Re: The foot of Thuridilla bayeri

May 14, 2010
From: John Fraser

Concerning message #21136:

Has anyone done DNA or recent radular tooth studies on T. gracilis, T. bayeri and other similar slugs? Here in Laulau Bay, Saipan, I find both what were/are called T. gracilis and T. bayeri. I have heard there are "expert" studies that have grouped them together but I cannot find the studies. Can you provide me with any leads? here are a couple examples from Laulau Bay. They are quite common there.

Locality: Laulau Bay, 30-40 feet, Saipan, Northern Mariana Islands, Pacific, 11 Jan 2010 and 25 Jan 2010, sand and rubble. Length: 10-20 mm. Photographer: John Fraser.

John Fraser

Fraser, J., 2010 (May 14) Re: The foot of Thuridilla bayeri. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear John,

I discuss the T. gracilis - T. bayeri question on the T. gracilis Fact Sheet, where you will also find a reference to Gosliner's major review of the genus. Gosliner doesn't think T. gracilis is identifiable from its original description. However he considers an animal which has been named T. ratna (Marcus 1965), and has the same colour pattern as what I am calling T. gracilis, to be a colour form of the blue-spotted T. bayeri. So we both consider th blue-spotted and unspotted forms to be the same species. The difference is that I consider T. gracilis to be very recognisable from its original description, and as it is a much older name than T. bayeri I call the species by its oldest name - T. gracilis.

Have a look at the earlier messages attached to the T. gracilis Fact Sheet as they show the range in colour variation in the species and include an on-going discussion on the pros and cons of one or more species. For example, Clay Carlson tends towards two species. In many opisthobranchs there are taxonomic puzzles like this which will require more study before we have a definitive answer. If you have large populations of both then perhaps you could look for biological differences. Can you see consistent colour differences? Do they interbreed? Are their intermediate colour forms? Are their egg masses different? What do they eat? etc etc.

Best wishes,
Bill Rudman

Rudman, W.B., 2010 (May 14). Comment on Re: The foot of Thuridilla bayeri by John Fraser. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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