411 Dive Log Australasia April 2025

Many non-nudibranch sea slugs still possess a shell that may be robust or light and fragile or it may be an external or internal remnant. Some examples Left: Bubble-shell headshield slugs have a well-developed shell, such as that of Hydatina physis . Centre: Sea Hares have an internal shell, sometimes partially visible through the mantle foramen, such as Aplysia concava . Right: Some sacoglossans have an external shell that in some families are well-developed. The example here is Lobiger viridis .

The naked soft bodies of most sea slugs certainly look like an inviting target for voracious predators. Most are slow moving, seemingly unprotected and many with colours and patterns that, incredibly, advertise their presence. Just what methods then, are employed to avoid becoming a meal? Shell Loss Although not all sea slugs have lost their shell most have as adults, or possess only a much reduced remnant and in losing the protective shell had to replace it with other forms of defence. And so they have. Some of these are quite sophisticated and are combined with particular behaviour patterns. This is not to suggest that the shell was lost and then they needed to develop those other forms. It would not have been an abandonment of the shell followed by an emergence of novel methods. The question should not be, why did they lose the shell but rather, how was it made possible that the shell could be reduced? It is reasonable to suppose that shell loss and development of alternative methods would have

unfolded as a trend towards shell reduction and a compensating trend towards reliance upon alternative methods including the enhancement of others already established, though rudimentary, as well the implementation of those novel. Changes in defensive methods facilitated the loss of the shell, i.e. they would have been a prerequisite, rather than just occurring due to the shell loss, although the reduction and eventual loss of the shell did diminish the anatomical restriction imposed by the shell therefore permitting radical body redesign, movement into different habitats and exploitation of other food sources. The two are inextricably entwined. The ability to exploit new food sources has led in many instances to the sea slugs, for example, utilising certain metabolites in those new diets for their own chemical defence purposes whether they are pigments for camouflage and warning colouration or directly as antifeedants in tissues and glands, against consumption by predators.

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DIVE LOG Australasia #11 - April‘25

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