October 24, 1999
From: Erik Schloegl
I just recently spent a week diving along the south coast of New South Wales (by the way, Tathra Wharf is a great site for ophistobranchs - on just one night dive I saw Tambja verconis, Berthellina citrina, Hypselodoris bennetti, Polycera capensis, Ceratosoma amoena and at least three species I'm still trying to identify - haven't scanned them yet). It was cold, but great! However, I had a rather traumatic experience at Burrewarra Point (near Bateman's Bay). I came across this beautiful nudibranch - but my camera was set up for wide angle! The trauma became worse when I tried to look it up in a book. Its distinguishing features were: approx. 6cm long, translucent white body with orange ridges, black gills and rhinophores. It looked like a Halgerda to me - the animal identified as Halgerda graphica in N. Coleman's book comes closest. However, it seems to me all species of Halgerda have tropical ranges. Is that true? One more info: the substrate was rocky reef, depth 26m, water temperature 15 C.
I just wrote a long involved answer to your question only to have my computer crash and lose it all. So here is a shortened, and probably much clearer, version.
It looks like you may have seen something interesting. Species of Halgerda are essentially tropical. One species, Halgerda willeyi is sometimes found as far south as Coff's Harbour in northern New South Wales but I know of no records further south. However if the currents are right, tropical larvae can get as far south as southern New South Wales, so it is possible that you did see a Halgerda. As long as suitable sponges are growing there as food, the water seems suitably warm for tropical animals to survive.
Halgerda graphica was described from South Australia. I will try and post a copy of the original picture next week, as I have not seen an animal matching the original illustration. The South Australian fauna is an interesting mixture of southern endemics and apparent tropical stragglers that, when conditions are right, are brought down the West Australian coast in the van Leeuwin current and then carried east in huge eddies of water that can reach as far as South Australia.
I'm afraid you'll just have to find another one.