Does cosmopolitanism result from overconservative systematics? A case study using the marine sponge Chondrilla nucula

The sponge species Chondrilla nucula has a simple morphology and a very wid e geographical distribution. To verify whether the latter might be an artif act of the former, samples of this species were collected across 10,000 km of its range, in the Mediterranean, the Caribbean, and the southwestern atl antic. The classical (spicule morphology) and molecular (allozymes) systema tic approaches were compared, to try to define the geographic limits betwee n populations and detect possible cryptic species. We found five distinct g enetic forms within C. nucula that sometimes showed morphological homogenei ty and other times plasticity. The difference in size of spicules could not be related to the clear-cut genetic differences, suggesting that the use o f spicule sizes for sponge systematics should be reappraised. The populatio n of one of the genetic forms along 3000 km of the Brazilian coast was high ly structured (F-ST = 0.21; N(e)m = 0.96). Our results reject the null hypo thesis of cosmopolitanism of C. nucula and indicate that the putative world wide distribution of some marine sponges, and possibly many other benthic i nvertebrates, may be the result of overly conservative systematics. Cryptic species appear to be particularly prevalent when genera are well defined b ut species are characterized by only a few morphological characters.