Mollusca: General Characteristics Molluscan Taxonomic Classes Polyplacophora Cephalopoda Bivalvia 7,650 sp Other 5 Classes ~1100 Gastropoda Scaphopoda Gastropoda 40,000 sp and Aplacophora Monoplacophora Bivalvia Very different, but with some important shared characters Cretaceous Triassic -Shell - Mantle Devonian - Ctenidium - Muscular foot Polyplacophora Rostriconchians Gastropoda Bivalvia Cephalopoda Scaphopoda Monoplacophora Fossil History of the Mollusca - Radular organ 1
Class Gastropoda SubClasses Prosobranchia 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 A coiled (planospiral) shell is top-heavy so in most larger species the coil is angled relative to the A-P axis. Torsion Ancestral Modern Pre-Torsional Gastropod Post-torsional Torsion occurs during the development of the veliger Early trochophore Front view Later trochophore Side view Veliger Before torsion after torsion 3
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 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 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 adaptive value of torsion (Pennington and Chia 1985) Results of Pennington and Chia 1985 Experiments Data are the mean number of larvae eaten. Torted larvae in white. --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) Crab larvae copepod fish hydromedusa spp ctenophore spp A B A B 10 20 30 -- 5 replicates, 25 larvae of each type, allowed experiments to run for 15 hr Conclusions??? 4
Problems Caused by Torsion A problem of plumbing Other Molluscan Groups: Evolutionary Tendencies: Solutions to sanitary problems that resulted after torsion Archeogastropod C. Bilateral D. Bipectinate Meso- and Neogastropods E. monopectinate F. Opisthobranchs G. Pulmonates 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 Other Molluscan Sub Classes Varying degrees Of detorsion In Bubble Shell Opisthobranchs 5
Other Molluscan Sub Classes: land and f.w. snails and slugs very few marine What features seem to be adaptations to terrestrial life? Adaptations to life In terrestrial Environments? Simplified phylogeny of Gastropoda and Torsion 18s r- DNA phylogenies support the monophyly of and Is Prosobranchia a valid taxon? coiled shell torsion detorsion lungs Shell loss Gills detorsion Shell loss Land slugs Monoplacophora ancestor In 28s and 18s rdna phylogenies Bye- Bye!! pulmonates are nested within the!! Schroedl et al. 2010 Land snails Prosobranchia Thalassas vol. 27:101-112 6