Origin and means of dispersal of cosmopolitan Pontodrilus litoralis (Oligochaeta: Megascolecidae)

Provenance and means of distribution of Pontodrilus litoralis (Grube, 1855), circummundane on shorelines in the tropics and warmer oceans, have always been enigmatic. Origin was argued for in the region of southern India or Australia/New Zealand where congeners occur, or the Mediterranean from whence the species was first described. Primary dispersal by ocean rafting was assumed, but synonymy of 20 names under P. litoralis and absence of geographic variability imply recent and consistent global recruitment and/or remarkably conservative morphology. Perhaps the synonymy is too broad or, a converse view, especially when Pontodrilus is placed under Megascolecidae sensu Blakemore, 2000 rather than Acanthodrilidae, is of an ancient species with precursors close to the original forms of this family, living in a relatively constant and stable littoral habitat. The current review is based on new records from recent ecological survey and taxonomic analysis of Pontodrilus and related genera for which the restoration of Pontodril-inae/-idae Vejdovsky, 1884 may be merited. An argument is presented for a dual mechanism of dispersal of P. litoralis, involving both historical human transportation (via ship’ sand-ballast) and natural rafting (of euryhaline cocoons on flotsam). A homeland is proposed in southern India or Australia or, possibly, both – that would indicate this species’ emergence >100 Ma ago.