Stylocheilus swarmimg in the Caribbean

June 20, 2005
From: Jim Chambers


Howdy Bill,
Thanks for your interest! Our message in Digitaldiver.net was in response to Mike Roberts' message concerning the mass swarming of Stylocheilus striatus in Hawaii [message #14074]. We observed a similar phenomenon recently in the Caribbean.

My wife/dive buddy Deborah and I saw the sea hares on Friday, May 20 (the full moon was on the 23rd, so it probably was not Lunar-related). We were on the north side of Little Cayman Island (about 90 miles ENE of Grand Cayman) at the site of an old shipwreck, the Soto Trader. The surrounding bottom is mostly patch corals and featureless sand. Depth was about 40 feet, and there was no current. Water temperature was 80 F (27 C). We noticed what at first appeared to be some clumps of algae moving along the bottom (a common sight), but on closer inspection, we saw that the "algae" was actually at least three separate groups of Blue-Ring Sea Hares [Stylocheilus striatus] (actually we didn't know what they were until we got back on shore and looked them up in our book, since we had never seen a sea hare before). Each group was about the size of a large dinner plate, and there were a lot of "trailers" behind each main group. The largest sea hares were about 4-5 cm (stretched out), and there were many smaller ones about 2-2.5 cm. My best guess is that each group had at least 200-300 sea hares, and they were moving perhaps one foot per minute (rough guess) away from shore. Unfortunately, we were running low on air, so we didn't have a chance to investigate further.

The next day, we met Judie Clee with R.E.E.F. She was staying on Little Cayman, and the divemaster had mentioned what we had seen to her, so she came by to talk with us and see our photos (Deborah also has a short video clip of the sea hares). After Judie returned home to Bermuda, she forwarded the information to at least one marine biologist. So far, no one seems to have any idea what the migration was about, so it appears to be a real mystery waiting to be solved. I have attached my photos. Please feel free to use any of this information and photos however you like. If it helps to solve the mystery, that would be great.

Deborah and I are already planning to return to Little Cayman the same time next May in the hope of seeing this again. It's a long shot, but we love Little Cayman and we'd return for a lot less reason than that!

Thanks again for your interest!
Jim Chambers

jimbo_limited@mindspring.com

Chambers, J., 2005 (Jun 20) Stylocheilus swarmimg in the Caribbean. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/14080

Dear Jim,
It's certainly interesting to have the same phenomenon reported from two oceans at much the same time. Have a look at my comments to Mike Robert's message [#14074] about what we know about it. I've added an inset on one of your photos to show the little blue rings which are sometimes found on this species.
Best wishes,
Bill Rudman

Rudman, W.B., 2005 (Jun 20). Comment on Stylocheilus swarmimg in the Caribbean by Jim Chambers. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/14080

Factsheet

Stylocheilus striatus

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