1 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 Very different, but with some important shared characters - Mantle -Shell - Ctenidium - Muscular foot - Radular organ
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Polyplacophora Cephalopoda Mollusca: General Characteristicsbosch/Mollusca1.pdf · 1 Mollusca: General Characteristics! Gastropoda 40,000 sp Bivalvia 7,650 sp Other 5 Classes ~1100
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Very different, but with some important shared characters
- Mantle
-Shell
- Ctenidium - Muscular foot
- Radular organ
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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.
Class Gastropoda SubClasses Prosobranchia Opisthobranchia
Pulmonata
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
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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.
Ancestral Modern Pre-Torsional Gastropod Post-torsional
Torsion
Torsion occurs during the development of the veliger
Early trochophore
Later trochophore
Front view
Side view
Veliger
Before torsion after torsion
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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)
--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.
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Archeogastropod Meso- and Neogastropods
F. Opisthobranchs G. Pulmonates
Solutions to sanitary problems
that resulted after torsion
D. Bipectinate
A problem of plumbing
E. monopectinate C. Bilateral
Problems Caused by Torsion
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