The status of Pleurobranchus hirasei

July 25, 2002
From: Nishina Masayoshi

Dear Bill,
Do you think Pleurobranchus hirasei Baba, 1971 is synonym of Pleurobranchus peroni?
Best Regards,
Nishina Masayoshi

Masayoshi, N., 2002 (Jul 25) The status of Pleurobranchus hirasei. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Nishina,
I would consider this species to be a synonym of A. peroni. Baba notes that in 'general configuration and colouring of the mantle' P. hirasei is very similar to P. peroni and P. albiguttata. However he considers they differ in having the shell embedded in the anterior part of the mantle while in P. hirasei it is in the posterior part of the mantle.

I don't know of any study on the subject but from my experience of pleurobranchs with small internal shells, they can sometimes be absent, sometimes dissolve in preservative, and are somewhat variable in position. It is possible that if there is some dissolving of the shell after preservation then the shell 'cavity' in the skin becomes larger than the shell and allows some movement. There is also quite a bit of body contraction when these animals are preserved, so what is in front of a central point in a preserved animal may have been posterior when it was alive. If all other features are the same, I suspect we should be saying that the shell position in a species is variable rather than using the shell position as the sole character of a species.

• Baba, K. (1971) Pleurobranchus hirasei n.sp., Proposed for a mollusc formerly known as Oscanius testudinarius: Hirase, 1927, from Japan (Opisthobranchia: Notaspidea). Venus, The Japanese Journal of Malacology, 30(1): 23-28,Pl.3.

Best wishes,
Bill Rudman

Rudman, W.B., 2002 (Jul 25). Comment on The status of Pleurobranchus hirasei by Nishina Masayoshi. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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