The protected zooxanthellate gorgonian coral Isis hippuris Linnaeus, 1758 is a conspicuous component of coral reefs and seagrass beds throughout the Central Indo-Pacific. It is a highly plastic species, yet such plasticity has historically challenged species identification and taxonomic assignment. Therefore, of the 20 species assigned to the Isis genus over the years, I. hippuris remains the only accepted species.
Order ALCYONACEA Lamouroux, 1812
Sub-Order CALCAXONIA Grasshoff, 1999
Family ISIDIDAE Lamouroux, 1812
Sub-Family ISIDINAE Lamouroux, 1812
Isis hippuris Linnaeus, 1758
(Figure 1 [below])
See Bayer & Stefani, 1987 for list of references [p. 55]
Type Material – Unfound, however ‘authentic’ specimens were collected and fully defined from Amboina, Indonesia (Milne-Edwards & Haime, 1857).
Figure 1. Isis Linnaeus, 1758 comparisons showing variability and a touch of confusion. (A) Isis hippuris Linnaeus, 1758 colony in Ellis & Solander, 1786; (B) sclerites of: i. I. hippuris and ii. Isis reticulata in Nutting 1910; (C) I. reticulata in Nutting, 1910; (D) Isis minorbrachyblasta Zou et al., 1991 colony and (E) sclerites. Note, image plate and caption from Rowley et al. 2015.
The species I. hippuris is the type species of the family Isididae Lamouroux, 1812 [nom. correct. Kükenthal, 1915 (pro Isididae Lamouroux, 1812)], which is characterised by a unique axis of alternating calcareous internodes and proteinaceous (gorgonin) nodes. But to add to the confusion (see Phylogenetics) I. hippuris is phylogenetically unrelated to the rest of the family! Thus the Isididae is polyphyletic. It is the construction of the axis that gives the game away - the Isis axis is simply not made the same as all the other Isidids!Figure 2. Denuded axes of Isis hippuris Linnaeus, 1758 (left, young end-branch and right, thicker mid-branch) showing the intermittent proteinaceous (gorgonin) nodes and calcareous internodes giving a bamboo appearance. Image by SJ. Rowley.
The history of I. hippuris is quite interesting being one of the 'marine plants' of Rumphius in 1741. Prior to this time, I. hippuris was originally (pre-Linnaean) named Hippuris saxea Clusius, 1605 ("the stony horse-tail") and has for centuries been admired for its distinct articulated axis. Considered of such value, it eventually was named after the most important ancient Egyptian deity, the Goddess Isis. To this day, the axis can be found in jewelry, art, souvenirs, and even Royal collections.
Figure 3. A variety of commercial creations from the axes of Isis hippuris Linnaeus, 1758. Images modified from Cooper et al. 2011.
Nonetheless, even though humans have and continue to exploit this species characteristic trait of a jointed axis for commercial jewelry and souvenirs, it is, in fact, a protected species throughout Indonesia. I hippuris has been under a 5-year moratorium (Ministry of Marine and Fisheries Ministerial Decree No. 46/KEPMEN-KP/2014, Dermawan et al. 2015) by the Indonesian government due, in part, to ecological and experimental research.
Figure 4. Location map of (a) the Wakatobi Marine National Park, SE Sulawesi, Indonesia, and (b) comparative study sites within the region showing contrasting reef environments relative to natural and anthropogenic disturbance. Image adapted from Rowley 2018a.
The type material, from which a species is described, has been lost. Nonetheless, specimens of I. hippuris were collected and fully described from Amboina, Indonesia by Milne-Edwards & Haime and reported in 1857. I. hippuris is commonly present on shallow reefs and seagrass beds throughout the Indonesian archipelago. In the Wakatobi Marine National Park (WMNP), SE Sulawesi, two distinct morphotypes were revealed during the initial ecological surveys of my PhD (Rowley 2014, 2018b). These highly abundant morphotypes were found in distinct reef environments: long-branched bushy colonies on degraded reefs, and short-branched multi-planar colonies on healthy reefs.
