Well-connected worms: genetic connectivity of annelids (Melinnidae and Ampharetidae) across a biogeographical break in Australia’s eastern abyss.

Marked species composition changes are observed between shallow-water (0–200 m) temperate and tropical fauna, however, this transition is not well documented in deep-water fauna (> 200 m). Along the east coast of Australia there is an apparent biogeographic tropical to temperate transition between 30–40°S for bathyal and abyssal fauna. This has been recorded for brittle stars (Echinodermata: Ophiuroidea) and certain benthic megafauna taxa combined but not tested for other taxa individually or tested with genetic data. During the 2017 RV Investigator expedition, a series of beam trawl and epibenthic sledge samples were taken from 13 sites along a south to north latitudinal transect along the east coast of Australia, from Freycinet Marine Park, 42°S, to Coral Sea Marine Park, 24°S, from 1,000 to 4,800 m depth. Three of the most abundant segmented worms (Annelida: Polychaeta) species, Melinnopsis gardelli, Melinnopsis chadwicki (Melinnidae) and Jugamphicteis galatheae (Ampharetidae) were picked out and morphologically identified, and the COI genetic marker was sequenced for 88 specimens. Melinnopsis gardelli was recorded across the biogeographical break from 42°S to 24°S. An AMOVA for the north and south populations revealed significant evidence of populations structuring. The other two species, Melinnopsis chadwicki and J. galatheae, were recovered from north and south respectively of the biogeographic break. One haplotype of M. gardelli was shared between two sampling locations south of the break, Jervis Marine Park and Freycinet Marine Park (distance 735 km), similarly, haplotypes were shared for M. chadwicki between locations 726 km apart and J. galatheae 950 km apart. We found genetic evidence of a break occurring at 28–35°S dividing M. gardelli populations in the north and south, additionally our data appeared to show a third central population for M. gardelli. However, the break was not strongly defined as M. gardelli was found to bridge across the transition. Our results indicated deep-sea annelid populations are well connected along sections of the eastern Australian margin, suggesting the designated deep-water Marine Parks function as an important network for these organisms.