Photo by Andrew Taylor
Photo by Guido Zsilavecz
This flatworm is the most commonly seen one on Cape Town's reefs. Often well hidden due to its colour and surface texture, it is the frilly margin which generally gives its presence away. The projections on its back not only serve as camouflage, however, they also increase the surface area through which to absorb oxygen.
It is easy to mistake this worm for a nudibranch: many nudibranchs also have projections on their back, and the worm also has rhinophore-look-alike tentacles at the front. In many ways it is thus similar to, for example, a gasflame nudibranch. The main way to tell it is a flatworm lies in its name: it is rather flatter than about any nudibranch.
The carpet flatworm usually moves about slowly, but can also cover larger distances by swimming vigorously. It does so by undulating the margins of its body. Generally it swims in a head-up manner, with a "wave" of the body margin travelling down, creating lift. The swimming manner is similar to that of a spanish dancer nudibranch, and very elegant, although not particularly efficient. While fish are interested in the worm, it is quickly spat out again if sucked in, much like nudibranchs. We have tried inducing the swimming a few times by lifting one from a reef, but it just curled up and sank - it thus does not seem to be a defensive action, but more likely a way of finding a new food source. The worm is a carnivore, and eats small crustaceans and molluscs.
Sequence of shots taken from a short video, showing the wave travelling down the sides of the body, creating lift and allowing the worm to swim. Images by Guido Zsilavecz
Two Oceans, A guide to the marine life of Southern Africa, GM Branch, CL Griffiths, ML Branch and LE Beckley, 1994.