Phylum Mollusca

• Second largest animal phylum (> 100,000 species)

• Oysters, snails, clams, octopus, squid

• Two unifying characteristics– External calcium carbonate shell– Muscular foot for locomotion

Generalized Molluscan Shell

• Outer periostracum (organic layer – conchiolin

• Inner layers largely calcium carbonate with some organic material in a matrix

Class Monoplacophora

• Originally known from Cambrian to Devonian fossils

• Living representatives found 1952• Since, 20 more species found (1800 – 7000 m deep)• Single bilaterally symmetrical shell• Probably share common ancestor with other

Mollusca except Polyplacophora• Members probably actually gave rise to the other

molluscan groups (bivalves, gastropods, cephalopods)

• Small – 3 mm to 3 cm long

Fig 12-12

Class Polyplacophora

• Chitons

• Retain many features of generalized mollusc

• Some adaptation for predominant lifestyle (intertidal zone)

• Shell structuring (layering) different from other molluscs (Conchifera)

Fig 12-8

Class Gastropoda

• Most diverse group (~60,000 species)

• >15,000 described fossil species

• Most extensive adaptive radiation of any mollusc group

Class Gastropoda

• Three “groups” – phylogeny revision• Prosobranchs – most common members

when think of snails– Terrestrial, freshwater, and marine*– Common feature – operculum

• Opisthobranchs– Sea slugs, sea hares– Many members lost shell

• Pulmonates– Many terrestrial species, also freshwater, a few

marine

Major Changes from Generalized Mollusc

• Development of head

• Dorsoventral elongation

• Shell – from shield to retreat

• Torsion

• Conispiral coiling and asymmetry

Fig 10-15

Monoplacophoranancestor

Planispiral coiling

Torsion

• Weight of shell over head, mantle cavity posterior

• Torsion – 180o counterclockwise rotation of visceral mass, shell, mantle, mantle cavity

• Occurs in larvae not adult

• First gastropods

• Detorsion

Costs of Conispiral Shell

• Loss of a gill, nephridium, atrium

• Mantle cavity (anus and nephridiopore) now anterior and near mouth

• Compensation - changes in water flow or shell structure

• See Figure 12-20 (mantle cavity evolution) and 12-21A (abalone)

Shell

• Apex, whorl, columnella, aperature, siphonal canal

• Spire, body whorl, outer lip, inner lip

• operculum

Locomotion

• Most move using foot

• Most have ciliated sole and secretory glands (mucus producing)

• Hard-bottom dwelling and terrestrial, and large soft-bottom snails - undulating wave of muscle contractions (Figure 12-30)

Feeding

• Most often thought of as algal scrapers (radula)

• Deposit feeders

• Suspension feeders

• Scavengers

• Predators

• Parasites

Class Bivalvia

• Oysters, clams, mussels ~8,000 species (1,300 freshwater, rest marine)

• Benthic filter-feeders (a few exceptions)– No radula– Enlarged gills

• Compressed body• Shell

– Two valves– Hinged dorsally– Completely encloses body

Class Bivalvia

• Rostroconch ancestor

Fig 12-96

Fig 12-100

Class Cephalopoda

• ~700 living species, 10,000 fossil species

• Highly specialized

• Pelagic (octopus secondary)

• Shell – coiled, internalized, reduced, or lost

• Closed circulatory system

• Visual eye

Fig 12-68