418 Dive Log Australasia JUNE 2026.pdf

Examples of deimatic or startle display: Left: The Spanish Dancer nudibranch Hexabranchus lacer unrolls it mantle margins to reveal startling bright contrasting colouration. This behaviour can be further escalated into a swimming response by dorso-ventral flexions. Right: Sakuraeolis nungunoides bristles its cerata as a startle response to some perceived antagonism. The sudden erection of the cerata with bright tips is both startling and increases its size suddenly. The action is not a bluff as the cerata tips contain cnidosacs with stinging cells obtained through their diet.

Startle response Direct contact or even the detection of a pressure wave causes some species to elicit a startle response. This is known as Deimatic Behaviour and involves a rapid startle display in an attempt to put the predator off further approach. Depending upon the anatomy of the sea slug this can take different forms. The Spanish Dancer nudibranchs of the Hexabranchus genus, on being aggravated, suddenly unfurl the lateral folds of the mantle to reveal a brilliant red and white colouration on the skirt, the display of which is enhanced by continuous rhythmic undulations. This display, as well as being startling and of warning colouration, is augmented with dorso-ventral contractions of the whole body producing a quite mesmerizing spectacle that doubles as a swimming avoidance response. Some Lobiger species belonging to the sap sucking Sacoglossa possess elongate parapodial extensions that may be suddenly unfurled and extended to reveal brightly colored patches and appearing to also double the animal’s size. Additionally, those extensions may be autotomized if sufficient distressed to distract a predator.

A number of aeolids will upon being disturbed straighten and bristle their cerata directing the tips towards the threat. The cerata of some have brightly colored tips such as the bright red or orange of Sakuraeolis nungunoides . The ceratal tips of many aeolids contain cnidosacs full of stinging cells. The operation of these will be discussed later. Next issue: More reaction defences

Lobiger viridis commences its deimatic display by splaying out laterally the tall parapodial appendages significantly increasing its size. This will be followed by unfurling the edges to reveal orange colouration.

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DIVE LOG Australasia #418 June ‘26

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