One nudi looks like an open gas flame blazing underwater. Another one appears on the sea floor like a ghost with rabbit ears. One looks like strings of neon noodles clinging to the reef. Another looks like puffs of tiny white clouds. And one even looks like a little marine rhinoceros with orange-tipped horns. These little marine slugs are so utterly strange and out-of-this world…
– Paul Steyn, National Geographic author
Nudibranchs are super-gorgeous. There are thousands of different known species, in seas all over the world, and new species are still being discovered. Not many people have seen them in person because most of them are very small, are well camouflaged and you have to be a diver with sharp eyes to spot them. Fortunately for us landlubbers, the extraordinary variety of their colours and shapes fascinates scuba divers, especially the special breed of underwater macro-photographers. So, brilliant images are quite easily found on the Internet.
South African nudibranchs span a huge range, from tropical animals to those that prefer cooler or even cold water. Some can be found in shallow rock pools and others prefer the deep ocean. Some specimens are known from only a single Cape reef.
We still have a limited understanding of which nudibranchs live where, what they eat, and who eats them. Humans don’t spend that much time underwater and nudibranchs lead short but interesting lives.
Nudibranchs are marine snails without shells. The name literally means “naked gills”, referring to the fact that they are “naked” without the shells we associate with other gastropods (snails) and they possess very prominent breathing appendages that resemble gills.
Nudibranch larvae are not completely nude. They start life with a rudimentary shell. But maintaining a shell requires a lot of energy, and so they have evolved other defence mechanisms.
Nudibranchs are carnivores that graze on sponges, anemones, and corals. Some species will assimilate the toxins and stinging cells of their victims (known as “nematocysts”) and use them for their own defence. When they eat the flesh of an anemone, the nematocysts are not digested but are relocated to the slug’s own skin.
The autonomous little harpoons don’t realise where they are and continue to function as normal, stinging any predators that attack the sea slug. Through this remarkable adaptation, a soft unarmed creature makes itself dangerous prey.
The nudibranch’s spectacular forms and colours are additional defences. The bright colours warn potential predators that nudibranchs are unappetising and even dangerous to eat because of the toxins they either make themselves or assimilate from their food. They are also good at hiding among the sponges and soft corals on the reefs they inhabit.
Nudibranchs are hermaphrodites, meaning that each animal is both male, producing sperm, and female, producing eggs. The male body parts develop first so that the nudibranch can store sperm from another individual until its eggs are ready to be fertilised. Once this has been done, each lays a ribbon of eggs which is unique to the species. From these eggs, larval nudibranchs hatch and begin a new life cycle.
The beautiful colours of nudibranchs are wasted on other nudibranchs. They have very rudimentary eyes and can only discern light and dark. Not so for their sense of smell. They have antenna-like appendages on their heads called “rhinophores”. These are extremely sensitive chemoreceptors, meaning they can smell out their prey at a distance. These little stalk- or club-like structures constantly “taste” the water for traces of food. Rhinophores are unique to nudibranchs and their close cousins, the sea hares.
Nudibranchs can handle stinging corals and tentacled anemones, but some foes make for a more unequal match. Ocean acidification, as a result of increasing CO2 in the atmosphere, cripples nudibranch larvae. The juveniles have a small shell in their planktonic stage, which is weakened by acidification, making them much more vulnerable to predators. Warming water has also brought about ecosystem changes.
In areas of intensive coastal development, you have the impacts of climate change, and you have greater pollution, human disturbance and habitat destruction. When all these things happen at once, their effects compound. Nudibranchs, having been around for millions of years, have no defence against humans and their destructive habits. If nothing changes these beautiful, charming little marine snails could be lost before many of them have even been found.
About the Author
Whale Coast Conservation passionately lives by its slogan “Caring for your environment”.
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