Molluscan Taxonomic Classes
Mollusca: General Characteristics
Bivalvia
7,650 sp
Polyplacophora
Cephalopoda
Scaphopoda
Other 5 Classes ~1100
Gastropoda
and
Aplacophora
Monoplacophora
Gastropoda
40,000 sp
Bivalvia
Very different, but with some important shared characters
Cretaceous
Triassic
-Shell
- Mantle
Devonian
- Ctenidium
- Muscular foot
Polyplacophora Rostriconchians Gastropoda
Monoplacophora
Bivalvia
Cephalopoda
Scaphopoda
Fossil History of the Mollusca
- Radular
organ
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
A coiled (planospiral) shell is top-heavy so in most larger
species the coil is angled relative to the A-P axis.
Ancestral
Pre-Torsional
Torsion occurs
during the
development of
the veliger
Torsion
Gastropod
Modern
Post-torsional
Front
view
Early
trochophore
Side
view
Later
trochophore
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)
--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)
Results of Pennington and Chia 1985 Experiments
Data are the mean number of larvae eaten. Torted larvae in white.
Crab larvae copepod
fish
10 20 30
hydromedusa spp
A
B
ctenophore spp
A
B
-- 5 replicates, 25 larvae of each type, allowed
experiments to run for 15 hr
Conclusions ???
4
Problems
Caused
by
Torsion
Other Molluscan Groups:
Opisthobranchia
A problem of plumbing
Evolutionary Tendencies:
Archeogastropod
Meso- and Neogastropods
•  Shell internal, reduced or lost
•  Limited torsion in development
C. Bilateral
D. Bipectinate
E. monopectinate
•  Loss or reduction of
mantle cavity
•  Loss of operculum
Solutions to
sanitary
problems
that resulted
after torsion
http://week.divebums.com/2008/Mar10-2008/index.html
•  Ctenidia replaced by gills
F. Opisthobranchs
Other Molluscan Sub Classes
Opisthobranchia
G. Pulmonates
Varying degrees
Of detorsion
In Bubble Shell
Opisthobranchs
5
What features seem to be
adaptations to terrestrial life?
Other Molluscan Sub Classes:
Pulmonata
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
Pulmonata
lungs
Is Prosobranchia a valid taxon?
Land snails
Pulmonata
coiled
shell torsion
Opisthobranchia
Shell loss
Gills detorsion
Prosobranchia
Monoplacophora
ancestor
In 28s and 18s rDNA phylogenies
“pulmonates” are nested within the
Opisthobranchia!!
Bye- Bye Opisthobranchia!!
Schroedl et al. 2010
Thalassas vol. 27:101-112
6