First impressions of the Southern Great Barrier Reef: Diversity is high, with a few species familiar to us from Polynesia, but most are new. Decapod abundance seems lower in comparison, but that could just be the microhabitats that we dove on yesterday. One thing does appear to occur in serious abundance: parasitism. In the samples from Biocode on Moorea, it is an unusual occurance for us to encounter a parasite on a crustacean or seastar- perhaps one in one hundred. Here in Australia the rate seems much much higher- perhaps one in five.
Here is an externally parasitic snail, Thyca, on the seastar Lincka multiflora
And here is an internally parasitic snail, Stylifer, on the same species of seastar.
Understanding the pattern behind these differences is the trick. Part of it may be that diversity begets diversity, creating what Phillipe Bouche has called "the russian dolls of biodiversity." This is where one organism may have a commensal organism living with it, that commensal may have a parasite, and that parasite may have a parasite. We certainly see it with coral, where one species of coral may host many symbionts and each of these may have their own set of commensals and parasites. By increasing the number of species of structural species (corals in this case, but could just as easily be trees in a rainforest) by one, we increase the over-all diversity by many.
This Pink Coral Guard Crab (Trapezia serenei) is a good example. It lives exclusively in one family of corals. If you look at the carapace, you can see that it is asymetrical. In this case that is indicative of a Boporid- a crustacean parasite on crustaceans that has infected the crab.