Blue-Ring Sea Hares in Little Cayman

Deborah and I just returned from a great trip to Little Cayman. The new Aquatica D70 housing was flawless, and the SB800 strobe in the Subal housing was also great.

We had a superb divemaster (Steve) at Southern Cross Club who showed us critters that we had never seen before. On our own, however, we saw what may have been an extraordinary sight. On May 20, near the sunken Soto Trader, we saw several thousand Blue-Ring Sea Hares in what appeared to be a mass migration. I had never seen a single sea hare, much less several streams of thousands of them. Here are some photos that I took.

This one is in one of the large groups:

The next two are of some of the bigger ones that were about 4 cm in length:

We've asked a number of people, and no one, including the divemasters, knew anything about such a migration. While on Little Cayman, we ran into Judie Clee with R.E.E.F., and she forwarded the information to a friend, Wolfgang Sterrer, curator of the Bermuda Natural History Museum. Here is his response:

Judie – I wish I knew! Mass migrations across the reef are known of Panulirus argus, and organised homeward-bound journeys from the little slug Onchidella, as quoted in my ‘Bermuda’s Marine Life’ (p. 143). But I’ve never heard of such a thing in other slugs. They are attracted to the scent of their food plants (red algae, in many cases), so maybe this is where they were heading.

Anyone ever see anything like this or have any ideas?

Thanks for any information.

Jim

Hi Jim,

WOW!

I think that these guys look pretty young and I would guess that they are probably a cohort of brothers and sisters that fortuitously recruited from their pelagic larval stage in bulk. Normally the pelagic larval stage of marine life results in mass mortality - but maybe on this occassion the conditions were ideal and loads of brothers and sisters happened to survive!

On Grand Cayman Lettuce Leaf Seaslugs are sometimes super abundant (although not quite this much) in a small area. At Turtle Farm reef Giles and I have been there when you can count 50 or so in a square metre over an area at least of a couple of tennis courts. Not quite this number - but i have always put these slug episodes down to an exceptional recruitment as they tend to be small-ish ones too.

Very interesting. Thanks for posting.

Alex

p.s. I have always assumed that these guys have a pelagic larval stage, but I don;t know for sure. If they don't then a predator free hatching and youth could also explain the super-abundance.

A single sea-hare mating would produce lots of young. All this talk reminds me of this massive seahare I found while exploring the mangroves in Cayman that was over a foot long. You can see it dwarfs an upside-down jellyfish in the foreground! It could have easily taken my arm off

Alex

Good grief, that's a whopper! I'll have to start looking in those mangroves.

We were low on air, or we would have observed the apparent migration a bit more. There were at least three distinct streams of sea hares all moving approximately parallel away from the shore. Each main group was about 15 inches in diameter, with lots of "trailers" going along behind. The speed was pretty fast (for sea hares), about 12-16 inches per minute, and they definitely appeared to be on a mission. I would take a wild guess that the three streams included at least 500-1000 sea hares.

A mystery to be solved!