The effects of depth, distance, and the Mid-Atlantic Ridge on genetic differentiation of abyssal and hadal isopods (Macrostylidae). Genetic differentiation of Macrostylidae in the Vema Fracture Zone

The deepest oceans are commonly assumed to represent a continuous environment due to homogeneity of environmental factors and the lack of barriers. Yet, Mid-Ocean Ridges and hadal trenches are bathymetric features that provide a discontinuation of the largest habitat on Earth. During the Vema-TRANSIT expedition in 2014/2015 to the tropical North Atlantic, a transatlantic transect was studied that followed the full extent of the Vema Fracture Zone in an east-west direction and terminated in the Puerto Rico Trench (PRT). The aim of this study was to test whether large bathymetric features represent barriers to dispersal and thus may potentially lead to differentiation and eventually speciation. These features were represented by the Mid-Atlantic Ridge (MAR) and the ~3000 m transition from the hadal PRT to the adjacent abyss. Using the presumably poor dispersers Macrostylidae (Crustacea, Isopoda) as a model, genetic differentiation and MOTU-based (molecular operational taxonomic unit) differences in community structure were tested for, comparing the areas east and west of the Mid-Atlantic Ridge as well as the bathyal and hadal depth zones. The MOTU distribution showed that macrostylid species can have distribution ranges covering distances of more than 2000 km and across oceanic ridges and trenches. There was, on the other hand, a clear signal for geographic structuring coinciding with the east-west division of the Atlantic by the Mid Atlantic Ridge as well as with the abyss-hadal zonation. This supports the hypotheses that depth gradients as well as oceanic ridges shape benthic communities by hindering dispersal. Additionally, positive associations between genetic- and geographic distances showed that the vast size of the deep-sea itself is a factor responsible for creating diversity.