Seagrass grazers coming out of their shells

The Calopiidae, with its only genus Calopia, is a family of snails known only from Australia. With translucent shells covering brightly coloured animals, calopiids would perhaps be better known were it not for their minute size. Favouring seagrass and algae in shallow estuarine settings, they form part of a micro-mollusc fauna that is poorly known, yet of great ecological importance. In our new study, we combined the use of mitochondrial genes and shell measurement statistics to shed further light on the evolutionary history and biogeography of the family. It turns out that genes and shells do not necessarily tell the same story, and this affects how we make decisions about classification.

Although sounding like a Greek deity, the name Calopia is in fact an amalgamation of the words calcareous, meaning something partly or mostly composed of calcium carbonate, and operculum, the ‘trap door’ many gastropods use to seal themselves inside their shells. Calopiids are also characterised by the complex tentacles on their head, and ‘tail-like’ structure at the back of their foot, called a metapodial tentacle. Although their shells are minute (some adults barely grow to one millimetre in length), their stature is not a reliable means of identification – microgastropods of that size and with simple, smooth shells, can belong to several distantly related groups.

It is also a small family in that it until very recently consisted of only four species – Calopia minutissima, C. laseroni, C. imitata and C. burni, the latter two largely separated based on the appearance of their shells. However, a recent AMRI study suggests that C. imitata and C. burni may be considered one species, reducing the number of known calopiids even further.

By targeting three populations of the widespread C. imitata, each separated by roughly the same geographical distance (Bowen, Brisbane, Sydney) and the narrow-range C. burni from Port Philip Bay in Victoria, we studied mitochondrial genes and shell dimensions from each population. We found that genetic differences among all four localities, in both species, turned out to be negligible. Additionally, we found that shells between populations of C. imitata differ statistically just as much from each other as they do to C. burni.

Considering the lack of genetic differentiation, these shell statistics suggest that C. burni, rather than being a species in own right, may be considered a distinct population of C. imitata. Given that these localities span several climatic zones, differences in shell dimensions are therefore possibly ecophenotypic – meaning that they are attributed to environmental, rather than genetic, factors. Such findings emphasise the utility of genetics to complement anatomical and morphological characters, especially for microgastropod groups that are difficult to study due to their size and simple shells.

Calopiids, and other micro-invertebrates, are crucial benefactors to their habitats as they commonly feed on algae and bacteria. If unchecked, algal growth may lead to eutrophication, where blooms can potentially block sunlight and also keep oxygen from reaching deeper water, causing significant ecosystem disruption. Seagrass environments in particular are important habitats for a multitude of fish, mammals and turtles, and their root systems ensure that coasts are protected from erosion. The study of their inhabitants, no matter how small and seemingly insignificant, is therefore advantageous as it helps us understand their complexity and make informed conservation decisions.

Dr Anders Hallan
Scientific Officer, AMRI

More information:

Ponder, W.F. (1999). Calopia (Calopiidae), a new genus and family of estuarine gastropods (Caenogastropoda: Rissooidea) from Australia. Molluscan Research, 20: 17–60.

Hallan, A., Golding, R. & Burghardt, I. (2015). A molecular phylogeny of the Calopiidae (Caenogastropoda: Truncatelloidea) with a taxonomic reassessment of Calopia imitata Ponder, 1999. Journal of Molluscan Studies. DOI:10.1093/mollus/eyv023