DIVE LOG AUSTRALASIA ISSUE 413 AUGUST 25

STAYIN ALIVE ...Continued THE DEFENCES OF SEA SLUGS - Part 3 Passive Defences Continuing the discussion on Appearance Form and Colour

background food source. Colour though, is not always everything. Sometimes the form presented by the mimic is so good that the colour hardly matters. At the other extreme, some are so plain as to be almost transparent and therein is the key to their particular form of camouflage - the substrate shows right through their body making them almost invisible against that background. Enhancing appearance through Behaviour Naturally, behaviour too plays an important role in making the camouflage process function most efficiently whereby, for example, the sea slugs might wrap themselves over and around the contours of their food or hold aloft and even move their cerata to resemble the branches of their soft coral or hydroid food through which they are moving and preying upon. Another aspect that works in favour of the sea slugs,

Part of the colour matching process can involve the retention, in the skin, by the sea slug, of ingested pigments obtained from their host, a process referred to as dietary camouflage . In some instances these pigments have been shown to also be noxious in nature. However the colour may not only be derived from pigments. Many of the sacoglossan sea slugs store chlorophyll plastids in their parapodia or cerata enabling them to closely match the colour of their host. Some of the aeolids too, adopt the colour of their food source. Some examples include the egg-eating Favorinus species that take on the colour of the ingested eggs and the coral eating Phyllodesmium species that store zooxanthellae from their diet within their cerata thus matching the

Above: Hexabranchus lacer, one species in a recently identified species complex within Hexabranchus. Left: Although bright red under the strobe lighting, in natural light it is usually quite drab and not contrasted against the background substrate. Notice the rolled-up margins of the mantle, that dorsally are the same in colour and pattern as the main notum.

Right: When this species is disturbed it unrolls the mantle margins revealing a bright red and white pattern to warn potential predators of its toxic nature. This of course is a reactive defence but is included here to demonstrate that species’ dual nature of appearance having both a passive drab “camouflage” colouration and also aposematic warning colouration as the situation dictates. Later, in the reactive defences section of this series, we will see it can take this even a step further.

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DIVE LOG Australasia #413 - August ‘25

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