Torres Strait represents a major biogeographical barrier between the Indian and Pacific Oceans and is well known for acting as an intermittent land bridge in line with periodic ice age glaciations over the past 250,000 years. The periodic closing and opening of the Strait, which now connects the Arafura and Coral Seas, promoted the fragmentation and secondary contact of several marine populations including sponges, the less famous residents of the Great Barrier Reef. The same events favored more than once the selection of lucky genetic variants capable of persisting in refugia zones east and west of the Strait.
To put everything into a context: as of today, the distance across the Torres Strait from Cape York to New Guinea is approximately 150 km (93 mi) with its northwestern region reported to be really shallow (10-5 meters). During the last glacial maximum, the Gulf of Carpentaria and the Strait were part of a large highway between Australia and New Guinea. Humans were one of the many species to make (successfully) use of the connection with the Australian continent!
Abstract: Morphological delineation of sponge species is hindered by the narrow range of fixed diagnostic characters and our limited knowledge of how much phenotypic plasticity the sponge body plan assumes in response to environmental conditions. Here, we make use of the partial mitochondrial cytochrome c oxidase subunit I (COI) gene and the second internal transcribed spacer (ITS2) region to assess the taxonomic validity of colour morphotypes observed in the elephant ear sponge Ianthella basta (Pallas, 1776) across its distribution range in northern Australia and explore levels and patterns of genetic diversity among populations of the species collected from both sides of the Torres Strait. Molecular phylogenetic analyses revealed congruent topologies consistent with three evolutionarily significant units (ESUs) that were independent of the morphology of the sponge. ESU I includes previously morphologically and genetically delineated western Pacific specimens of I. basta (Guam), and probably corresponds to the type specimen originally described from Indonesia. ESU I occurs in almost all sampling sites across northern Australia, suggesting considerable levels of connectivity among reefs throughout the Torres Strait. ESUs II and III are each exclusively associated with a geographic region of high sponge species richness separated by Torres Strait, and probably represent the result of historical population fragmentation.
- Cryptic speciation and phylogeographic relationships in the elephant ear sponge Ianthella basta (Porifera, Ianthellidae) from northern Australia. 2012. N Andreakis, HM Luter and NS Webster. Zoological Journal of the Linnean Society. 166: 225-235.
Deep genetic divergences among Indo-Pacific populations of the coral reef sponge Leucetta chagosensis (Leucettidae): founder effects, vicariance, or both? 2008. Wörheide G, Epp LS, Macis L. BMC Evolutionary Biology 8: 24.
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