Werner
de Gier
Guest Writer
The pteropods are a group of gastropods somewhat related to nudibranchs. The name “Pteropoda” comes from “wing-footed” and already gives us a clue on the biology of these snails. The sea butterflies (Thecosomata) are just one of the two groups making up the pteropods, the other group being the sea angels (Gymnosomata). The two groups are classified based on their biology and body structures. While thecosomes have fragile shells, their naked counterparts have completely lost their shell.
Sea butterflies are highly specialized for their pelagic lifestyle, the way they move through the water and catch their prey are both directly linked to their free-living habits. While most thecosomes still have a curled shell (like you would see in most snails), there have been reports of individuals having “straightened-out” shells. The Orthoconcha is a group of sea butterflies with such straight shells, it is thought the symmetrical shells correlate directly to their planktonic lifestyle.
The “flying” snails have developed modified flaps on their foot called parapodia that they use to actively swim through the water and escape from predatory sea angels. The way these snails swim resembles the flight of small insects. They appear to make a figure-eight pattern in their swimming cycle, just like flies. This is the first time this way of “flight” is found in another group of animals, a perfect example of convergent evolution.
