419 Dive Log Australasia AUGUST 2026

Although it does not possess the protective gill pockets of the cryptobranch dorids the phanerobranch dorids of the Hexabranchus genus can contract their six separately inserted gill branches significantly.

A number of reasons have been put forward for the development of bioluminescence, both offensive and defensive explanations, including luring or illuminating of prey, warning coloration, startling (deimatic behavior), sacrificial distraction, counter-illumination, as a “smoke screen” if particles are released, and even mate attraction. These nudibranchs however, emit the light not as a steady glow but in rapid flashes meant to produce a startle response and therefore a distraction or hesitancy in potential predators, that have triggered it by physical disturbance when encountering the nudibranch. In Plocamopherus the aforementioned structures that produce the most intense bioluminescence emissions are normally situated around the delicate and vulnerable gill that, in these phanerobranch nudibranchs, cannot be withdrawn into a protective pocket below the mantle surface. There is no doubt that in this situation it is employed in a defensive role and not as a prey lure or mating signal. This phenomenon might, in the case of these slow-moving nudibranchs, also be diverting the predator away from that highly valuable and vulnerable gill to another, perhaps even sacrificial, portion of the body. Specimens are

often seen with one or more of these globular knobs missing or in a stage of regeneration. The process of creating bioluminescence requires significant energy expenditure as does the process of regenerating sacrificial parts, so it can be reasonably assumed that it is providing an acceptable return by way of protection. Retraction response Often, if the whole animal is not equipped to react suddenly to avoid a threat it may be able to contract or retract vital appendages that, for more efficient functioning, are normally completely exposed to the environment. These include the gill and rhinophores that in the cryptobranch nudibranchs, for example, are able to retract completely and quickly for protection into pockets located under the mantle surface. Some species, notably the dendronotinids, although not possessing actual pockets for retraction are able to contract their rhinophores down somewhat into the raised sheaths that surround them. So too, some of the side-gilled sea slugs, are capable of contracting their rolled rhinophores back under the anterior edge of the mantle from which they usually project. ____________________

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DIVE LOG Australasia #419 August ‘26

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