Barriers in a sea of elasmobranchs

Background The interplay of animal dispersal and environmental
heterogeneity is fundamental for the distribution of biodiversity on
earth. In the ocean, the interaction of physical barriers and dispersal
has primarily been examined for organisms with planktonic larvae. Animals
that lack a planktonic life stage and depend on active dispersal are
however likely to produce distinctive patterns. Methods We used available
literature on population genetics and phylogeography of elasmobranchs
(sharks, rays and skates), to examine how marine barriers and dispersal
ecology shape genetic connectivity in animals with active dispersal. We
provide a global geographic overview of barriers extracted from the
literature and synthesize the geographic and hydrologic factors, spatial
and temporal scales to characterize different types of barriers. The three
most studied barriers were used to analyse the effect of elasmobranch
dispersal potential and barrier type on genetic connectivity. Results We
characterized nine broad types of marine barriers, with the three most
common barriers being related to ocean bathymetry. The maximum depth of
occurrence, maximum body size and habitat of each species were used as
proxies for dispersal potential, and were important predictors of genetic
connectivity with varying effect depending on barrier type. Environmental
tolerance and reproductive behaviour may also play a crucial role in
population connectivity in animals with active dispersal. However, we find
that studies commonly lack appropriate study designs based on a priori
hypotheses to test the effect of physical barriers while accounting for
animal behaviour. Main conclusions Our synthesis highlights the relative
contribution of different barrier types in shaping elasmobranch
populations. We provide a new perspective on how barriers and dispersal
ecology interact to rearrange genetic variation of marine animals with
active dispersal. We illustrate methodological sources that can bias the
detection of barriers and provide potential solutions for future research
in the field.