DIVE LOG JUNE 2025 issue 412

Above: Left to Right: A difficult to perceive, undescribed species of Thordisa nudibranch upon its sponge prey. The nudibranch ringed for identification. The nudibranch, photographed off its food source, becomes readily apparent on a different background. Colour and texture can be powerful aids to concealment out in the open.

Disruptive Colouration and Patterns Of the sea slugs that aren’t brightly coloured or patterned many are drably coloured and yet others exhibit disruptive colouration - disruptive that is, against the background of their habitat. The colours, patterns, surface contours and overall shape break up their outline on the substrate making them less conspicuous. Removed from that background for whatever reason, they become incongruously obvious. Much misunderstanding was promulgated by early taxonomists of the relationship of colours and patterns of the sea slugs they described to their normal habitat as this was done without knowledge of their habitat. Underwater photographers are often tempted to relocate certain subjects to a different

background, to that which is normal, in order to obtain better definition and create a more “appealing” or defined image of the isolated subject. This can be misleading if trying to understand their true habitat situation, appearance against their natural habitat or food source. Background Imitation When you can’t hide under a rock because your food is growing out in the open and you spend the greater proportion of your life exposed upon that food source, it makes sense to be able to blend in and be inconspicuous to potential predators by impersonating the appearance of that food because it is not that food the predator is looking for. Many and varied are the textures, colours, patterns and notal appendages possessed by the sea slugs to achieve that.

Above: The nudibranch Miamira alleni - An excellent example of a sea slug whose presentation mimics a life form in their habitat that is not related to their food source. The large nodular appendages resemble the branches of a xeniid octocoral, common in their habitat, an appearance that probably goes a long way towards explaining why such a large, not uncommon, nudibranch went undiscovered for so long. It is a sponge feeder in the Chromodorididae family.

Above: What looks like a partially eaten sponge colony is actually a nudibranch. This undescribed species of Atagema has contours, texture and colouration in the form of a sponge colony. There are only a few large predators of sponges due to their toxic and spicule-laden nature, certain nudibranchs being some of the few to have adapted to the diet and also turned it to their protective advantage, as will be explained later.

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DIVE LOG Australasia #412 - June‘25

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