Environmental filtering and neutral processes shape octocoral community assembly in the deep sea
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The ecological and evolutionary processes that interact to shape community structure are poorly studied in the largest environment on earth, the deep sea. Phylogenetic data and morphological traits of octocorals were coupled with environmental factors to test hypotheses of community assembly in the deep (250–2500 m) Gulf of Mexico. We found lineage turnover at a depth of 800–1200 m, with isidids and chrysogorgiids at deeper depths and a diversity of species from across the phylogeny occupying shallower depths. Traits, including axis type, polyp shape, and polyp retraction, differed among species occupying the shallowest (250–800 m) and deepest (1200–2500 m) depths. Results also indicated that octocoral species sort along an environmental gradient of depth. Closely related octocoral species sorted into different depth strata on the upper to middle slope, likely due to barriers imposed by water masses followed by adaptive divergence. Within any given depth zone down to 2000 m, the phylogenetic relatedness of co-existing octocorals was random, indicating that stochastic processes, such as recruitment, also shape community structure. At depths >2000 m, octocorals were more closely related than expected by chance due to the diversification of chrysogorgiids and isidids, which retain conserved traits that impart survival at deeper and/or colder depths. Polyp density, size, and inter-polyp distance were significantly correlated with depth, particularly in plexaurids and isidids, highlighting trait lability across depth and supporting that environmental gradients influence octocoral morphology. Our community phylogenetics approach indicates that both environmental filtering and neutral processes shape community assembly in the deep sea.
KeywordsCommunity assembly Phylogenetics Niche Trait lability Octocorallia
We thank the R/Vs Ron Brown, Seward Johnson, Nancy Foster, Atlantis, ROV Jason II, ROV Seaeye Falcon, HOV Johnson-Sea-Link, and HOV Alvin. Thanks to C. Doughty, R. Falco, and N. Remon for help with DNA extractions. C. Fisher, R. Sanders, T. Shank, J. Brooks, and TDI-Brooks provided support. Particular thanks to A. Freestone for helpful suggestions. O. Breedy, S. Cairns, P. Etnoyer, S. France, S. Herrera, B. Horvath, C. McFadden, K. Moore, J. Thoma, J. Sánchez, L. Watling, and L. van Ofwegen provided taxonomic expertise.
Author contribution statement
AMQ and EEC conceived and designed the study. CEG conducted the morphological trait analyses. AMQ performed the research, analyzed the data, and wrote the article with contributions from CEG and EEC.
Compliance with ethical standards
Funding was provided by BOEM and NOAA-OER (BOEM contract #M08PC20038) for the Lophelia II project led by TDI-Brooks International. AMQ was supported by the NOAA Nancy Foster Scholarship Program, Temple University Dissertation Completion Grant, and the Lerner-Gray Fund of Marine Research. CEG is under support from the Fulbright-Colciencias Doctoral Scholarship program.
Conflict of interest
The authors declare that they have no conflict of interest.
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