Thuridilla virgata from Reunion Island

February 7, 2008
From: Hugues Flodrops

Concerning message #13238:

Dear Bill,
Happy New Year 2008 and thanks for your forum. I've noticed that there was few record of Thuridilla virgata. I confirm your impression that it's an Indian Ocean endemic species with this record. It's a second record from Reunion Island (First one was found by Emmanuel Eby at Saint-Leu). Reunion Island:

Locality: Grande ANse, 2 mètres, Reunion Island, Indian Ocean, 25 December 2007, Muddy. Length: 25 mm. Photographer: Hugues Flodrops.

Best regards.

Flodrops, H., 2008 (Feb 7) Thuridilla virgata from Reunion Island. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Hugues,

Thanks for another record of this species. You raise an interesting quandary when you say your find confirms this species is an Indian Ocean endemic. Logically you can't prove that it is absent from the rest of the world by showing it is present in one part. Which raises the question of how to you prove an animal is endemic to a certain area? How many times do you have to 'not find' it elsewhere before you can say it is really absent?

There is no easy answer to this. It is easier for land animals because most of them don't seem to travel far, or if they do it is in a fairly regular way so a few years regular sampling can usually give you a good idea of what is present and what is absent. In the marine environment though where animals are often distributed in water currents as larvae, we can find major variations in distributions over many years. And then again there is the problem of poor sampling in many parts of the world. As we have seen on the Forum, an animal described from Sri Lanka 150 years ago and never seen again can suddenly be found in Thailand, or a species apparently endemic to part of the east Atlantic will be found on the east coast of Nth America.

T. virgata seems as though it may be endemic to the western Indian Ocean but as soon as we say it definitely is it will turn up in Thailand or Western Australia or even in the Pacific.

Best wishes,
Bill Rudman

Rudman, W.B., 2008 (Feb 7). Comment on Thuridilla virgata from Reunion Island by Hugues Flodrops. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


Thuridilla virgata

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