In this Minute of Relaxation, we would like to introduce you to one of the most interesting species of Nudibranch, known as solar-powered Melibe (Melibe engeli). Nudibranchs are molluscs in the class Gastropoda, which includes snails, slugs, and sea hares. Many gastropods have a shell. Nudibranchs have a shell in their larval stage, but it disappears in the process of becoming an adult.
Melibe engeli is a nudibranch
Nudibranchs can thrive nearly everywhere, from shallow, temperate, and tropic reefs to Antarctica and even hydrothermal vents. At present, there are well over 3000 species of nudibranchs known to science, but new species are still being discovered. They come in all shapes and sizes, not to mention wild colour pattern variations, which makes them so popular with divers and snorkellers. The word nudibranch comes from the Latin word nudus (naked) and Greek branchiae (gills), which refers to the gill-like appendages which protrude from the backs of many nudibranchs.
Rhinophores: Chemical sensors to see the world
Although they possess eyes, their eyesight is thought to be limited to picking up only light and dark shapes. 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.
Radula: The „teeth“ of nudibranchs
However strange it may seem, this colourful family 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.
Solar-powered Melibe (Melibe engeli)
With so many vibrantly coloured and interesting family members, what makes the Melibe engeli stand out? It’s not its size, this creature grows up to around 5 cm. And this Melibe doesn’t have any bright colour patterns. But where its carnivorous cousins are going mostly after static prey, this particular species of Melibe is an active hunter, feeding on shrimps, crabs and other small crustacea which they catch by throwing the inflated oral hood over the substrate like a fisherman casting his net. This active “fishing” practice is a joy to observe. However, they lack a radula, which means that their swallowed prey remains alive in the gut until killed by digestive juices.
Full of energy: Solar-powered (Melibe engeli)
But what makes this creature even more special, is that it doesn’t rely only on catching its prey for sustenance, it also hosts algae farms in its tissues, that through photosynthesis 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.
One of the mysteries to science
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. What a beautiful world we live in, and how fortunate we are to be able to observe all these different adaptations to life on our planet!