Gorgonian Biogeography

Biogeography is the geographic distribution of taxa and the evolutionary processes that bring these patterns of distribution about. The Coral triangle - most notably the Indo-Malay-Philippine (IMP) region - is the most biodiverse biogeographic region on the planet; recognised as the global epicenter of shallow-water marine biodiversity. The origin of such biodiversity remains the source of much intrigue and investigation with a variety of hypotheses being proposed. Most significant are:

  1. The Center of Origin hypothesis, which predicts that the IMP is a centre of speciation where species disperse marginally (see Ekman 1953
  2. The Center of Overlap hypothesis, which predicts that species diversity is due to dispersal overlap in all directions from numerous biogeographic provinces (see Woodland 1983
  3. The Center of Accumulation hypothesis, which predicts that peripheral speciation through dispersal is extended unidirectionally by prevailing currents into the IMP (see Ladd 1960
  4. The Center of Survival by Paulay (1990), who hypothesises that species are buffered by extinction in contrasting peripheral locations.

All models likely pull in unison with biodiversity feedback between the various hypothesised processes contributing to the current high biodiversity that we see today (see Bowen et al. 2013). 

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Figure 1. Tropical Indo-Pacific biogeographic provinces defined by more than 10% endemic species. The Coral triangle is depicted in royal blue. Image from Bowen et al. 2016

Yet how these hypotheses translate to gorgonian corals, particularly at different depths is unknown. This is largely due to lack of field-sampling effort and the funds in which to do so. Nonetheless, for well over a decade I have collected nearly 7,000 gorgonian specimens (see Collections), the majority of which from between 0 - 160 m depth throughout the Indo-Pacific. When combined with historical data from expeditions of the 19th and 20th centuries and museum collection across the Indo-Pacific, significant ecological and biogeographic patterns emerge. 

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Figure 2. Gorgonian ecology and biogeography on Indo-Pacific shallow and mesophotic reefs. (a) Gorgonian species richness with increased depth. (b) Gorgonian biogeography with depth comparisons: green circles are records up to 29 m depth, blue circles from 30 - 200 m depth. Data from Rowley 2014 in part, Rowley et al. 2019 in part, Sánchez et al. 2019 in part, Rowley unpublished data.

As depth increases so does gorgonian species diversity and abundance (Figure 2a). From this two key groups emerge; 'depth specialists', staying within a particular depth range, and 'depth generalists' that span remarkable depth ranges and typically have wide geographic range sizes. These ecological patterns appear relatively consistent throughout the Indo-Pacific. Furthermore, gorgonian biogeography reveals two more patterns; that shallow reef gorgonian diversity and abundance attentuate eastward across the Pacific, and that gorgonian diversity and abundance at deeper mesophotic depths remain high across the Pacific (Figures 2b & 3). Such biogeographic patterns are not particularly new in the marine realm (e.g., Hoeksema 2007 and Veron et al. 2009 for corals). However, these patterns suggest that shallow and mesophotic gorgonians have different biogeographic distributions and, therefore, evolutionary histories. 

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Figure 3. A zoomed in yet coarse look at the distribution of gorgonian corals from west (left) to east (right); the Philippine Archipelago, within the Wakatobi Marine National Park, Indonesia, to Enewetak Atoll and Bikini (Pikinni) Atoll in the Marshall Islands. Green circles are records up to 29 m depth, blue circles from 30 - 200 m depth. Data from Rowley 2014 in part, Rowley et al. 2019 in part, Sánchez et al. 2019 in part, Rowley unpublished data.

Such biogeographic patterns, however, could simply be a lack of sampling effort. And to be quite frank, the only significant way one can satisfactorily research such patterns and investigate the systematic and evolutionary questions that arise, is to sample widely, thoroughly, and with great repetition. Henceforth, the collection expands and the pursuit for funding persists. 

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Figure 4. Gorgonian corals (forefront Acanthogorgia sp.) and a variety of other octocoral, zoanthid, and associated taxa at 135 m depth, Pakin Atoll, FSM. Image by SJ. Rowley.

The oceanic islands and atolls of the Pacific are havens for evolutionary novelty. - new pathways to create opportunities for evolutionary innovation, ultimately increasing biodiversity. Because selection favours different traits in different populations, evolutionary novelty is a scene on the oceanic islands and atolls of the Pacific.....

This page is currently in progress and will be completed by the end of October 2019.  Please feel free to contact me with any questions, otherwise relevant publications can be found below. Thank you for your patience and please return for updates soon. 

 

Project Publications

Rowley SJ, Roberts TE, Coleman RR, Joseph E, Spalding HL, Dorricott MKI (2019) Pohnpei, Federated States of Micronesia. In Loya Y, Puglise K, Bridge T (Ed.), Mesophotic Coral Ecosystems. Coral Reefs of the World, vol 12. Springer, Cham. DOI: 10.1007/978-3-319-92735-0_17

Sánchez JA, Dueñas LF, Rowley SJ, González FL, Vergara DC, Montaño-Salazar SM, Calixto-Botia I, Gómez CE, Abeytia R, Colin PL, Cordeiro RTS, Pérez CD (2019) Gorgonian Corals. In Loya Y, Puglise K, Bridge T (Ed.), Mesophotic Coral Ecosystems. Coral Reefs of the World, vol 12. Springer, Cham. DOI: 10.1007/978-3-319-92735-0_39

Rowley SJ (2018) 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 (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