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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

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

http://week.divebums.com/2008/Mar10-2008/index.html

Other Molluscan Sub Classes Opisthobranchia

Varying degrees Of detorsion In Bubble Shell Opisthobranchs

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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!!

Bye- Bye Opisthobranchia!!

Schroedl et al. 2010

Thalassas vol. 27:101-112