Thuridilla virgata from Reunion Island

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