Mollusca: General Characteristics


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Mollusca: General Characteristics

  • Gastropoda
  • 40,000 sp
  • Bivalvia
  • 7,650 sp
  • Other 5 Classes ~1100
  • Polyplacophora
  • Cephalopoda
  • Molluscan Taxonomic Classes
  • Gastropoda
  • Scaphopoda
  • Bivalvia
  • and
  • Aplacophora
  • Monoplacophora
  • Cretaceous
  • Triassic
  • Polyplacophora Rostriconchians Gastropoda Bivalvia Cephalopoda
  • Scaphopoda
  • Monoplacophora
  • Devonian
  • Fossil History of the Mollusca
  • 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.
  • Halkieria
  • Recognized by paleontologists as a stem mollusc from the early to middle Cambrian
  • Very different, but with some important shared characters
  • - Mantle
  • -Shell
  • - Ctenidium
  • - Radular organ
  • Class Gastropoda
  • SubClasses
  • Prosobranchia
  • Opisthobranchia
  • Pulmonata
  • Modern Prosobranch Gastropod
  • 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
  • P.o.D.
  • Clay Brice, in Wilson,1994
  • 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
  • Ancestral Modern
  • Pre-Torsional Gastropod Post-torsional
  • Torsion occurs
  • during the
  • development of
  • the veliger
  • Early
  • trochophore
  • Later
  • trochophore
  • Front
  • view
  • Side
  • view
  • Veliger
  • Before torsion after torsion
  • Are there adaptive advantages to Torsion?
  • Three hypotheses proposed
  • 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
  • New perspectives (Louise Page Univ. of Victoria)
    • “asymmetry hypothesis”
    • anterior re-arrangement/enlargement of the
    • lateral mantle cavity in a monoplacophoran
  • --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
  • Results of Pennington and Chia 1985 Experiments
  • Crab larvae copepod fish hydromedusa spp ctenophore spp
  • A B A B
  • 10 20 30
  • Conclusions ???
  • Data are the mean number of larvae eaten. Torted larvae in white.
  • Problems caused
  • byTorsion
  • F. Opisthobranchs G. Pulmonates
  • Solutions to sanitary problems that resulted after torsion
  • D. Bipectinate
  • A problem of plumbing
  • E. monopectinate
  • C. Bilateral
  • 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
  • Varying degrees
  • Of detorsion
  • In Bubble Shell
  • Opisthobranchs
  • Other Molluscan Sub Classes:
  • Pulmonata
  • land and f.w. snails and slugs
  • very few marine
  • Adaptations
  • to life
  • In terrestrial
  • Environments?
  • What features seem to be
  • adaptations to terrestrial life?
  • Simplified phylogeny of Gastropoda
  • and Torsion
  • Monoplacophora
  • ancestor
  • torsion
  • lungs
  • Land slugs
  • Pulmonata
  • Land snails
  • Pulmonata
  • Shell loss
  • Shell loss
  • Opisthobranchia
  • Prosobranchia
  • detorsion
  • coiled
  • shell
  • Gills detorsion
  • 18s r- DNA phylogenies support the monophyly of Opisthobranchia and Pulmonata
  • Is Prosobranchia a valid taxon?
  • 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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