Variations on a theme, Part I

The sea snail Columbella rustica (Linné, 1758) is a very common inhabitant of Maltese rock pools and shallow platforms. It has a large range, extending from the Mediterranean down to the coasts of Angola and the eastern Atlantic islands.

The shell is highly variable, even within relatively short distances in this range, since it relies directly on the habitat. Specimens from the northwest of Malta tend to be mostly small and dark, while others from the sandy beaches of eastern Malta are light coloured and much larger (photograph below). This can be explained by natural selection - yellow specimens are better camouflaged against the sand, while mechanical damage to the shells by waves is not a pressing problem in a soft substrate such as sand. The small compact individuals from rocky shores, on the other hand, are exposed to very rough currents, which may dislodge and batter the shells onto rocks, therefore a smaller surface area works better in this regard. The darker colour also conceals shells in the algae better than light ones.

In addition to population-scale differences, there is also some variability on the individual level. The above photograph, on the extreme left, shows a 'freak' elongated specimen with convex whorls. A 'turriculated' variety from the same area is figured below (photo by Constantine Mifsud).