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advantage of any current thus putting a greater distance between the predator and themselves. This “swimming” method of avoidance is only possible by having become unencumbered by the loss of the shell. This response is also elicited through direct contact by the predator. The swimming may be somewhat efficient as in the eel-like lateral contractions of Bornella anguilla for example, the vigorous parapodial flapping of some of the gastropterids, such as Sagaminopteron ornatum , and some of the sea hares or just plain thrashing around without necessarily any real progress being achieved, such as exhibited by Plocamopherus ceylonicus . This latter method however, although inefficient, does serve to lift them off the substrate making them a moving rather than a static target and also temporarily away from the perceived threat, perhaps into any current or surge present.

Others are known to just simply release their attachment to the substrate and drift away in the current or surge of the water column. As well as swimming, sea hares are known to change movement method and increase speed by “galloping” as a form of locomotion avoidance behaviour. This also causes breaks in the continuity of the slime trail put down by the sea hare and may facilitate confusion in a slime-following predator. Some Aplysiidae species of Notarchus are known to use a very primitive form of jet propulsion. By filling the mantle cavity with water and then expelling the water out through a gap in the parapodial join by muscular contraction of those parapodia, to lift themselves, albeit clumsily, off the substrate and somersaulting away.

The sea hare Aplysia concava changes from its more sedate crawling method of locomotion into galloping mode for a more rapid escape. This method also lays a discontinuous slime trail to confuse a following predator. Follow the sequence clockwise from upper left.

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DIVE LOG Australasia #418 June’26

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