Testing the depth-differentiation hypothesis in a deepwater octocoral

Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of The Royal Society for personal use, not for redistribution. The definitive version was published in Proceedings of the Royal Society B: Biological Sciences 282 (2015): 20150008, doi:10.1098/rspb.2015.0008.The depth-differentiation hypothesis proposes that the bathyal region is a source of genetic diversity and an area where there is a high rate of species formation. Genetic differentiation should thus occur over relatively small vertical distances, particularly along the upper continental slope (200-1000 m) where oceanography varies greatly over small differences in depth. To test whether genetic differentiation within deepwater octocorals is greater over vertical rather than geographic distances, Callogorgia delta was targeted. This species commonly occurs throughout the northern Gulf of Mexico at depths ranging from 400-900 m. We found significant genetic differentiation (FST=0.042) across seven sites spanning 400 km of distance and 400 m of depth. A pattern of isolation by depth emerged, but geographic distance between sites may further limit gene flow. Water mass boundaries may serve to isolate populations across depth; however, adaptive divergence with depth is also a possible scenario. Microsatellite markers also revealed significant genetic differentiation (FST=0.434) between C. delta and a closely-related species, C. americana, demonstrating the utility of microsatellites in species delimitation of octocorals. Results provided support for the depth-differentiation hypothesis, strengthening the notion that factors co-varying with depth serve as isolation mechanisms in deep-sea populations.Funding was provided by BOEM and NOAA-OER (BOEM contract #M08PC20038) for the Lophelia II project led by TDI-Brooks International. AMQ was funded by the Dr. Nancy Foster Scholarship program, Temple University Dissertation Completion Grant, and the Lerner-Gray grant for marine research.2016-04-2