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Mollusca: General Characteristics

Mollusca: General Characteristics. Other 5 Classes ~1100. Bivalvia 7,650 sp. Gastropoda 40,000 sp. Molluscan Taxonomic Classes. Polyplacophora. Cephalopoda. Gastropoda. Scaphopoda. Monoplacophora. Bivalvia. and Aplacophora. Cretaceous. Triassic. Devonian.

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Mollusca: General Characteristics

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  1. Mollusca: General Characteristics Other 5 Classes ~1100 Bivalvia 7,650 sp Gastropoda 40,000 sp

  2. Molluscan Taxonomic Classes Polyplacophora Cephalopoda Gastropoda Scaphopoda Monoplacophora Bivalvia and Aplacophora

  3. Cretaceous Triassic Devonian Polyplacophora Rostriconchians Gastropoda Bivalvia Cephalopoda Scaphopoda Monoplacophora Fossil History of the Mollusca

  4. Figure 1. Consensus phylogeny of mollusc classes as supported by the phylogenomic anlayses of Kocot et al.[1] and Smith et al.[2].The Mollusca are divided into two major clades: i) the Aculifera, which possess spicules and show repetition of structures along their antero-posterior axis; and ii) the Conchifera, which (at least primitively) possess a shell. Within Conchifera, there remains some uncertainty over the most likely position of the Scaphopoda (tusk shells). Monoplacophora was placed by the analysis of Smith et al.[2] as sister group of the Cephalopoda. The Cambrian fossil genus Latouchella is indicated at the base of the Conchifera. The Cambrian fossil genera Wiwaxia and Halkieria (reconstruction) possess dorsal sclerites and a radula-like structure and are supported as stem group molluscs by some palaeontologists.

  5. Halkieria Recognized by paleontologists as a stem mollusc from the early to middle Cambrian

  6. Very different, but with some important shared characters -Shell - Mantle - Ctenidium - Muscular foot - Radular organ

  7. Class Gastropoda SubClasses Prosobranchia Opisthobranchia Pulmonata

  8. Modern Prosobranch Gastropod

  9. P.o.D. Clay Brice, inWilson,1994 • When not hunting the proboscis retracted inside • Senses prey by chemical signals & extends searching proboscis • -moves venom filled radular tooth to end of proboscis • http://www.jsonline.com/blogs/news/106131413.html

  10. Evolution of Gastropoda: • Possibly from now extinct monoplacophoran (A class of molluscs represented today by one deep water genus) • Involves coiling and re-alignment of the shell • Also, in an independent evolutionary event, a twisting of the viscera relative to the A-P axis of the body as defined by the head and the foot: torsion

  11. Ancestral Modern Pre-Torsional Gastropod Post-torsional

  12. Torsion occurs during the development of the veliger Front view Early trochophore Later trochophore Side view Veliger Before torsion after torsion

  13. Are there adaptive advantages to Torsion? Three hypotheses proposed 1. To bring adult coiled shell into better balance along a-p axis Mantle cavity is Beneath this area

  14. Are there adaptive advantages to Torsion? • Two hypotheses have been proposed • 1. To bring adult coiled shell into better • balance along a-p axis • 2. Allows larva to pull its head and velum • into the mantle cavity first • New perspectives (Louise Page Univ. of Victoria) “asymmetry hypothesis” anterior re-arrangement/enlargement of the lateral mantle cavity in a monoplacophoran

  15. Design of experiments to test adaptive value of torsion (Pennington and Chia 1985) --Fed a choice of pre-torted and newly torted veliger larvae of the abalone Haliotis to seven potential predators: crab larvae, copepod, fish, ctenophore (2), hydromedusa (2) -- 5 replicates, 25 larvae of each type, allowed experiments to run for 15 hr

  16. Results of Pennington and Chia 1985 Experiments Data are the mean number of larvae eaten. Torted larvae in white. Crab larvae copepod fish hydromedusa spp ctenophore spp A B A B 10 20 30 Conclusions ???

  17. Problems caused byTorsion

  18. A problem of plumbing Archeogastropod Meso- and Neogastropods D. Bipectinate C. Bilateral E. monopectinate Solutions to sanitary problems that resulted after torsion F. Opisthobranchs G. Pulmonates

  19. Other Molluscan Groups: Opisthobranchia • Evolutionary Tendencies: • Shell internal, reduced or lost • Limited torsion in development • Loss or reduction of • mantle cavity • Loss of operculum • Ctenidia replaced by gills http://week.divebums.com/2008/Mar10-2008/index.html

  20. Other Molluscan Sub Classes: Opisthobranchia

  21. Varying degrees Of detorsion In Bubble Shell Opisthobranchs

  22. Other Molluscan Sub Classes: Pulmonata land and f.w. snails and slugs very few marine Adaptations to life In terrestrial Environments?

  23. What features seem to be adaptations to terrestrial life?

  24. Simplified phylogeny of Gastropoda and Torsion 18sr- DNA phylogenies support the monophyly of Opisthobranchia and Pulmonata Shell loss detorsion Land slugs Pulmonata lungs Is Prosobranchia a valid taxon? Land snails Pulmonata coiled shell torsion Opisthobranchia Shell loss Gills detorsion Prosobranchia Monoplacophora ancestor In 28s and 18s rDNA phylogenies “pulmonates” are nested within the Opisthobranchia! (slugs) Bye- Bye Opisthobranchia!! Schroedl et al. 2010 Thalassas vol. 27:101-112

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