Nembrotha purpureolineata mating

May 4, 2005
From: Roberto Sozzani

[Note added 4 March 2008: This is the new species Nembrotha aurea - see message #21419]

Dear Bill,
I send these photos because they show the mating ability of this species, even if not side by side on the same level. I find it really amazing!

Locality: Rinja island. Indonesia. Depth: 18 metres. Length: 7 cm. June 2003. healthy reef. Photographer: Roberto Sozzani

Best regards
Roberto Sozzani

Sozzani, R., 2005 (May 4) Nembrotha purpureolineata mating. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Roberto,
Thanks for these interesting photos. We have a number of mating photos of this species on the Forum but yours shows a rather interesting feature. The large disc-shaped structure which extends out when mating, is not just the penis but is the whole of the genital vestibule or pouch [which usually forms an internal pocket in the body wall]. In your close-up, the actual penis can be seen as a small papilla [my arrows show direction of sperm transfer] which arises from the centre of the flattened disc. The female opening is off to the side of the disc. In many opisthobranchs only the penis extends out like this and the female genital opening is at the base of the penis. So this is quite a different arrangement. What I am not clear about is where the eggs are extruded. It is possible there is a third opening? The eggs are not layed during mating so perhaps the same genital aperture is used but it is hidden within the genital vestibule during egg laying.

Why this is particularly interesting is that most of out knowledge of the functioning reproductive system is derived from dissections of preserved animals. Because various ducts extend and contract in relation to other ducts during mating and egg laying it is often difficult to interpret how parts of the system work.

Best wishes,
Bill Rudman

Rudman, W.B., 2005 (May 4). Comment on Nembrotha purpureolineata mating by Roberto Sozzani. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


Nembrotha aurea

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