411 Dive Log Australasia April 2025

Helpless? No true nudibranch possesses a shell as an adult. The shell is lost when as a veliger larva it settles out of the plankton and metamorphoses into the “naked” adult form. Similarly, those with direct development crawl out of the egg capsule leaving the shell behind. The two major types of true, shell-less and seemingly defenceless, nudibranchs, Left: A typical Dorid nudibranch. Centre & Right: Cladobranchs - an Arminid and an Aeolid as examples.

No change happens in isolation. As already mentioned, there are many selection pressures on a species and the combination of all of these has resulted in the species we see today – not just what species but also how they appear, how they behave and how they survive. The various defensive methods to be described in this series are divided into two broad groups – passive and reactive. PASSIVE DEFENCES Passive defences are those that are not specifically designed to react when the sea slug is threatened or attacked, examples being lifestyle to avoid predators; structural, such as, protective appendages around the gills that absorb damage thereby diminishing impact on the vital gills themselves; Spicules and toughened flesh; repugnant chemicals in the flesh that render it unpalatable; Colours and patterns that warn of toxic bodies; crypsis or camouflage, that serve to reduce the chance of the sea slug even being encountered by any predator.

REACTIVE DEFENCES Reactive defences are on the other hand those that respond directly to the threat or attack including, retraction of vital body parts; release of acid from glands in the mantle; clouding the water with noxious ink; autotomy whereby specific portions of the body are cast off as a sacrifice distraction; avoidance behaviour such as swimming; deimatic behaviour where the sea slug, for example, unfurls its mantle to reveal startling colours and patterns; discharges, chemical and nematocyst. However, no defensive method can be considered absolute i.e. preventing predation completely. They serve but to mitigate the frequency or severity of predation on individuals or the species as a whole. The specialist predator will always have developed means and methods to overcome or circumvent a specific type of defence. - References supporting this article will be appended to a related article to be placed on NudibranchDomain.org in the near future. We will examine all of these methods in future issues.

DIVE LOG Australasia #11 - April‘25 22

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