Here in Madagascar we are lucky enough to have a plethora of different organisms to see once we submerge ourselves into the big blue. From these many different organisms, my favourite things to find on a dive are NUDIBRANCHS! Nudibranchs, or sea slugs, are found in all of the world’s oceans, from the polar seas, temperate Europe, and tropical Madagascar. They are the most diverse group of Opisthobranchs (marine gastropods) ad comprise of around two thirds of known opisthobranchs of the Indo-Pacific. Around 2000 species of nudibranchs are known to live in the Indo-Pacific. This makes diving here in Madagascar a dream for any nudi lover!
Nudibranchs can differ greatly in their appearance, from beautiful, brightly coloured large individuals such as the Spanish dancer, to well camouflaged and small individuals. Nudibranchs are carnivorous and feed on other invertebrates such as sponges, soft coral, anemones, and hydroids. Many species are highly specialised and feed on a single organism, or specific part of organism, while others are generalists and will eat whatever they come across.
The term nudibranch means ‘naked lungs’. The aptly named organisms have external gills that encircle their anus which they use for respiration. In other families, the slugs can have finger like projections from their body called Cerata which have evolved to perform a number of functions, one of which is respiration. Nudibranchs also have two rhinophores on the front end of their bodies which are used to collect chemosensory information.
An interesting fact about some nudibranchs belonging to the Aeolid suborder is that they can feed on stinging marine invertebrates, and store the nematocysts (stinging cells) in their cerata, to use as a defence mechanism! Another awesome nudibranch is Elysia chlorotica. This sea slug has the ability to incorporate genes from the algae that it feeds on, and thus can photosynthesise! This allows the animal to rely on sunshine for nutrition if there’s ever a shortage of its food. The sea slug steals chloroplasts from the algal food; ad embeds these into their digestive cells. If only humans could evolve such a feat! These fun invertebrates aren’t just boring slugs! They’re amazing, highly specialised animals, that aren’t just a pretty sight to look at, but also fascinating to learn about.
On our most recent beach clean we came across a beached beautifully coloured and ornate Spanish dancer (the largest nudibranch), which happened to still be alive! So through a team effort we moved the nudibranch back into the water and let it be on its way. This was the first Spanish dancer sighting for many of our volunteers, sadly not in its natural habitat. But I believe everyone was pleased knowing we’d done our best to save the little dude!
Here in Madagascar we really are spoilt with nudibranchs. It’s almost like playing ‘Where’s Wally’ on each dive. During my first weeks here I could only spot one or two per dive; now I’ve learned what different species feed on and can thus spot more than 10 different species on each dive! YAY!
By Tali Nachoom – Madagascar Assistant Research Officer
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