Halgerda toliara? from Nosy Be Madagascar

Dear Ugo,
As you will see from looking at earlier messages concerning Halgerda toliara and Halgerda formosa I have had some difficulty in distinguishing some animals as one or the other. We know so little about variation in both species that new photos are always welcome. When I first saw your photo I thought that the extensive network of ridges on the mantle were very unusual for H. toliara, although the yellow median line was typical, and then I noticed what may turn out to be a good way to separate the two species. In your animal, the basal third of each gill is white while the upper two thirds is all black. This of course is not always obvious if the gills are not fully extended. if you look at photos of what we are calling H. formosa you will see that the gills are also black and white but the black is in the form of a black line along the inner edge of each gill, running right to the base of the gill and into the central circum-anal ridge. Gill colour is usually quite constant within a species so these quite obvious differences may be a useful way of separating animals which look a bit like both species

Your animal also raises the question again of whether H. toliara is a pale form of Halgerda wasinensis [see message #6223] ? Which is all very interesting to me but probably just a bit confusing to you. My advice would be to call it H. toliara, but remember that this is a work in progress - there is no final, definitive answer.

Best wishes,
Bill Rudman