Nudibranches are minute, slug-like molluscs that are dotted over the floors of all the oceans, from Antarctica to the Florida Keys, and Mozambique to the Arctic. Officially, they are shell-less molluscs, part of the sea slug family and in the Gastropod class. There are over 3000 species and more being discovered daily – many as yet unidentified on our reefs! It is virtually impossible to find the same one twice. So, if you like small colourful things, that potentially no-one else in the world has ever seen, our Nudi dive sites are the place for you.
Nudibranch (or Nudibranchia) literally means “naked gills’, a fitting name as they breathe through feathery gills standing out on the back, a little like external lungs. There are two known types: the ‘Dorid’ and the ‘Aeolid’. The Aeolid (also spelt Eolid) are covered in Cerata (finger-like attachments), which are used for breathing, digestion and defence. In contrast, the Dorid only have gills (no Cerata) on their back and breathe through these.
Nudi’s are Bethnic animals, spending virtually all their time on the sea floor or reefs. Only a minority can swim freely. They have one ‘foot’, which is used to hold on to the bottom. Much like a snail, this foot leaves a slimy trail along the floor. Occasionally, they will release the foothold on the reef and suck air into the stomach (to float), allowing them to tumble in the current before resettling in a new location.
Fascinatingly, Nudi’s have few natural defences, but use poison from their prey for protection. They will identify any irritating or toxic compounds in their prey, and store it in their Cerata before it can be digested. The poison remains in the Cerata until required for hunting or defence, when it is secreted through skin cells or glands.
The prey Nudis’s consume is also responsible for their wide-array of colours; for example the Spanish Shawl (type of Nudi) eats a species of hydroid which possesses a pigment that gives this kind of Nudi its distinctive purple, orange and red colouration. Nudi’s are also carnivorous. They largely feed on algae/anemone/barnacles/corals/sponges and fish eggs, but have been known to be cannibalistic too. They are a particular predator of the Portuguese Man of War. Despite this wide array of prey, some Nudi’s are very fussy, and entire species or families will consume just one kind of prey! Like the Frogfish, Nudi’s use a Radula to eat.
Given the relative immobility of Nudi’s, they are not ideal animals for mating. As a result, they have evolved to be Hermaphrodites (so do not have to wait for a Nudi of the opposite sex to pass for mating), but cannot self-fertilize. They will deposit eggs within a gelatinous spiral. Eventually the eggs hatch into free-swimming larvae, which settle on the floor as adults (comparative to many underwater species, a simple reproductive cycle).
Nudibranches can survive at almost any depth, as well as any temperature. Most commonly though, they are found in shallow, tropical waters. Lifespan varies too, from one month to one year.