Gigantonotum album from South China Sea

Dear Alexei,

If I have identified you animal correctly this is a very interesting find. As soon as I saw your photo I thought Gigantonotum album. And why on earth would I remember such a thing? .. well my wife says its because I am very strange - but in this case its because I have been preparing a few pleurobranch pages for the Forum and this is a problem genus, related to Pleurobranchaea, which still needs to be sorted out.

Gigantonotum album Lin Guangyu & Tchang Si, 1965 was named from a specimen collected south of Hainan Island in 220 m, which is not far from where you found yours. It is characterised by the broad mantle which covers the foot and the gill. Willan (1977) synonymised Gigantonotum with Pleurobranchella, a genus erected by Thiele (1925) for a species from the Nicobar Islands, P. nicobarica. However Marcus & Gosliner (1984) report that Pleurobranchella nicobarica lacks a central radular tooth while G. album has one, so the proposed synonymy remains tentative. The anatomy of Gigantonotum is therefore in need of further investigation so I suspect your specimen will be in demand, or if you plan to dissect it yourself, your findings will be of great interest.

• Guangyu, L. & Tchang Si. (1965) Etude sur les mollusques Pleurobranchidae de la cote de Chine. Oceanologia et Limnologia Sinica, 7(3): 265-277
• Marcus, E. and Gosliner, T. M. (1984) Review of the family Pleurobranchaeidae (Mollusca, Opisthobranchia). Annals of the South African Museum  93: 1-52,Figs1-25.
• Thiele, J. (1925) Gastropoda der Deutschen Tiefsee Expedition. II Teil. Wissenschaftliche ergebnisse der Deutschen Tiefsee-expedition auf dem Dampfer “Valdivia” 1898-1899, 17(2): 38-382, pls. 13-46.
• Willan, R. C. (1987) Phylogenetic systematics of the Notaspidea (Opisthobranchia) with reappraisal of families and genera. American Malacological Union Bulletin, 5: 215-241.

Best wishes,
Bill Rudman