In many temperate regions of the world estimating the number of sea slugs is reasonably easy because there has been a history of many people studying the marine biology of the area and the species that live there often have restricted distributions. For example the opisthobranch fauna of the north Atlantic coast of Europe from the English Channel north is well documented. When new species are discovered it is a major event. So estimating the total number of species found there (about 170) is relatively easy. A book such as T. E. Thompson, 1988, Molluscs: Benthic Opisthobranchs. Synopses of the British Fauna (8), Linnean Society, London, gives a fairly comprehensive coverage. You would perhaps think that the Mediterranean, being an enclosed sea, would be well known but it is amazing how many new species are regularly found. It clearly has a complicated geological history and its fauna is related to that of southern France, Spain and Portugal, the west coast of Africa and even the Caribbean.

Which finally brings me to your question about the Great Barrier Reef. Although we may think of it as a distinct entity, in biological terms it cannot be separated from the vast tropical Pacific and Indian Oceans. Although some species are only found in the Pacific or Indian Oceans, and in some instances certain species are restricted to a small part of one of these oceans, many seem to have a wide distribution throughout the region. So just because a particular species found elsewhere in the Indo-West Pacific has not yet been recorded from the Great Barrier Reef it doesn't mean it doesn't occur there. I know of no "official list" of opisthobranchs from the Great Barrier Reef but there are a number of "unofficial" working lists various workers have compiled and they range between 3-500 species of nudibranch. We do not know enough about the the other sea-slugs, in particular the Bubble-Shells, even to make a reasonable guess of their numbers. In other parts of the tropical Indo-West Pacific similar "unofficial lists" include similar numbers, but not necessarily the same species. If you want a guess I would suspect there are about a 1000 species of nudibranch in the Indo-West Pacific and a similar number of other Sea-Slugs. I would not like to guess how many of these would be found at any one place. But I must emphasise that we need to learn a lot more about the fauna before I could give you anymore than a guess.

If anyone has good species lists or estimates of species numbers from anywhere, I would be happy to post them on the Forum. Perhaps we could get some consensus figures? .. Bill Rudman