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Mollusca: General Characteristics

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Mollusca: General Characteristics Molluscan Taxonomic Classes Polyplacophora Cephalopoda Mollusca: General Characteristics Other 5 Classes ~1100 Scaphopoda Bivalvia 7,650 sp Gastropoda and Gastropoda Aplacophora 40,000 sp Monoplacophora Bivalvia Very different, but with some important shared characters Cretaceous -Shell Triassic - Mantle Devonian - Ctenidium - Muscular foot Polyplacophora Rostriconchians Gastropoda Bivalvia Cephalopoda - Radular Monoplacophora Scaphopoda organ Fossil History of the Mollusca 1 Class Gastropoda SubClasses Prosobranchia Opisthobranchia Pulmonata 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. Modern Prosobranch Gastropod 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 2 Coiling is a way to maximize volume while minimizing height Torsion Ancestral Modern A coiled (planospiral) shell is top-heavy so in most larger Pre-Torsional Gastropod Post-torsional species the coil is angled relative to the A-P axis. Torsion occurs Front Early during the view trochophore development of the veliger Later Side trochophore view Veliger Before torsion after torsion 3 Are there adaptive advantages to Torsion? Are there adaptive advantages to Torsion? Three hypotheses proposed Two hypotheses have been proposed 1. To bring adult coiled shell into better 1. To bring adult coiled shell into better balance along a-p axis balance along a-p axis 2. Allows larva to pull its head and velum Mantle cavity is Beneath this area into the mantle cavity first 3. New perspectives (Louise Page Univ. of Victoria) “asymmetry hypothesis” anterior re-arrangement/enlargement of the lateral mantle cavity in a monoplacophoran Design of experiments to test Results of Pennington and Chia 1985 Experiments adaptive value of torsion Data are the mean number of larvae eaten. Torted larvae in white. (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 hydromedusa spp ctenophore spp crab larvae, copepod, fish, 10 20 30 A B A B ctenophore (2), hydromedusa (2) -- 5 replicates, 25 larvae of each type, allowed experiments to run for 15 hr Conclusions ??? 4 Problems A problem of plumbing Other Molluscan Groups: Caused Opisthobranchia by Torsion Evolutionary Tendencies: Archeogastropod Meso- and Neogastropods • Shell internal, reduced or lost • Limited torsion in development • Loss or reduction of D. Bipectinate C. Bilateral E. monopectinate mantle cavity http://week.divebums.com/2008/Mar10-2008/index.html • Loss of operculum Solutions to • Ctenidia replaced by gills sanitary problems that resulted after torsion F. Opisthobranchs G. Pulmonates Varying degrees Other Molluscan Sub Classes Of detorsion Opisthobranchia In Bubble Shell Opisthobranchs 5 Other Molluscan Sub Classes: What features seem to be Pulmonata adaptations to terrestrial life? land and f.w. snails and slugs very few marine Adaptations to life In terrestrial Environments? Simplified phylogeny of Gastropoda and Torsion 18s r- DNA phylogenies support the monophyly of Opisthobranchia and Pulmonata detorsion Shell loss Land slugs lungs Is Prosobranchia a valid taxon? Pulmonata Land snails coiled Pulmonata shell torsion Opisthobranchia Shell loss Gills detorsion Prosobranchia Monoplacophora ancestor In 28s and 18s rDNA phylogenies Bye- Bye Opisthobranchia!! “pulmonates” are nested within the Opisthobranchia!! Schroedl et al. 2010 Thalassas vol. 27:101-112 6 .
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