DIVE LOG JUNE 2025 issue 412

Above: A number of aeolid nudibranchs of the Phyllodesmium genus have developed intricate cerata that in structure and colouration mimic the branches and polyps of their soft octocoral prey. Here, on the Left , is Phyllodesmium rudmani with cerata that closely resemble, Right , the polyps of Xenia sp. soft coral, when in the closed position including the darker shading along the lines of closure. When amongst their host they can be most difficult to recognise, however, the rhinophores and the fact that the “polyps” that don’t open and close in the coral’s feeding method are the give-away to those in the know. Mutation and selection pressure in the habitat has caused many species to evolve an Removed from the background of their normal substrate their now gaudy or obvious

appearance whereby those textures, colours, patterns and appendages cause them to be camouflaged upon their background food source, that is, in a sense “mimicking” it, with some species being nigh impossible to discern thereon, unless one knows what to look for and where to look. Highly derived examples include some species of Phyllodesmium with cerata that have evolved to closely match not only the colour of tentacles of their host soft coral but also structurally their size, shape and texture.

appearance belies the effect they otherwise achieve. The pelagic nudibranchs of the Glaucus genus exhibit a particular form of camouflage called countershading . They float on the surface of the ocean exhibiting a dark upper colouration and a silvery underside making them less obvious against their background from both upward and downward perspectives. To be continued …

Above: The countershading of the pelagic aeolid nudibranch Glaucus atlanticus. The animal actually floats upside down so the view from the surface, looking down, is actually the ventral side of the animal - Left. This surface is mainly dark blue with some lighter colours to represent reflection. Right The view from beneath is the dorsal side -and is silvery in colour to blend with the reflective view looking up towards the water’s surface.

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DIVE LOG Australasia #412 - June‘25

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