Re: Ascobulla and Cylindrobulla - are the differences real?

November 10, 2006
From: Kathe R. Jensen

Concerning message #18254:

Dear Bill,

I don't think we really disagree on this. Paula Mikkelsen would like to include Cylindrobulla in the Sacoglossa, and I would like to see them just outside (sister group). The important character is not the shape of the radular teeth. Bosellia mimetica, which definitely feeds on Halimeda, has short and broad teeth, not too different from those of Cylindrobulla, which may feed on Halimeda. The important character is the formation of the ascus-muscle as part of the pharynx. In Cylindrobulla the odontophore is long and "free" inside the pharyngeal bulb (as in most cephalaspids). In Ascobulla and all other sacoglossans longitudinal muscles surround the descending limb of the radula and an ascus is formed at the posterior end. In Cylindrobulla the old, used teeth just pile up at the posterior end of the radula. The formation of the ascus-muscle and the fundtional separation of ascending and descending limbs are what made the piercing-sucking feeding mechanism possible, and thus are true "innovations", which allowed a radiation of species. For schematic drawings of pharynx musculature see Jensen 1993 and 1996.

  • Jensen, K.R. (1993). Sacoglossa (Mollusca: Opisthobranchia) - specialist herbivores and partial predators: integrating ecological, physiological and morphological data. [In:] The Marine Biology of the South China Sea. Proceedings of the First International Conference on the Marine Biology of Hong Kong and the South China Sea. Hong Kong, 28 Oct. - 3 Nov. 1990 (Ed. B. Morton). Hong Kong University Press, Hong Kong, pp. 437-458.
  • Jensen, K.R. 1996. The Diaphanidae as a possible sister group of the Sacoglossa (Gastropoda, Opisthobranchia). [In] Origin and evolutionary radiation of the Mollusca (J. Taylor, ed.). Oxford University Press, Oxford, pp. 231-247.

Best wishes,
Kathe

krjensen@snm.ku.dk

Jensen, K.R., 2006 (Nov 10) Re: Ascobulla and Cylindrobulla - are the differences real?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/18263

Dear Kathe,
Thanks for your quick response. I've added a few more references below if anyone wants to explore this more deeply. You are right, I should have said 'foregut anatomy' because obviously the teeth aren't independent of what makes them work. I guess what seems to have got lost in the phylogenetic discussion is that the Cylindrobulla - Ascobulla leap (I hesitate to say quantum leap) looks to me like an evolutionary moment, frozen in time.

In one of your interesting reviews of sacoglossan evolution you say: "The synapomorphies delimiting the Sacoglossa from the Cylindrobullidae are almost all connected to the anterior alimentary system ...... It is unlikely that these synapomorphies developed independendently .... It is apparently the acquisition of these 'key innovations' which have enabled the Sacoglossa to speciate profusely, whereas the Cylindrobullidae has remained a monogeneric group." (Jensen, 1997: 318)

I guess it is how you look at this. To me the Cylindrobullidae were not an evolutionary failure - it was their evolution of a new foregut which gave rise to the Sacoglossa. To say they are not sacoglossans reminds me of 19th century melodramas where the socially successful son is embarassed by his lower class parents so cuts off all links to them.

In the review I mention above you conclude that "the ancestral sacoglossan probably fed on calcified, filamentous ancestors of Halimeda" (Jensen, 1997: 329). This again fits with our knowledge of Cylindrobulla. I realise that to include Cylindrobulla within the Sacoglossa would cause problems in defining the clade. My belief is that that is a problem we should face. I realise that it is probably a problem for cladistic methodology as well - but surely cladistics is our tool, not our master.

  • Jensen, K.R. (1980) A review of saccoglossan diets with comparative notes on radular and buccal anatomy. Malacological Review, 13: 55-77.
  • Jensen, K.R. (1997) Evolution of the Sacoglossa (Mollusca, Opisthobranchia) and the Ecological associations with their food plants. Evolutionary Ecology, 11: 301-335.

Best wishes,
Bill Rudman

Rudman, W.B., 2006 (Nov 10). Comment on Re: Ascobulla and Cylindrobulla - are the differences real? by Kathe R. Jensen. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/18263

Factsheet

Ascobulla, Cylindrobulla spp.

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