In this minute of visual meditation, we would like to take a minute to relax and focus on nudibranchs in general instead of going into the specifics of the two species filmed in the waters of Bali (Indonesia). Divers lovingly call them nudis, short for nudibranchs, which makes them even cuter and brings out their true nature: beautiful, colourful, and exotic on the one hand, mysterious, bizarre and toxic on the other.
Take your minute to relax with nudibranchs
What to see in nudis?
Different families of nudibranchs (further split into genus and species) form the order Nudibranchia within the large taxonomic class Gastropoda, commonly known as snails and slugs. While all nudibranchs are sea slugs, not all sea slugs are nudibranchs. The name nudibranch originates from the Latin „nudus“, meaning „naked“, and the Ancient Greek βράγχια (bránkhia) for „gills“, referring to the gill-like appendages which protrude from the backs of many nudibranchs.
Although they possess eyes, their eyesight is thought to be limited to picking up light and dark shapes only. They view the world through chemical receptors in the shape of tentacles on their heads. These tentacles are called rhinophores and they allow nudibranchs to smell food, find potential mates, and predators and provide them with some sort of situational awareness.
Nudibranchs can thrive nearly everywhere, from shallow, temperate, and tropic reefs to Antarctica and even hydrothermal vents. At present, there are well over 3,000 species of nudibranchs known to science, but new species are still found. Discoveries of large numbers of bioactive compounds suggest that sea slugs are an excellent biomedicine source which has fueled the research into this order within the animal kingdom.
They are what they eat
However strange it may seem, these colourful families of sea slugs are carnivores, whose prey consists of sponges, coral, anemones, hydroids, barnacles, fish eggs, sea slugs, and other nudibranchs. To eat their food, most nudibranchs possess a radula, which is a toothed structure that they use to “chew” their food up. Some species suck out their prey after predigesting their tissue with selected enzymes, rather like a spider. Nudibranchs are very picky about what they eat, individual species or families of nudibranchs may eat only one kind of prey. Nudibranchs get their vivid colours from the food they eat, which in turn advertises to would-be predators, that they are poisonous, or at the very least foul-tasting. In any case, enough to be left alone by most.
The characteristic of aeolid nudibranchs, like the ones in this clip, are long, narrow bodies with numerous horn-like extensions which are called cerata and serve as gills. The form of the cerata extends the surface for respiration significantly and is also used for defence. Various species feed on hydroids and their stinging cells (nematocysts) pass through the digestive system of some aeolids and are built into the tips of their cerata (watch “Take a Minute II” for more details).
Nudibranchs have a shell in their larval stage, but it disappears in the process of becoming an adult. They come in all shapes and sizes, not to mention wild colour pattern variations, which makes them so popular with divers and snorkellers. Some are very hard to tell apart, others strikingly different from anything you have ever seen before. While some stand out, others are highly camouflaged.
Specialities of different kinds of nudibranchs
They also vary in sizes from massive beasts such as the“Moon-headed sidegill slug” (Euselenops luniceps) presented in „Take a Minute XIX“ to tiny speaks of some millimetres, like Costasiella kuroshimae a.k.a. “Shaun the Sheep” (shown in „Take a Minute VI“). This tiny creature has the ability to extract the chloroplasts from the food it eats and stores them in its cerata. This process is called kleptoplasty, and it enables “Shaun” to harvest/feed the energy that is released by the photosynthesis of these accumulated chloroplasts.
This is also the second feeding strategy of Melibe engeli („Take a Minute XXVI“). Through photosynthesis, the algae farms in its tissues produce nutrients for the Melibe in situations when food is scarce. The mutualistic symbiosis between different species of nudibranchs and unicellular photosynthetic dinoflagellates of the genus Symbiodinium (often known as ‘zooxanthellae’) has been known to science for quite some time. Most “solar-powered” nudibranch species take up Symbiodinium from their prey of soft or hard corals and cultivate them inside the cells of their digestive glands. But since Melibe engeli feeds exclusively on small crustaceans, science is still baffled as to how this nudibranch picks up the symbiont zooxanthellae for its emergency solar farms.
The search is on – not only for divers and snorkelers.