418 Dive Log Australasia JUNE 2026.pdf

STAYIN ALIVE THE DEFENCES OF SEA SLUGS - Part 8 Reactive Defences

Reactive Defences Although nudibranchs and other sea slugs do not possess true eyes there are a number of sensory organs that enable them to detect predatory advances, some from at a distance and others closer to home. Upon detecting a perceived threat they are able to react in several ways that include some or many of the following depending on the species. These reactions/responses to perceived or real threats include: Avoidance response Startle response

Together with their passive defences the sea slugs can employ one or more of the above categories of response to enhance their chances of survival. Avoidance response Experiments have shown that, in the nudibranchs tested, those nudibranchs were able to detect the presence of specific predators “upstream” from their position and react by changing the direction of their travel. The rhinophores, or even the oral tentacles, in these instances have registered a chemical cue from the predator that the nudibranch has recognized and subsequently undertaken an avoidance response. Apart from just changing direction, some can burrow and a certain few are also able to “swim”, or at least release from or lift themselves above the substrate to take

Illumination response Retraction response Autotomy response Discharge response - Chemical discharge - Nematocyst discharge

Some examples of a swimming escape response: Left: Sagaminopteron ornatum takes flight up into the water column by flapping its parapodia. Right: Aplysia argus is one of the few sea hares with parapodia that are not substantially joined together along the midline enabling it to flap them in a synchronised manner for a controlled swimming action. DIVE LOG Australasia #418 June ‘26 19 www.divelog.net.au