Allegory of a cave crustacean: systematic and biogeographic reality of Halosbaena (Peracarida: Thermosbaenacea) sought with molecular data at multiple scales

Halosbaena Stock, 1976 are small crustaceans found in a number of distant, isolated subterranean locations in the Northern (Caribbean and Canary Islands) and Southern Hemispheres (Christmas Island and north-western Australia in Cape Range, Barrow Island and Pilbara regions). This distribution is surprising for an animal that produces few eggs, has no free-living larval stage, and succours their young in a dorsal brood pouch. It is usually explained by the passive movement of ancestral populations on tectonic plates as the ancient Tethys Ocean spread. We used molecular data (one mitochondrial and three nuclear genes) to reconstruct phylogenies and time-trees to understand their biogeography at the global scale and at four diminishing scales within the Southern Hemisphere. We found: (1) a basal split between species in the Northern and Southern Hemispheres, but the inferred ages of divergences between species are not old enough to be associated with the spread of the Tethys; (2) a recently discovered species from Christmas Island which is the sister to Australian mainland taxa; (3) the one described species from mainland Australia, H. tulki, probably constitutes at least five separate species that reflect local geography (Cape Range west, Cape Range east, Barrow Island, Pilbara low elevation, Pilbara high elevation); (4) the Pilbara high elevation taxon is likely not old enough to have been stranded high inland during an Eocene marine transgression; and (5) phylogeographic breaks within Cape Range west are congruent with other breaks in sympatric cave species.