Ascobulla and Cylindrobulla - are the differences real?

November 9, 2006
From: Bill Rudman

Denis Riek's message [#17552] seemed an appropriate time to post some thoughts I have on the genera Cylindrobulla and Ascobulla.

Photo: Heron Is, Capricorn Group, Great Barrier Reef, Queensland, Australia, June 1983. Photographer: Bill Rudman

The genus Cylindrobulla was proposed (Fischer, 1857) for small opisthobranchs with a thinly calcified, bubble-shaped shell. When live specimens were later studied they were found to have a cephalaspid like head shield and over the years they have been considered primitive opisthobranchs, and linked to the cephalaspideans, Akera-like aplysiids, diaphanids and sacoglossans. The anatomy of a number of species has been studied and while many have a sacoglossan-like radula, two west Atlantic species, including the type species, C. beaui Fischer, 1857, have a rather different shaped radular more reminiscent of the diaphanids. This led to the erection of a new genus, Ascobulla Marcus, 1972, for those species with a sacoglossan-like tooth.

More recently, a number of cladistic analyses of the phylogenetic relationships of these animals have concluded that while Ascobulla is a good, if primitive, sacoglossan, Cylindrobulla is quite unrelated, perhaps a sister group to the sacoglossans, perhaps a diaphanid or even a cephalaspidean. I am very uncomfortable with this hypothesis. One unfortunate aspect of cladistic analysis is that often the need for characters, leads to the loss of the whole picture - a case of not being able to see the forest because the trees get in the way. I have pored over all the literature on these animals and the only difference is the shape of the radular morphology. In some analyses the structure of the radula teeth is broken into 5 or more characters, but when we get down to reality, what we are dealing with is a single evolutionary event. Not surprising to me is the fact that this difference in morphology is directly correlated with the known feeding biology. Both species of Cylindrobulla [with solid triangular denticulate teeth] feed on species of calcified Halimeda, while those with the dagger-shaped sacoglossan teeth [Ascobulla] feed on algae of the genus Caulerpa, which have a relatively soft easily piercible cell wall. It seems to me that the simple interpretation of the Ascobulla - Cylindrobulla controversy is that they are an extremely closely related group of primitive sacoglossans. One simple scenario is that the ones feeding on Caulerpa the group that evolved the classic sacoglossan dagger-shaped tooth after moving from Halimeda - which would make them are basal to all sacoglossans. If so the two on Halimeda may be a link to the Diaphanidae, which has often featured in discussions on the origin of the sacoglossans.

Whatever the scenario, it seems very strange to me that this group of animals with so much in common, can be split into two totally unrelated phylogenetic groups on the shape of the radula. Surely changes like this are how evolution works. We only have to look at the amazing radula changes that occur in the Turridae and other toxoglossan gastropods to realise that in some phylogenetic groups, differences in usually stable characters, like tooth morphology, should not automatically be considered evidence in favour of phylogenetic distance.

Whatever the 'truth' is, I can see no way to separate species of Ascobulla and Cylindrobulla from photographs, so unless photographs can be linked to anatomical studies of the actual specimens photographed, I will place them all on this page.

• Clark, K. B. and DeFreese, D. (1987) Population ecology of Caribbean ascoglossa (Mollusca: Opisthobranchia): a study of specialized algal herbivores. American Malacological Union Bulletin 5: 259-280.
• DeFreese, D. E. (1987) Ecology and burrowing behaviour of Ascobulla ulla (Opisthobranchia: Ascoglossa). The Veliger 30: 40-45.
• Fasulo, G., Gaglini, A., and Terreni, G. (1982) Nota su recenti ritrovamenti di un mollusco poco noto, Cylindrobulla fragilis (Jeffreys, 1856) (Gastropoda: Opisthobranchia) e considerazioni sulla sua collacazione sistematica. Notizario CISMA 3: 27-32.
• Hamatani, I. (1971) A new species of Cylindrobulla, sacoglossan opisthobranch, from California: with a comparison with C. japonica Hamatani, 1969. Publications of the Seto Marine Biological Laboratory 19: 111-117.
• Hamatani, I. (1969) A new species of the rare shelled sacoglossan genus Cylindrobulla, from Middle Japan (Opisthobranchiata). Publications of the Seto Marine Biological Laboratory 17: 171-174.
• Ichikawa, M. (1993) Saccoglossa (Opisthobranchia) from the Ryukyu Islands. Publications of the Seto Marine Biological Laboratory 36: 119-139.
• Jensen, K. R. (1983) Factors affecting feeding selectivity in herbivorous ascoglossa (Mollusca: Opisthobranchia). Journal of Experimental Marine Biology and Ecology 66: 135-148.
• Jensen, K. R. (1989) A new species of Cylindrobulla from Phuket, Thailand, with a discussion of the systematic affiliation of the genus. PhuketMarineBiologicalCenter, Research Bulletin 52: 1-11.
• Jensen, K.R. (1996)  Phylogenetic systematics and classification of the Sacoglossa (Mollusca, Gastropoda, Opisthobranchia). Philosophical Transactions of the Royal Society of London, B 351, 91-122.
• Jensen, K. R. and Wells, F. E. (1990) Sacoglossa (= Ascoglossa) (Mollusca, Opisthobranchia) from southern Western Australia . In: Wells, F.E., Walker, D.I., Kirkman, H. & Lethbridge, R. Proceedings of the Third International Marine Biological Workshop: The Marine Flora and Fauna of Albany, Western Australia. WesternAustralianMuseum, Perth, Vol. 1: 297-331.
• Lin, G. (1978) A new species of Cylindrobulla (Opisthobranchia) of the Xisha Islands, Guangdong Province, China. Oceanologia et Limnologia Sinica 9: 95-98.
• Marcus, E. and Marcus, E. (1956) On the tectibranch Gastropod Cylindrobulla. Dos Anais da Academia Brasiliera de Ciencas 28: 119-130.
• Marcus, E. and Marcus, E. (1970) Opisthobranchs from Curacao and faunistically related regions. Studies on the Fauna of Curacaoand other CaribbeanIslands 33: 1-129.
• Marcus, E. (1972) On some opisthobranchs from Florida. Bulletin of Marine Science 22: 284-308.
• Melvill, J. C. (1918) Descriptions of thirty-four species of marine mollusca from the Persian Gulf, Gulf of Oman, and Arabian Sea, collected by Mr. F. W. Townsend. Annals and Magazine of Natural History (9), 1: 137-158, 2 pls.
• Mikkelsen, P. M. (1998) Cylindrobulla and Ascobulla in the western Atlantic (Gastropoda, Opisthobranchia, Sacoglossa): Systematic review, description of a new species, and phylogenetic reanalysis. Zoologica Scripta 27: 49-71.

Rudman, W.B., 2006 (Nov 9) Ascobulla and Cylindrobulla - are the differences real?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/18254

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