Colpodaspis thompsoni mating

Dear Ayumi,

Thanks for this interesting observation which is a valuable addition to your earlier message [#18191] on their egg-laying. The white tube is the penis of the posterior animal. Like all sea slugs, Colpodaspis is hermaphrodite, with a full set of male and female organs. In most of the more primitive shelled opisthobranchs the penis is on the right side of the head, alongside the mouth, and when not in use it is hidden in a pocket in the body cavity. Our only knowledge of its reproductive system is Greg Brown's (1979) detailed description of a preserved specimen. Your photo is the first evidence we have of just how far the penis can extend in this species.

In your photos we can see the penis disappearing into the flap-like exhalent siphon of the anterior animal. The exhalent siphon, leads directly to the enclosed mantle cavity, and the the female genital opening.  Although Colpodaspis is hermaphrodite, the arrangement of its reproductive organs means that it is probably too difficult for it to act simultaneously as both male and female during mating. This is quite different from the case in most nudibranchs, which have the male and female organs very close together. Having the two organs close together means that two partners can easily act as male and female simultaneously during mating.

• Brown, G.H. (1979) An investigation of the anatomy of Colpodaspis pusilla (Mollusca: Opisthobranchia) and a description of a new species of Colpodaspis from Tanzanian coastal waters. Journal of Zoology, London, 187: 201-221.

Best wishes,
Bill Rudman