Nudibranchs are some of the hardest things to find on a dive, you have to move slowly and look very closely, but if you know what you are looking for and where to look you will find them. They range in size from 20mm – 600mm. The largest nudibranch ever recorded was a 52 cm Spanish Dancer, found in the Red Sea.
Nudibranch are sometimes called ‘sea slugs’, now while ALL nudibranchs are in fact ‘sea slugs’, not all sea slugs are nudibranchs.
Nudibranchs are related to snails; these colourful little creatures have ditched the homes they carry on their backs as they have evolved. Similar to land snails, nudibranchs also leave a slime trail, full of chemicals, which can warn other slugs of danger, or be used to find a mate.
Their name means ‘naked gills’ and describes the feathery gills and horns that most wear on their backs.
They are carnivores that slowly ply their range grazing on algae, sponges, anemones, corals, hydroids, barnacles, fish eggs and even other nudibranchs. But they are picky eaters. Individual species or families of nudibranchs may only eat one kind of prey
To identify prey, they have two highly sensitive tentacles, called rhinophores, located on top of their heads. To eat they use a radula, a toothed structure that they use to scrape their prey from the rocks they cling to; some suck out the prey after digesting its tissue with selected enzymes, rather like a wasp.
Nudibranchs derive their colouring from the food they eat, which helps in camouflage, and some even retain the foul-tasting poisons of their prey and secrete them as a defence against predators.
Their most common activity is eating and they dedicate most of their time to it, with approximately 3 – 5 hours a day inactive
As hermaphrodites, nudibranchs have both male and female organs. To mate, two nudibranchs come together side-by-side and exchange sperm sacs through a tube, a process that can last from seconds to a whole day depending on the species. Either way, it is not a romantic affair - they will separate and go their own ways before laying 1 to 6 egg masses in anti-clockwise spirals, some containing up to two million at a time. As most nudibranchs do not live longer than a year, this number of eggs is essential.
Nudibranchs have a few predators and are at risk only from other nudibranchs, turtles, craps and us humans.
Lacking any physical defence mechanisms and being soft-bodied organisms, nudibranchs have become prime candidates for their uses in marine pharmacology and biotechnology (the application of biological knowledge and techniques to develop products or otherwise benefit humans.) Already it is estimated that 24% of marine natural products come from nudibranchs and their relatives, the sea hare, with particular focus at the moment for their uses in anti-cancer drugs. As well as being collected for Aquariums and to eat
In addition to us humans being a threat, many nudibranch species are becoming rare and more difficult to be found due to water pollution, degradation or loss of suitable marine habitats and biodiversity decline. If a nudibranchs only primarily feeds on one thing like tunicates (marine invertebrate animals) and the coral reefs where these live die or are devastated by man then the nudibranchs will also die out.
Our job as divers to protect and to educate, so take those pictures show everyone how pretty they are. There are so many more species to be discovered, let's not wipe their environments out before we find out they existed. Without them, the scenery changes and we can no longer benefit from their beauty and the products they help us produce.
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