DIVE LOG AUSTRALASIA ISSUE 413 AUGUST 25

Aposematism is the opposite approach to camouflage and has some sea slugs exhibiting bright and conspicuous colours and patterns that actually attract attention. Aposematic colouration is intended as a clear warning to predators that the sea slug is noxious or distasteful and is to be avoided. This is of benefit to both prey and predator, as both will avoid misfortune. At first glance camouflage and aposematism seem opposing concepts, nevertheless there are sea slugs that employ both tactics, being normally reasonably camouflaged but turning on the warning signals when directly threatened. Hexabranchus adopts this dual strategy, as explained under deimatic behaviour in the reactive defence section. Right: Where it has laid its spawn on the bryozoans, the geometric shape of the spawn, the tight spiral, and its different colour is easily noticed by the human eye. Then it is a matter to closely inspect the bryozoan colony for the nudibranch. Here two spawn spirals are arrowed in white and two nudibranchs are arrowed in yellow. It must be admitted though that the more the bryozoan zooids are removed the more visible the animal becomes.

apart from those that are dedicated hunters and necessarily swift movers, is the fact that sea slugs mostly don’t move around too much. An almost static lifestyle, the lack of discernible movement while feeding, surely aids detection avoidance. Spawn Tip-off The one thing that often betrays their presence, to us as sea slug searchers anyway, is the spawn they lay on their food, although it is doubtful this aspect denounces them to a predator. The usually different coloured spawn arranged geometrically as a spiral is a most obvious cue to the human eye. Above: Idaliadoris maugeansis is a small semi-transparent nudibranch that feeds on encrusting colonial bryozoans. Left: This close up makes it a little easier to see the nudibranch on its host but from a little further away it can be difficult to detect, that is, to visually separate it from its host if you don’t know what to look for. The semi-transparency and its markings blur the lines.

Above: The discodorid nudibranch Jorunna hervei photographed upon its sponge food. Not only does the nudibranch emulate the texture, patterning and colour of the sponge quite closely, but its behaviour of flattening itself out and contouring its body to the shape of the sponge ensures that it is very difficult to detect. Often though, its spiral spawn laid nearby will be the only give-away - to the human eye anyway. The rhinophores are just discernible on the animal’s left and the gill to the right.

Above: The coral eating Tenellia viei exhibits behaviour that reduces its chances of detection. This aeolid nudibranch moves slowly over the hard coral consuming the polyps. It lays its crowded cerata out horizontally upon the coral’s surface thus mimicking to some extent the coral’s surface texture. Zooxanthellae obtained from its coral host diet are transported to its cerata where, as well as supplying food as by-products, their colour assists further in helping it to blend in.

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DIVE LOG Australasia #413 -August ‘25

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