Stylocheilus striatus swarming - Turks & Caicos Ids

July 29, 2005
From: Martin Banford

A friend of mine who runs a windsurfing school on the island phoned me last month to come check out something going on by his area of the beach. Some of the people swimming from the beach had found all these little things floating in the water and wondered what they were. They look very much like blue-ring sea hares to me, but I am just wondering if you have ever known them to congregate in this number. Usually when someone says there were millions, they are exaggerating, but if they had not managed to make up a million, they were not far off. There were large areas of dispersed ones, and then two long lines almost like battle fronts of these things, moving steadily eastward. My friend Lyn who is a dive instructor here is in the first pic to give some idea of the scale of the number of these things. I swam the length of this particular line shooting video. I had fins on so I was not hanging around, but the clip taken swimming from one end of the line to the other is 30 seconds long. One suggestion made was that they may have been newly hatched. They were very close in to the shore - some were even sloshing around in the shore break, but the main body of them were about 3 to 15m out from the edge of the water. I would be interested to hear your comments and suggestions as to what may have brought this about.

Locality: Grace Bay, Provo, Turks & Caicos Islands, Tropical West Atlantic. Depth: 0.5 - 3 m. Length: 3-4 cm. 17th June 2005. Sandy bottom. Photographer: Martin Banford

Thanks very much.
Martin Banford

caicosbeachbum@hotmail.com

Banford, M., 2005 (Jul 29) Stylocheilus striatus swarming - Turks & Caicos Ids . [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/14442

Dear Martin,
We have had a number of recent reports of this swarming behaviour in Stylocheilus striatus from both the Caribbean and Hawaii. If you have a look at my comments in Jim Chambers message [#14080] from the Cayman Ids, the links will lead you to further information on this spectacular occurrence.

As you will read, there is some debate about what is happening, but my feeling is that these swarms of sea hares, which are usually of adult animals - at the size you give, your animals are definitely adults - which for some reason are weakened, or have reached the end of their life span, and are dying. I suspected they haven't 'migrated' together, but have grown up together and are only noticeable at an adult size. Perhaps they have eaten out their food supply? It is true that they all seem to be marching in the same direction, but opisthobranchs usually follow the mucous trail of their own kind, so if they have all aggregated together it is quite natural they are following each other's trail, but it doesn't necessary follow that they are 'migrating' to a particular place. Your description of some of them sloshing around near the tide line adds weight to the idea they are weakened and near death.

It would be a great thing from someone to study, but many of these phenomenon are over so quickly, and occur so irregularly at any one place, that planning a research project on them would be very difficult

Best wishes,
Bill Rudman

Rudman, W.B., 2005 (Jul 29). Comment on Stylocheilus striatus swarming - Turks & Caicos Ids by Martin Banford. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/14442

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