gastropods

gastropods Gastropods are one class of the phylum Mollusca, a large phylum that contains such diverse organisms as the bivalves, octopus, and ammonites. The gastropods are the largest class of molluscs with about 1650 genera, and are by far the most varied, occurring in marine, freshwater, and terrestrial environments and occupying a variety of habitats. They normally inhabit a coiled shell and move by means of a muscular foot: however, in the slugs and marine nudibranchs the shell has been lost. Although gastropods are at their most successful today, they have a long geological history stretching back into the Cambrian.

Snails are perhaps the most characteristic gastropods. A helically coiled shell contains most of the organs, while the head and foot project outside (Fig. 1). The head bears the main sense organs, the eyes and tentacles, and also the mouth, which contains the radula, a rasping structure consisting of a tongue-like band with many small teeth. In some forms the radula is used to scrape algae from rocks, but in predatory gastropods it is used to bore holes into the shells of other molluscs. The foot is a muscular structure used to creep over hard substrates, propulsion being provided by waves of muscular contraction that pass along it. Both the head and foot can be pulled inside the shell for protection. The shell is secreted by the mantle, which lines it and also encloses the mantle cavity within which are the gills, the osphradium (a chemo-sensory organ), and openings of the digestive glands. The mantle cavity opens at the shell aperture, and may be partly drawn out to form a siphon whose function is to draw water into the gill and osphradium. In terrestrial gastropods the gill is lost and the mantle cavity has evolved into a vascularized lung.

Gastropod classification is largely based on the anatomy of living forms, fossils being placed by comparison of the morphology of their shells. A major characteristic of gastropods is the torsion of their bodies: the posterior part of the mantle cavity is rotated laterally and anteriorly so that it comes to lie above the head. This torsion is evident in the phylogeny of gastropods as they develop from bilaterally symmetrical ancestors with simple conical shells. Detorsion also took place later in forms such as slugs, which have lost their shells and reverted to a bilateral symmetry. The classification of gastropods is based chiefly on this feature. There are three subclasses: the Prosobranchia, which show full torsion, are marine, freshwater, and terrestrial gastropods that are generally herbivorous and include about 50 per cent of the members of the class; the Opisthobranchia are detorted gastropods in which the shell is absent or concealed in the mantle (e.g. nudibranchs); and the Pulmonata, which are also detorted but retain a spiral shell, have a mantle cavity converted to a vascularized lung, since they are terrestrial forms.

Unequivocal gastropods first occur in the Early Cambrian where Bellerophontacean gastropods with shells coiled in one plane are found. Problematic simple conical shells are known from the Early Cambrian; there is little agreement about their relationships, although it is probable that the earliest forms developed from a monoplacophoran ancestor that initiated torsion by its ability to withdraw deep into a narrow shell. This ability may have developed independently in several lineages; if so, the gastropods could be polyphyletic (developed from several different ancestors). The first helically coiled shells appeared by the end of the Cambrian and development continued until the Early Carboniferous (Mississippian), by which time the gastropod faunas were very diverse and included the first non-marine forms. The gastropods were affected by the Permian–Triassic extinctions, as were all marine invertebrates, but they continued to evolve through the Mesozoic, the pulmonates making a successful transition to land at this time.

Gastropods have been little used in biostratigraphy because of the relatively restricted range of their shell morphology; they have, however, proved useful in evolutionary studies. In the late 1970s work by Peter Williamson of the University of Bristol on the molluscan faunas of the Tertiary sediments of Lake Turkana in Kenya provided fine-scaled evolutionary data that supported the punctuated equilibrium (evolution in steps) model of speciation. Periodic regressions of the lake left peripheral isolates (isolated groups of organisms) in which major changes took place under the influence of environmental stress, contrasting with the slow rate of change in the main lake. Short bursts of rapid speciation thus alternated with long periods of stasis in the fossil record.

David K. Elliott

Bibliography

Boardman, R. S., Cheetham, A. H., and Rowell A. J. (eds) (1987) Fossil invertebrates. Blackwell Scientific Publications, Oxford.
Morton, J. E. (1967) Molluscs. Hutchinson, London.