It has recently been uncovered that the model reef coral Pocillopora damicornis is actually a taxonomic complex comprised of multiple species that are difficult to differentiate in situ. However, little is known about the distribution of these species across or within reef ecosystems. To better understand the spatial partitioning between the morphologically-similar sister species P. damicornis and P. acuta, 88 Pocillopora colonies were genotyped across five and three islands/atolls within the Austral and Cook Islands, respectively. A mix of newly described pocilloporid types was found across the two archipelagos, with nearly 33% of the specimens being identified not as the more commonly referenced a genotype of P. damicornis, but instead as P. acuta. Furthermore, P. damicornis was more likely to be found at depths greater than 15 m and in higher coral cover areas relative to P. acuta, suggesting that these closely-related species display distinct habitat preferences.

Given the significant threats towards Earth’s coral reefs, there is an urgent need to document the current physiological condition of the resident organisms, particularly the reef-building scleractinians themselves. Unfortunately, most of the planet’s reefs are understudied, and some have yet to be seen. For instance, the Kingdom of Tonga possesses an extensive reef system, with thousands of hectares of unobserved reefs; little is known about their ecology, nor is there any information on the health of the resident corals. Given such knowledge deficiencies, 59 reefs across three Tongan archipelagos were surveyed herein, and pocilloporid corals were sampled from approximately half of these surveyed sites; 10 molecular-scale response variable were assessed in 88 of the sampled colonies, and 12 colonies were found to be outliers based on employment of a multivariate statistics-based aberrancy detection system. These outliers differed from the statistically normally behaving colonies in having not only higher RNA/DNA ratios but also elevated expression levels of three genes: 1) Symbiodinium zinc-induced facilitator-like 1-like, 2) host coral copper-zinc superoxide dismutase, and 3) host green fluorescent protein-like chromoprotein. Outliers were also characterized by significantly higher variation amongst the molecular response variables assessed, and the response variables that contributed most significantly to colonies being delineated as outliers differed between the two predominant reef coral species sampled, Pocillopora damicornis and P. acuta. These closely related species also displayed dissimilar temporal fluctuation patterns in their molecular physiologies, an observation that may have been driven by differences in their feeding strategies. Future works should attempt to determine whether corals displaying statistically aberrant molecular physiology, such as the 12 Tongan outliers identified herein, are indeed characterized by a diminished capacity for acclimating to the rapid changes in their abiotic milieu occurring as a result of global climate change.