Cross-species breeding - Sea Hares

March 8, 2006
From: Jasmine Peters

I work at a nature center in south Florida. We occasionally get sea hares in our tanks when visitors bring them from the beach, wondering what they are. Right now we have about 6 breeding and egg laying Ragged Sea Hares (Bursatella leachii pleii) and one Black Atlantic Sea Hare (Aplysia morio ). This morning as I was checking the tanks, one of the Ragged Sea Hares was away from the breeding 'pile' for lack of a better word, and was enveloped by the Black Atlantic Sea Hare. The Ragged didn't seem to be trying to escape, but since the Black is about 3 times larger, it was completely covering the Ragged at times.
Any thoughts? I would love to know what to tell the visitors when they ask.

Locality: Salt Water Tank (2000 gallon, constantly flowing with sea water), N/A, Florida, USA, Atlantic Ocean, 06 March 06, Sandy bottom tank. Length: Black (25 cm), Ragged (10 cm)

Jasmine Peters

Peters, J. S., 2006 (Mar 8) Cross-species breeding - Sea Hares. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Jasmine,
You don't actually say the Aplysia was mating with the Bursatella unless you were using the term 'covering' in the euphemistic sense that horse breeders do. Opisthobranchs are no different from the rest of the animal kingdom in avoiding interspecies breeding as a rule. If you find two closely related species mating then it is worth considering that perhaps they aren't separate species. In this case, Aplysia and Bursatella  are quite distinct genera, so I am sure these two species are not close relatives. Strange things occur in tanks and crowded conditions. The most likely explanation is that the Aplysia just crawled over the Bursatella and decided it was a comfortable, or comforting, place to rest.

Unless you can see the Aplysia with its penis inserted in the Bursatella, I think this is just one of those things that happen. The animals often hide in crevices or aggregate together with their own kind. I guess in unnatural situations, aggregating with relatives is not surprising, but I wouldn't think it is necessarily a reproductive behvaiour
Best wishes,
Bill Rudman

Rudman, W.B., 2006 (Mar 8). Comment on Cross-species breeding - Sea Hares by Jasmine Peters. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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