A molecular phylogeny and historical biogeography of the marine sponge genus Placospongia (Phylum Porifera) indicate low dispersal capabilities and widespread crypsis

Very little is known about the dispersal of marine sponges but hypotheses based upon observations of larval life suggest limited dispersal ability. Until recently, the diversity of dispersal mechanisms employed by different species, coupled with the putative cosmopolitan distribution of some species, suggested that short larval life may not limit sponge dispersal. However, recent studies have found that cosmopolitan species frequently represent morphologically cryptic species complexes. Here, the historical biogeography of the circum-tropical marine sponge genus Placospongia is investigated using sequence data from the internal transcribed spacer region (ITS). ITS is the predominant marker used for fine-scale investigations in sponges (and many other taxa), but problems with paralogy (divergent copies within individual genomes) have brought the utility of this marker into question. Ninety-eight ITS subclones were sequenced from twenty-eight individuals of Placospongia sampled from twelve localities worldwide. Divergent ITS paralogs were detected but the data still reflected geographic (and presumably phylogenetic) structure. Placospongia populations sampled were found to harbor at least 9 unique evolutionary lineages, several of which are sympatrically distributed but morphologically indistinguishable, Most of the relationships reconstructed are consistent with the hypothesis that marine sponges do not disperse very far (by any means). Nevertheless, phylogenetic divergences between lineages were not clearly consistent with modem, or historical, geological or oceanographic processes. No component of the phylogeny presented clearly reflected the geological event of the rising of this Isthmus of Panama and populations from marine lakes in Indonesia were found to be indistinguishable from populations in the Seychelles. These biogeographically anomalous results highlight that ITS is useful as a tool to identify interesting preliminary patterns in widespread groups, but that these patterns should be tested using multiple independent loci. (c) 2005 Elsevier B.V. All rights reserved.