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Biology 136L – 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. Page 3 of 9
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? Page 4 of 9
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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)