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Biology 122L – Invertebrate zoology lab
Molluscan diversity lab guide
Author: Allison J. Gong
Figure source: Brusca and Brusca, 2003. Invertebrates, 2nd edition. Sinauer Associates,
Inc.
The Mollusca comprise a huge taxon, second only to the Arthropoda in terms of
number of extant species. They are successful in marine, freshwater, and terrestrial
habitats. There are probably close to 100,000 species of living molluscs, including such
diverse forms as snails, clams, slugs, and squids. There are also some 35,000 fossil
species. You have already dissected two common local marine molluscs – a snail
(Chlorostoma funebralis) and a bivalve (Mytilus californianus) – and should be familiar
with their internal and external anatomy. This understanding will help you make sense of
the diversity of body forms you will observe in representatives of other molluscan taxa.
With any luck I will have specimens from four of the seven extant molluscan classes for
you: Polyplacophora, Bivalvia, Cephalopoda, and Gastropoda.
In strictly pedagogical terms, we can use a non-existent creature called a
Hypothetical Ancestral Mollusc (HAM) as a starting point on which natural selection has
acted to produce the variety of molluscan body plans that we see today. This HAM was a
benthic animal adapted for life on hard surfaces, crawling around on its muscular ventral
foot and using its radula to scrape algal and detrital films. It was poorly cephalized with
an anterior head and had a single, cap-shaped shell that could be clamped down to protect
the visceral mass and other soft body parts. The mantle cavity enclosed several pairs of
bipectinate ctenidia, or gills. See pages 285-291 in your textbook (Ruppert et al, 2004)
for a diagram and description of the HAM.
As you make your observations, keep in mind these characteristics of the HAM. In
general, these are the features that biologists use to group all molluscs into a single
phylum, although you will find them modified in different ways among the animals that
you observe today. How are these modifications (adaptations, right?) reflected in the
current taxonomy of the group? Don't forget that the reduction of a given feature can be
just as adaptive as its elaboration. In living specimens, the most easily observed of these
features are the shell, foot, and (in some cases) the mantle. Other structures, such as the
ctenidium and radula, are difficult to observe without dissection, and you will probably
not be able to see them in the live animals available to you in this lab. However, this
should not keep you from speculating about the nature of these structures. What can you
infer from the fact that you CAN'T see them from the outside? Does this say anything
about their function? Or about their "importance" to the animal?
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General molluscan features
The diagram below demonstrates how some of the major features of the Phylum
Mollusca have been modified in the most frequently encountered classes. Which of these
adaptations can you detect in the animals you observe? Annotation for this lab (and all
diversity labs) should include comparisons of, for example, shell morphology in bivalves
vs. prosobranchs vs. pulmonates vs. cephalopods.
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Observations of the shell
The ancestral molluscan shell was a simple cap- or dome-shaped structure that the
animal could pull over its body. Modern limpets exemplify this type of shell. The shell
is secreted by glands in the dorsal mantle and is attached to the body by a few or several
retractor muscles – remember the columellar muscle in Chlorostoma? Some molluscs
(e.g., snails) can move their shells relative to the position of the body, while others
cannot. Some can be entirely enclosed within the shell(s). Others have internal shells, or
remnants thereof.
Many molluscs have elaborated on this simple shell design, and some have lost it
altogether. You will see examples of both extremes. In each case, consider what the
shell morphology tells you of the animal's habitat and lifestyle. What are the shell's
functions? Are these functions the same in all molluscs? Notice that a chiton's shell
consists of eight calcareous plates. How does this design enable the animal to clamp
tightly to its substrate? A snail's shell is coiled – in what way(s) is this feature adaptive?
Nudibranchs and some cephalopods have lost the shell entirely. How do these animals
manage without a shell? Do you think they are at a disadvantage compared to their
shelled relatives?
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Radula
The radula is one of the features unique to molluscs. In ancestral molluscs such as
prosobranch gastropods it is used for scraping, and in cephalopods it functions as a
conveyor belt to propel food into the mouth. Bivalves have lost the radula entirely. The
radula sits atop a cartilaginous structure called the odontophore, and various muscles
serve to move it in and out of the buccal cavity. Radular teeth are replaced continually,
as they erode while the animal feeds.
Radular scraping on a surface covered with algal film often produces a characteristic
zig-zag pattern. You can see these patterns on rocks in the intertidal. In addition to
scraping algal growth, grazing by the molluscan radula also removes newly settled larvae.
Some intertidal animals (e.g., Lottia gigantea) maintain territories of cleared space in this
fashion. A large female Lottia actively "farms" her territory, allowing algal scum to grow
on one part of her territory while she grazes elsewhere.
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Class Polyplacophora
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Class Gastropoda
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Class Bivalvia
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Anatomy of Loligo opalescens (Class Cephalopoda)