by Piter Kehoma Boll
Last week I presented a beautiful sea snail, the common sea butterfly, with wing-like parapodia that allows it to swim. The sea butterflies belong to a group of marine gastropods known as Pteropoda due to this foot modified into fins. There are two main groups of pteropods, Thecosomata, that have a shell, and Gymnosomata, that don’t have a shell. While the shelled ones are called sea butterflies, the naked ones are called sea angels or naked sea butterflies.
The most popular sea angel is Clione limacina, the common sea angel. Its body is mostly transparent and, like all pteropods, has two parapodia that look like wings which, together with its elongate and shell-less body makes it look like an angel indeed. Despite its angelic appearance, the common sea angel is a fearsome creature.
Being a predator, the common sea angel is specialized in eating the common sea butterfly. Both species share the same environment in arctic waters and their association is known for centuries. The way that the common sea angel captures and eats the common sea butterfly is impressive and kind of scary.
When the sea angel detects a sea butterfly nearby, it starts a pursue and everts six adhesive buccal cones from its mouth, forming a basket-like structure. This structure is used to capture the sea butterfly and, once the poor shelled snail is traped, the sea angel rotates the sea butterfly’s shell until its opening is directed to the predator’s mouth.
After that, the terror begins. The poor sea butterfly has withdrawn into its shell by this time, but the sea angel uses a group of chitinous hooks in its mouth to perforate the sea butterfly’s body and, helped by the radula, pulls the whole body of the prey from inside the shell, swallowing it entirely at once. It is likely a horrible death for the poor sea butterfly. After finishing swallowing one sea butterfly, the common sea angel can go after another one in about 2 minutes.
While the life cycle of the common sea butterfly is short, lasting only a year, that of the common sea angel is much longer. As a result, there are no adult sea butterflies to serve as food for the common sea angel from later autumn to early spring. For a long time it was thought that the common sea angel would spend this whole time without eating, and indeed it was found that it can survive long periods in starvation. However, analyses of the stomach content of the common sea angel revealed the presence of amphipods and eventually calanoid copepods, suggesting that it can rely on some alternative food sources in cases of extreme necessity. Their main food, however, is the common sea butterfly with no doubt. They start to feed on them when they are still larvae, always capturing and ingesting sea butterflies that have a size similar to theirs.
Will the common sea angel be able to survive on these other prey types if the populations of the common sea butterfly decline due to climate change? If find it unlikely and I hope we don’t need reach a point in which this becomes an option.
– – –
– – –
Böer M, Graeve M, Kattner G (2006) Exceptional long-term starvation ability and sites of lipid storage of the Arctic pteropod Clione limacina. Polar Biology 30:571–580. doi: 10.1007/s00300-006-0214-6
Conover RJ, Lalli CM (1972) Feeding and growth in Clione limacina (Phipps), a pteropod mollusc. Journal of Experimental Marine Biology and Ecology 9(3):279–302. doi: 10.1016/0022-0981(72)90038-X
Kallevik IHF (2013) Alternative prey choice in the pteropod Clione limacina (Gastropoda) studied by DNA-based methods. Master thesis in Biology, The Arctic University of Norway.
Lalli CM (1970) Structure and function of the buccal apparatus of Clione limacina (Phipps) with a review of feeding in gymnosomatous pteropods. Journal of Experimental Marine Biology and Ecology 4(2):101–118. doi: 10.1016/0022-0981(70)90018-3
– – –
* This work is licensed under a Creative Commons Attribution 4.0 International License.