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Nudibranch – Gambling behavior


If Darwin were to study these bewildering creatures, I am pretty sure he  would have said nudibranch took an ‘evolutionary gamble’ with life.

So named because of their ‘naked gills’, which can be seen perched on the anterior of the body, resembling a branching flower. Although nudibranch appear seemingly unprotected, these charming ‘slugs’ are not to be underestimated and are in fact highly evolved. Despite their lack of shell, they have adopted some interesting defence mechanisms.

Nudibranch diet differs between species, ranging from sea squirts, sponges to even one another, which they can identify using sensory organs called ‘Rhinophores’. The search for food is highly preoccupying, so once the prey is found, Nudibranch like to remain undetected. They can achieve this by camouflage, whereby they can imitate not only the form and colour but also by imitating the poisonous parts of the host.

Nudibranch gills
Photo by Marine Owen

Mating however is probably even more so compelling. Nudibranch are not only simulated by seasonal factors, but also by an elaborate courtship of touching and caressing, for example the Spanish Dancer. After ample stimulation two Nudibranch’s will align themselves head to tail, with their right hand sides touching, where their genitals lie. Even after the mating is completed, fertilisation is not always immediate, waiting until the environmental conditions are ideal. After fertilisation the pair will estrange one another, whilst either pair searches for a suitable substrate to lay their eggs upon.

All Nudibranch are hermaphrodites, which means they possess both male and female gonads and thus both partners can store the fertilised eggs until released. When laying their spawn, they will choose a location characteristic to each species, this is often dependent upon a camouflage factor. The eggs are laid in long strings/ribbons attached together by a layer of mucous called ‘albumen’ which offers a method of linkage and protection. Upon hatching, the eggs will develop onto a phase known as ‘planktonic veligers’ whereby the larvae begins to swim and feed, until a suitable substrate is located.

Nudibranch can be found worldwide across our oceans, in particular the Indo-pacific is home to a wide range of species. However they can be prone to osmotic stress, so cannot be found in areas such as estuaries or intertidal pools. Larvae possess a small shell during their juvenile stage, which can be exhausted with ocean acidification. Thus Nudibranchia presence can be a good indication of a healthy ecosystem. 

Rebecca Kierman

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