Mass Congregation of Aplysia fasciata

February 23, 2001
From: Hans Rothauscher


In June 2000 we saw masses of seahares [Aplysia fasciata] on the 'Canal du Rhône à Sète' in southern France, drifting with the current towards the Étang de Thau. They swam actively, grazed from rocks and the bottom of our boat. Next day they were gone.

The Canal, an extension of the Little Rhône, runs as an inland waterway along the coast through large shallow saltlakes and marshes. It is connected with the lakes through many floodgates. It has quite a current and I don't believe that it is the normal habitat of the seahares. I rather think that they normally live in the lakes.

Any suggestions as to what caused the temporary gathering? We believe that the animals were on their way to a mass marriage, probably in the Étang de Thau. This happened 2 days before the full moon, and we know that many invertebrates have such fixed annual reproduction "appointments".

Hans Rothauscher
http://home.t-online.de/home/rothauscher/

Rothauscher@t-online.de

Rothauscher, H., 2001 (Feb 23) Mass Congregation of Aplysia fasciata. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/3856

Dear Hans,
Thanks for the message. Finding large 'swarms' of Sea Hares is indeed an interesting experience, and it has been observed throughout the world with different species. The general feeling is that these 'swarms' are not migrations. Have a look at the messages on the Aplysia brasiliana page, where there are many reports of 'swarms' in Florida. Also have a look at the Mass Mortality page for more information and examples. In brief, these swarms appear to be the result of synchronised life cycles. Since they all spawn about the same time, large numbers reach maturity about the same time. You will certainly find many of the swarm mating, often in chains of 3 or 4, but they haven't formed the swarm to mate, they are mating because they are all close together.

Concerning your suggestion of 'mass marriages'. Many marine invertebrates do have mass reproductive events which are often synchronised by the phases of the moon. However these usually occur in groups of invertebrates [corals, polychaete worms, sea urchins] which are either unable to move, or do not mate or copulate. To reproduce they simply release their eggs and sperm in to the sea. The sea is a very big place, so by synchronising their spawning, these animals greatly improve the chances od a sperm finding, and fertilising, an egg of the same species. Not quite as romantic as the idea of synchronised mass marriages, but it works.
Best wishes,
Bill Rudman

Rudman, W.B., 2001 (Feb 23). Comment on Mass Congregation of Aplysia fasciata by Hans Rothauscher. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/3856

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