Figure 5. Contrasting environments with corresponding morphotypes of the zooxanthellate gorgonian Isis hippuris Linnaeaus, 1758 in the Wakatobi Marine National Park (WMNP), SE Sulawesi. The upper three images show the healthy reefs and I. hippuris phenotype, the lower three images show the unhealthy (human-impacted) reefs that are adjacent to a Bajo (sea gypsy) village, and the I. hippuris phenotype that dominates the shallow waters. Images by SJ. Rowley.
Determining the mechanisms by which these contrasting phenotypes came about became the dominant part of my research in the following years. Such morphological differentiation may be due to the I. hippuris holobiont having a high capacity to be plastic, or these morphotypes could, in fact, be two previously diverged species, or alternatively, they could be in a state of anthropogenically-driven incipient ecological divergence on degraded reefs. Figure 6. Contrasting morphotypes of the zooxanthellate gorgonian Isis hippuris Linnaeus, 1758 on healthy (left) and unhealthy (human-impacted) reefs of the Wakatobi Marine National Park (WMNP), Indonesia. Center image shows a transplant block used for experimental fieldwork by Rowley 2014, 2018a. Images by SJ. Rowley.
What I descovered through a suite of field experimentation and anaylses, was that the I. hippuris morphotypes of the WMNP are in a state of ecological divergence, where an inherently plastic phenotype (as seen on the healthy reefs) has become fixed over time when exposed to prolonged anthropogenic disturbance. This means that the two morphotypes of I. hippuris act as viable indicators of reef health, and as such, the methods and outcomes of this study has been used for reef restoration and the conservation management of I. hippuris morphotypes throughout Indonesia.
Figure 7. Isis hippuris Linnaeus, 1758 morphotypes and corresponding haplotype network. (a) Healthy reef phenotype, (b) unhealthy reef phenotype, and (c) haplotype network with ITS2 RNA predicted secondary structure demonstrating the phenotypic differences on a molecular level. Red and black arros indicate point mutations and loop differences respectively - the points at which natural selection is at work. Image adapted from Rowley 2014, Rowley et al. 2015.
Further evidence also reveals the phylogenetic significance of this research, providing insights into the polyphyletic nature of this curious group (see Phylogenetics). Many fascinating questions continue to arise with regard to I. hippuris, which I will continue to pursue.
This study was made possible through the support of the Wakatobi Government, the Indonesian staff at ALAM of the Wakatobi Marine National Park, and the Wallacea Foundation for their logistical support, Victoria University Wellington Doctoral Research Scholarship and the Coral Reef Research Unit. The State Ministry of Research and Technology (RISTEK) granted research permits to Prof. DJ. Smith, under whose auspices this work was conducted. Terima kasih banyak Dafiuddin Salim of the University of Lambung, Indonesia, for awareness of Isis hippuris current protection status, and the utility of this study for the conservation management of Indonesian reefs.
Rowley SJ (2018a) Acclimatory capacity of the Gorgonian Isis hippuris Linnaeus, 1758 to environmental change in SE Sulawesi, Indonesia. Journal of Experimental Marine Biology & Ecology. 500: 73-88. DOI:org/10.1016/j.jembe.2017.12.012
Rowley SJ (2018b) Environmental gradients structure gorgonian assemblages on coral reefs in SE Sulawesi, Indonesia. Coral Reefs. 37: 609-630. DOI:org/10.1007/s00338-018-1685-y
Rowley SJ, Pochon X. Watling L (2015) Environmental influences on the Indo-Pacific gorgonian Isis hippuris Linnaeus, 1758 (Alcyonacea: Isididae): genetic fixation or phenotypic plasticity? PeerJ 3:e1128; DOI:10.7717/peerj.1128
Rowley SJ (2014) Gorgonian responses to environmental change on coral reefs in SE Sulawesi, Indonesia. Doctoral thesis, Victoria University Wellington, New Zealand, pp. 213. DOI:10.13140/RG.2.1.5126.7682