Dear Liz,
I think Terry Gosliner, who has made extensive collections of opisthobranchs at Madang, considers his list from Madang to be the largest from one place, and he is probably right. So on existing evidence, you could say that Madang was the 'geographic center of nudibranch species diversity'. But what does that mean? In fact it just means that it is the place we have the largest known list of species. The danger is that it will be taken, incorrectly, to mean that Madang is the 'centre of opisthobranch speciation' - whatever that concept might mean.

I doubt that we can extrapolate from that, that Madang is the world centre of nudibranch speciation. The list is obviously evidence of a very large opisthobranch fauna in the region, and of Terry and his helpers' diligence, but before we can use present day abundance as evidence of past speciation events we have to take past geological history into account. Madang, and the whole Papua New Guinea region is a very active geological region on the edge of a continental plate, and was probably not there when species presently found there evolved. In more general terms, much of the present opisthobranch fauna, may have evolved when present continents were in quite different places, so talking of centres of speciation in terms of present geography is usually fairly meaningless. It may be possible to use such concepts with terrestrial animals with little ability to move far, or for marine animals, such as amphipods, which have direct developing larvae, but for animals with planktonic larvae, and a long earth history, such concepts a fairly meaningless. Present areas of faunal abundance are just as likely to be areas which, at present, have optimal ecological conditions for the species, as they are to be the actual area in which these species evolved.
Best wishes,
Bill Rudman