Phenotypic, ecological and genomic variation in common bully (Gobiomorphus cotidianus) populations along depth gradients in New Zealand's Southern Great Lakes

Depth gradients in lakes are often key drivers of population divergence and speciation in fishes. New Zealand has many deep lakes but no known profundal specialist fishes or cases of intralacustrine speciation. We sampled a native benthic fish, the common bully, from 5–90 m depth in four South Island lakes, to test for morphological, ecological, or genetic differentiation associated with depth. Deeper fish consistently had narrower bodies, while other morphological traits showed variable relationships with depth. Carbon and nitrogen stable isotope values of fish increased with depth, largely tracking isotopic trends with depth of benthic invertebrate prey. Genotyping-by-sequencing showed some genome-wide differentiation between two of the lakes, but no evidence for within-lake genetic structuring along depth gradients. These results indicate that individual bullies associate with shallower or deeper habitats within their lifetimes, but we found no evidence of progress toward genetic divergence within lakes. The apparent lack of intralacustrine genetic divergence in New Zealand's fishes may be explained by a combination of environmental factors and constraints intrinsic to its marine-derived freshwater fish fauna.

Methods
Common bullies (Gobiomorphus cotidianus) were collected at five depths in four deep New Zealand lakes. Samples were used to compare population structure (size and sex), morphology, and diet (gut contents and stable isotopes) across depths. In two lakes genotyping-by-sequencing (GBS) was used to test for genomic variation across depths.