Why have many molluscs lost or reduced their shells?
The dramatic ‘explosion’ of hard-shelled animals into the fossil record that occurred 500 million years ago clearly suggests that a tough shell is a useful accessory for an animal to have. It provides a reliable means of protection, both against predators and against the physical environment, and it can be modified for many other purposes (some creatures, for instance, have converted their shells into gas-filled flotation aids). The phylum that has undoubtedly been most successful in exploiting the advantages of having a hard, calcified covering is the molluscs, yet there are several types of mollusc whose shells have been greatly reduced in size, or even lost completely, during their evolution. In this essay, I will examine two major groups of molluscs within which secondarily shell-less forms have evolved, in an attempt to discover the reasons behind this puzzling characteristic. The gastropods With over 30,000 species surviving in a wide variety of habitats, the gastropods are by fa
