Gorgonian Responses to Environmental Change on Coral Reefs in SE Sulawesi, Indonesia
Authors:   , Rowley Sonia J. 
Published: November 2021

<p>Gorgonian corals (Cnidaria: Anthozoa: Octocorallia) are conspicuous, diverse and often dominant components of benthic marine environments. Intra- & interspecific morphological variability in gorgonians are influenced by environmental factors such as light, sedimentation and flow rates. Yet, little is known about the responses of gorgonian taxa to environmental parameters particularly in Indonesia, despite their high regional abundance and diversity. With a burgeoning human population and subsequent marine resource exploitation, reefs throughout the Indonesian archipelago are under rapid decline and often destroyed. Conservation surveys are however, underway with a tendency to overlook gorgonian taxa primarily due to unresolved taxonomic assignment leading to difficulties in field identification.  The aims of this study were to: 1) characterise gorgonian diversity and ecology across a gradient of habitat quality within the Wakatobi Marine National Park (WMNP), SE Sulawesi, Indonesia, 2) assess morphological and genetic variability between morphotypes of the ubiquitous zooxanthellate isidid Isis hippuris Linnaeus 1758 from healthy and degraded reefs, 3) determine if I. hippuris morphotypes are environmentally induced (plastic) or genetically derived through reciprocal transplant experiments (RTEs) between contrasting reefs and thus, 4) identify mechanisms of plasticity capacity or divergence through phenotypic trait integration in response to environmental change.  Ecological surveys revealed considerable gorgonian diversity with a total of 197 species and morphotypes from 42 genera, and 12 families within the suborders Calcaxonia and Holaxonia and the group Scleraxonia, with current estimates of over 21 new species and 28 new species records for the region. Gorgonian abundance and diversity increased with reef health and bathymetry. However, a clear loss of gorgonian diversity existed with increased sedimentation and reduced light due to anthropogenic disturbance. In particular, two distinct I. hippuris morphotypes were highly abundant between environmental clines: short-branched multi/planar colonies on healthy reefs, and long-branched bushy colonies on degraded reefs. Comparative morphological and molecular analyses using ITS2 sequence and predicted secondary structure, further corroborated haplotype differences relative to morphotypes between environments. However, unsatisfactory assignment of I. hippuris morphotypes to previously described alternatives (Isis reticulata Nutting 1910, Isis minorbrachyblasta Zou, Huang & Wang 1991) questions the validity to such taxonomic assignments. Phylogenetic analyses also confirm that the polyphyletic nature of the Isididae lies in its type species I. hippuris, being unrelated to the rest of its family members.  A one-year RTE revealed three key results, that: 1) reduced survivorship of healthy reef morphotypes on degraded reefs implied the onset of lineage segregation through immigrant inviability, 2) prominent phenotypic traits were at the morphological and bio-optical levels revealing high phenotypic plasticity in healthy clones, and relative insensitivity to environmental change in degraded reef morphotypes, indicative of local adaptation leading to incipient ecological divergence, and 3) photoacclimation at the bio-optical level was not attributed to endosymbiont diversity or shuffling, with all test colonies possessing a novel clade D1a Symbiodinium.  While it is clear that gorgonian taxa within the WMNP are of exceptional diversity and abundance, responses to environmental perturbation highlight three pertinent, testable ideas. Firstly, increased species richness specifically with depth in azooxanthellate taxa, invite tests of deep-reef refugia previously established through geological change. Secondly, ecological assessment targets research on informative taxa for focused systematics and mechanisms of phenotypic divergence. Thirdly, exploring intrinsic and extrinsic interactions that define the host-symbiont relationship and differential biological success using physiological and next generation sequencing approaches. These objectives would provide considerable insight into the evolutionary processes to environmental change, accelerated by anthropogenic encroachment.  Taken together, this work signifies that gorgonian corals within the WMNP are of foremost diversity and concern, exhibiting informative ecological and mechanistic responses to environmental perturbation. This evidence elicits tests of deep-reef refugia, priority systematics, mechanisms of ecological divergence and physiological assessment. Such tests inevitably expand our understanding of the intrinsic and extrinsic associations of gorgonian taxa to environmental change from an historical and predictive perspective yielding benefits to conservation assessment and management.</p>