411 Dive Log Australasia April 2025

Above Left: The variation and elaboration of sea shells almost knows no bounds. Many have gone down the path of a heavier shell or thickened ridges (such as exhibited here with a Harp shell) or spines and radiating processes that prevent crushing or making getting a grip on the shell more difficult. Sea slugs, however, have taken different paths in defence methods. Above Right : The primitive Ringicula headshield sea slug has a well-developed shell that is inflated, thick and robust but also possesses extra weapons in its armoury - compound toxic defensive glands located in the posterior region of its headshield.

Perpetually Evolving Life is a continual battle of defence and offence between prey and predator, the one for protection to survive, the other to feed to survive. Both of these elements are perpetually evolving, simultaneously, by natural selection through the process of selection pressure, and it must be said, that this is not just limited to different species where one is the prey and the other the predator. Within the one species there are pressures acting upon it in its own dual

roles as both predator and prey. There are many of these pressures on a species population - some subtle, some manifest. Let’s ignore climate, food sources (although many of the defences employed by sea slugs are dependent upon their diet), habitat change and disease for the moment and look at predation. Predation has to be considered a manifest type of selection pressure. Those individuals in the population that get to pass on their genes have survived predation.

Above: Although the shelled molluscs have been a very successful group the external shell is not the ulti mate defence. Here we see the compromised defence of some shelled sea slugs that have been attacked and penetrated through the external shell by other molluscs using a “drilling” radula - Left, Pupa affinis and Centre, Tornatina sp. In the Right image the shell of this Roxaniella leucampyx has been cracked most probably by a crab or the strong jaws of a bottom-feeding fish, but has managed to effect an escape00

DIVE LOG Australasia #411 - April‘25 20

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