Results from the nearest-taxon (NTI) and net-relatedness (NRI) indexes.

<p>NTI for habitat type (a) and reef type (b) for Caribbean shallow-water octocoral communities. * Values significantly different from 0 under a two-tailed t-test and interval 95% confidence interval.</p

Discriminant component analysis, for the most abundant Caribbean octocoral species in every sample, to determine morphologic similarities according to the habitat type and reef type.

<p>For the SM_5 m habitat neither species was more abundant with regard to the rest of the habitats. SESmetric for the four continuous and one binary traits (the analysis does not allow to run multistate traits) according to habitat type (c) and reef type (d). * = Greater absolute correlation between each variable and any discriminant function; s = Data significantly different from 0 with two-tailed t-test and 95% confidence interval.</p

Octocoral Species Assembly and Coexistence in Caribbean Coral Reefs

<div><p>Background</p><p>What are the determinant factors of community assemblies in the most diverse ecosystem in the ocean? Coral reefs can be divided in continental (i.e., reefs that develop on the continental shelf, including siliciclastic reefs) and oceanic (i.e., far off the continental shelf, usually on volcanic substratum); whether or not these habitat differences impose community-wide ecological divergence or species exclusion/coexistence with evolutionary consequences, is unknown.</p><p>Methods</p><p>Studying Caribbean octocorals as model system, we determined the phylogenetic community structure in a coral reef community, making emphasis on species coexistence evidenced on trait evolution and environmental feedbacks. Forty-nine species represented in five families constituted the species pool from which a phylogenetic tree was reconstructed using mtDNA. We included data from 11 localities in the Western Caribbean (Colombia) including most reef types. To test diversity-environment and phenotype-environment relationships, phylogenetic community structure and trait evolution we carried out comparative analyses implementing ecological and evolutionary approaches.</p><p>Results</p><p>Phylogenetic inferences suggest clustering of oceanic reefs (e.g., atolls) contrasting with phylogenetic overdispersion of continental reefs (e.g., reefs banks). Additionally, atolls and barrier reefs had the highest species diversity (Shannon index) whereas phylogenetic diversity was higher in reef banks. The discriminant component analysis supported this differentiation between oceanic and continental reefs, where continental octocoral species tend to have greater calyx apertures, thicker branches, prominent calyces and azooxanthellate species. This analysis also indicated a clear separation between the slope and the remaining habitats, caused by the presence or absence of <i>Symbiodinium</i>. K statistic analysis showed that this trait is conserved as well as the branch shape.</p><p>Discussion</p><p>There was strong octocoral community structure with opposite diversity and composition patterns between oceanic and continental reefs. Even habitats with similar depths and overall environmental conditions did not share similar communities between oceanic and continental reefs. This indicates a strong regional influence over the local communities, probably due to water transparency differences between major reef types, i.e., oceanic vs. continental shelf-neritic. This was supported by contrasting patterns found in morphology, composition and evolutionary history of the species between atolls and reef banks.</p></div

Sorensen beta diversity between habitat type (a) and reef type (b) for Caribbean octocoral communities.

<p>Circles represent groups that can be explained by distribution patterns. <i>P</i>–value of Pearson’s correlation with Mantel test of phylogenetic community dissimilarity and environmental condition, between habitat type (c) and reef type (d). *Significant correlation.</p

Geographic distribution of sampled coral reefs in the Western Caribbean (Colombia).

<p>Each color represents a different reef type: four atolls, one barrier reef, four reef banks and one fringe reef (map contours were tablet-digitized from the nautical charts COL 005 and 261, CIOH, República de Colombia).</p

Unrooted star phylograms of <i>Symbiodinium</i> spp.

<p>Topologies were obtained with Bayesian inference (support for major clades are Bayesian clade credibility/maximum likelihood 100 bootstrap replicates). A. ITS2 phylogenetic hypothesis. B. cp 23S phylogenetic hypothesis. Terminal branch names correspond to the zooxanthellae clade letter plus GenBank accession numbers except for new free-living sequences.</p

Ordination plot of Saba Bank transects.

<p>A non-metric multidimensional scaling (MDS) plot based on Bray-Curtis dissimilarity measures derived from species abundance values in 8 quantitative transects. Fore-reef sites (A, CV) are significantly different (ANOSIM, <i>P</i><0.05) from plateau sites (D, E, and Void), but not significantly different from each other. There is good separation between fore-reef and plateau sites, with slight prospect of a misleading representation (2D stress <0.10).</p

A map of Saba Bank, in the Netherlands Antilles.

<p>Saba Bank is shown relative to its nearest neighbor, Saba. A pale band of color representing 11–20 m depths represents a “fore-reef” extending 50 km along the southeast perimeter of the Bank. Red stars indicate the location of 8 quantitative transects, four on the fore-reef (A sites and CV sites) and four in the plateau (E3, E4, D9, Void) of Saba Bank's interior region. The dashed lines indicate Overall Bank, where a zonation scheme with random sites was in effect. Void was random, Conch Valley was non-random.</p

Summary of the sampling scheme. Reefs (sites), depths and number of samples (environmental and <i>Sparisoma viride</i>) collected.

<p>Summary of the sampling scheme. Reefs (sites), depths and number of samples (environmental and <i>Sparisoma viride</i>) collected.</p

Low-diversity gorgonian assemblage on the Saba Bank plateau.

<p>An image from one of the “E” sites in the Saba Bank plateau region shows a large sea plume (∼1.5 m- <i>Pseudopterogorgia acerosa</i>) and scattered sea fans <i>Gorgonia mariae</i> attached to a hard carbonate platform covered with a thin veneer of sediment. The plateau region is distinct from the fore-reef, consisting of reef flat and lagoon bottom environments <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0010668#pone.0010668-VanderLand1" target="_blank">[4]</a>.</p