Phylum Mollusca

1. 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

3. Generalized Molluscan Shell • Outer periostracum (organic layer – conchiolin • Inner layers largely calcium carbonate with some organic material in a matrix

7. 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

8. Fig 12-12

9. Class Polyplacophora • Chitons • Retain many features of generalized mollusc • Some adaptation for predominant lifestyle (intertidal zone) • Shell structuring (layering) different from other molluscs (Conchifera)

10. Fig 12-8

11. Class Gastropoda • Most diverse group (~60,000 species) • >15,000 described fossil species • Most extensive adaptive radiation of any mollusc group

12. 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

13. Major Changes from Generalized Mollusc • Development of head • Dorsoventral elongation • Shell – from shield to retreat • Torsion • Conispiral coiling and asymmetry

14. Fig 10-15 Monoplacophoran ancestor

15. Planispiral coiling

16. 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

20. 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)

21. Shell • Apex, whorl, columnella, aperature, siphonal canal • Spire, body whorl, outer lip, inner lip • operculum

22. 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)

23. Feeding • Most often thought of as algal scrapers (radula) • Deposit feeders • Suspension feeders • Scavengers • Predators • Parasites

25. 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

26. Class Bivalvia • Rostroconch ancestor

31. Fig 12-96

34. Fig 12-100

35. 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

37. Fig 12-68