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 Monoplacophoran ancestor

Planispiral coiling

Torsion Weight of shell over head, mantle cavity posterior Torsion – 180 o 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 (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

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