Acclimatory capacity of the Gorgonian Isis hippuris Linnaeus, 1758 to environmental change in SE Sulawesi, Indonesia

Abstract

Coral reefs within the Indonesian archipelago are some of the most biodiverse yet anthropogenically compromised marine ecosystems. Within the Wakatobi Marine National Park (WMNP), SE Sulawesi, Indonesia, pronounced environmental clines are either caused or exacerbated by marine resource subsistence and destruction. The protected zooxanthellate gorgonian (sea fan coral) Isis hippuris Linnaeus, 1758 however, thrives on degraded reefs, with distinct morphotypes across contrasting reef environments within the region. To investigate if I. hippuris morphotypes are environmentally induced (plastic) or genetically derived (fixed), reciprocal transplant experiments (RTEs) were conducted across environmental gradients of light attenuation and anthropogenic disturbance. Phenotypic traits were measured and grouped into modules (colony, polyps, sclerites, optical parameters) to assess the physiological responses and endosymbiont specificity of each I. hippuris morphotype to environmental change. Trait modules were then modelled for tests of integration to determine phenotypic traits that interact to delimit I. hippuris morphotypes in response to environmental perturbation. After one-year, survivorship was lowest in clones transplanted from healthy to the degraded reef, suggesting the onset of immigrant inviability. Multivariate analyses revealed that phenotypic traits in healthy source colonies consistently showed significant trait plasticity, whereas residents from the impacted site were relatively insensitive to environmental change. Of the 38 phenotypic traits assessed, 17 were identified as driving test dissimilarities, most notably in branching dynamics, polyp density, capstan and spindle sclerite dimensions, and Symbiodinium chlorophyll a light energy absorbance efficiency (for photosynthesis). Specifically, photoacclimatory responses were integrated at the morphological and bio-optical levels, with chlorophyll a light harvesting efficiency maintained during reduced pigment density resulting from an increase in host sclerite articulation that maximized the internal light field in healthy clones on degraded reefs. Variable optical responses were not attributed to endosymbiont type, as all test colonies possessed a novel Symbiodinium Clade D1a. In summary, patterns of phenotypic variability within the I. hippuris holobiont likely represent incipient ecological divergence, with a high capacity for plasticity that has become fixed through ongoing anthropogenic disturbance on degraded reefs.