Sea Hares - strange behaviour.

January 12, 2004
From: Krista Recsei

Hi! I was wondering if I could get some info on some interesting behaviour my mum (Jacquie Recsei) and I observed by some Sea Hares. (i think sea hares, could be sea slugs?) They were in the large rockpools on a beach north of Sydney, on the coast next to Lake Macquarie [New South Wales, Australia]. It actually would be more accurate to say they WEREN'T in the rockpools. They were biggish (5-10cm long), very round, dark brown, with no real colouration other than that. During low tide, just as the tide began to come in, I noticed that maybe about fifty of these sea slug/hares were moving out of the water into the blazing hot sun. If moved back into the water they promptly moved themselves out again. Why would they be doing this? I read, on this site, about mass death with some sea slugs but it sounded different to this, as they didn't seem to be dying? Any information on this would be good.
Krista Recsei

Recsei, K., 2004 (Jan 12) Sea Hares - strange behaviour. . [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Krista,
This doesn't sound like mass mortality to me. I'm afraid I can't give you an answer to why they were doing this but I can suggest a few possibilities.
The first possibility is that this has something to do with the heat. As you know we have been having many days over 30 deg C recently and this makes living intertidally a fairly risky business for animals which live there. At first sight, rockpools seem to be a good place to escape from the heat and the possibility of drying out, but unfortunately its not that simple. The water in rockpools heats up at low tide on a hot day. This means that there is less oxygen in the water and it also means that the sea weeds in the pool become quite stressed and can secrete chemicals into the water which are sometimes quite unhealthy for animal life. My first bet would be that the sea hares were finding life in the pools fairly uncomfortable and so were attempting to escape a slow cooking.

Another possibility is that they were responding to their normal biological rhythms or 'biologcial clock'. Many tidepool animals seem to become very active just before the the tide turns. Animals that have been hiding for 3 or 4 hours will suddenly appear and start crawling around. I know of no studies on why they do this but I suspect it is part of an activity rhythm they have which coincides with the return of fresh seawater as the tide returns.

People often make the mistake of thinking that we know everything about the environment. In fact as your strange sea hares show us, we still have a lot to learn.
Best wishes
Bill Rudman

Rudman, W.B., 2004 (Jan 12). Comment on Sea Hares - strange behaviour. by Krista Recsei. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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