Chelidonura amoena piggyback

July 12, 2007
From: Kristin Anderson

Dear Bill

Although we see Chelidonura amoena nudibranchs quite often on a wide variety of our sites - both singly and in groups, I have not seen them "riding" each other before. Just thought it was interesting.

Locality: Ningaloo Reef, 12m, Western Australia, Indian Ocean, 16 June 2007, limestone/coral reef. Length: 3 cm. Photographer: Kristin Anderson.

Kristin Anderson

Anderson, K.J., 2007 (Jul 12) Chelidonura amoena piggyback. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Kristin,
I can't say I have seen Chelidonura doing this either, but it does remind me of a photo I have on the Fact Sheet for Aplysia cf parvula of a small animal sitting on a larger one for some time with its penis inserted in the female opening of the larger animal. As you know all opisthobranchs are hermaphrodite with functional male and female reproductive systems. In quite a few cases the male system matures before the female system so younger animals can be fuctional males before their 'female half' matures. In the case of the Aplysia I am pretty sure this 'mating' could have been successful. So seeing your photo I wondered if this was a similar case. It's possible, as the female genital opening of the larger animal is just below the small animal's head, but I can't see any sign that its penis is extended.

On a more general point you talk of Chelidonura as a 'nudibranch'. I know nudibranch is sometimes used to mean seaslug but it has a precise scientific meaning and applies only to one group of sea slugs, namely the shell-less carnivorous group which includes the dorids, aeolids, arminids, tritoniids and their relatives. A general name for seaslugs is opisthobranch - and that includes the nudibranchs, pleurobranchs, cephalaspidean bubble shells, the aplysiid sea hares, and sacoglossans and other minor groups.

Best wishes,
Bill Rudman

Rudman, W.B., 2007 (Jul 12). Comment on Chelidonura amoena piggyback by Kristin Anderson. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


Chelidonura amoena

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