Abstract
As conspicuous modular components of benthic marine habitats, gorgonian (sea fan) octocorals have perplexed taxonomists for centuries through their shear diversity, particularly throughout the Indo–Pacific. Phenotypic incongruence within and between seemingly unitary lineages across contrasting environments can provide the raw material to investigate processes of disruptive selection. Two distinct phenotypes of the Isidid Isis hippuris Linnaeus, 1758 partition between differing reef environments: long-branched bushy colonies on degraded reefs, and short-branched multi/planar colonies on healthy reefs within the Wakatobi Marine National Park (WMNP), Indonesia. Multivariate analyses reveal phenotypic traits between morphotypes were likely integrated primarily at the colony level with increased polyp density and consistently smaller sclerite dimensions at the degraded site. Sediment load and turbidity, hence light availability, primarily influenced phenotypic differences between the two sites. This distinct morphological dissimilarity between the two sites is a reliable indicator of reef health; selection primarily acting on colony morphology, porosity through branching structure, as well as sclerite diversity and size. ITS2 sequence and predicted RNA secondary structure further revealed intraspecific variation between I. hippuris morphotypes relative to such environments (ΦST = 0.7683, P < 0.001). This evidence suggests—but does not confirm—that I. hippuris morphotypes within the WMNP are two separate species; however, to what extent and taxonomic assignment requires further investigation across its full geographic distribution. Incongruence between colonies present in the WMNP with tenuously described Isis alternatives (Isis reticulata Nutting, 1910, Isis minorbrachyblasta Zou, Huang & Wang, 1991), questions the validity of such assignments. Furthermore, phylogenetic analyses confirm early taxonomic suggestion that the characteristic jointed axis of the Isididae is in fact a convergent trait. Thus the polyphyletic nature of the Isididae lies in its type species I. hippuris, being unrelated to the rest of its family members.
Highlights
Reef biodiversity reflects that of its environment and geological history, those within the Indo–Pacific Coral Triangle some of the most diverse of all
Branches, including the branch tips, were consistently longer and thicker, at Sampela; polyp parameters were relatively invariable despite significantly high polyp density
The two distinct I. hippuris morphotypes within the Wakatobi Marine National Park (WMNP) are phenotypically segregated through trait integration between healthy and degraded reefs, likely reinforced through reproductive strategy
Summary
Reef biodiversity reflects that of its environment and geological history, those within the Indo–Pacific Coral Triangle some of the most diverse of all Intense competition in such environments may lead to niche partitioning through resource acquisition, leading to ecological divergence. Distinct morphotypes of the isidid gorgonian Isis hippuris Linnaeus, 1758 exist between healthy (Ridge 1) and degraded (Sampela) reefs within the Wakatobi Marine National Park (WMNP), SE Sulawesi, Indonesia; short-branched multi/planar colonies, and long-branched bushy colonies, respectively (Fig. 1; Rowley, 2014; Rowley & Watling, in press) Whether such morphological differentiation is a consequence of a capacity to be plastic, plasticity as an adaptation (Hoogenboom, Connolly & Anthony, 2008), or has become genetically fixed leading to two species, is unclear
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