about 30 species in OH• Each segment has small bundles of tiny chaetae• Most live in silt and mud in ponds, lakes, etc.• As deposit feeders, feces deposited atop the

bottom; helps keep surface aerobic• Some are very tolerant of low oxygen; Tubifex

worms are long and red; can build up very high densities (8,000/m2)

Class: Hirudinea (Leaches)

Leeches

• Primarily aquatic (~69 species in N.A.); only a few marine

• Many are highly colored (but fade in alcohol)

• Body: soft, muscular, flattened; 34 segments

• Two suckers: one anterior (includes the mouth), one posterior

Natural History

• Most live in shallow waters with much plants, debris, or stones; in suitable habitat, can reach density of 700/m2

• Move inchworm-style; many can swim• All feed on fluids from other organisms either as piercer-

predator or as external parasite– Most feed on invertebrates– Blood suckers have 3-toothed jaws and secrete hirudinin, an

anticoagulant; most feed on frogs, turtles, or fish– May not feed again for two years

• Always reproduce sexually; some make cocoons;• Many require years to reach maturity• Are both predators and prey