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Widespread polyphyly in stony corals (order Scleractinia) has prompted efforts to revise their systematics through approaches that integrate molecular and micromorphological evidence. To date, these approaches have not been comprehensively applied to the dominant genera in mesophotic coral ecosystems (MCEs) because several species in these genera occur primarily at depths that are poorly explored and from which sample collections are limited. This study is the first integrated morphological and molecular systematic analysis of the genera Leptoseris and Pavona to examine material both from shallow-water reefs (<30 m) and from mid- to lower-MCEs (>60 m). Skeletal and tissue samples were collected throughout the Hawaiian Archipelago between 2–127 m. A novel mitochondrial marker (cox1-1-rRNA intron) was sequenced for 70 colonies, and the micromorphologies of 94 skeletons, plus selected type material, were analyzed. The cox1-1-rRNA intron resolved 8 clades, yet Leptoseris and Pavona were polyphyletic. Skeletal micromorphology, especially costal ornamentation, showed strong correspondence and discrete differences between mitochondrial groups. One putative new Leptoseris species was identified and the global depth range of the genus Pavona was extended to 89 m, suggesting that the diversity of mesophotic scleractinians has been underestimated. Examination of species’ depth distributions revealed a pattern of depth zonation: Species common in shallow-water were absent or rare >40 m, whereas others occurred only >60 m. These patterns emphasize the importance of integrated systematic analyses and more comprehensive sampling by depth in assessing the connectivity and diversity of MCEs.

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The vertical zonation of dominant megabenthic, photosynthetic taxa suggests that differential photosynthetic capabilities enable specialized, low-light zooxanthellate corals to dominate at depths where shallow-water corals become light limited. This study examines the ecophysiology of deep-water (68−113 m) Leptoseris spp. and shallow-water (2−15 m) Porites spp. zooxanthellate corals from Hawai'i by comparing spectral absorbance properties and photosynthetic pigment concentrations to the available light spectra in their respective environments. Photo synthetically active radiation reaching Leptoseris spp. was 3 to 11% of surface irradiance compared to 41 to 90% reaching Porites spp. Optical measurements indicated that Leptoseris spp. exhibited lower reflectance (i.e. higher absorptance) compared to Porites spp. and were chromatically adapted to the wavelengths of photons available at depth. Despite the decreased spectral reflectance, deep-water Leptoseris spp. exhibited significantly lower areal photosynthetic pigment concentrations than did shallow-water Porites spp. Based on morphological comparisons of the skeletons of both coral genera, we hypothesize that Leptoseris spp. skeletons may cause incident light to travel through the coral tissue several times, thereby increasing photon-pigment interactions without increasing pigment concentrations. This superior light harvesting efficiency exhibited by Leptoseris in an energy limited environment (enabled by skeletal design rather than pigment physiology) may in part explain why the dominant genus of reef-building corals in Hawai'i cannot compete successfully with specialized low-light corals at extreme depths. KEY WORDS: Photobiology · Mesophotic · Coral ecology · Ecophysiology · LeptoserisResale or republication not permitted without written consent of the publisher

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The first published report of the snowflake coral in Hawai'i comes from Pearl Harbor in 1972; subsequent identification as the Caribbean octocoral Carijoa riisei led to the general conclusion that it was introduced to Hawai'i from a Caribbean source. In an attempt to confirm the source of the Hawaiian population, we used mitochondrial (mtDNA) and nuclear (nDNA) sequence data to compare Hawaiian populations with samples of Carijoa (N = 244) collected worldwide. In addition, cumulative vessel traffic patterns from 1940-1979 were compiled for the Pacific Ocean to determine maritime connectivity to and from Hawai'i during the assumed time of introduction. Carijoa sampled from throughout Hawai'i (N = 94) share none of the Caribbean mtDNA haplotypes and only a single nDNA allele (0 of 28 and 1 of 27, respectively), indicating that the Hawaiian populations derive from Indo-Pacific rather than Caribbean-Atlantic origins. Furthermore, both mtDNA and nDNA show significant isolation-by-distance patterns overall and among the Pacific and Hawaiian sampling regions. These data indicate that Carijoa is native to the Indo-Pacific. Published reports of geographically wide-ranging invasions of Carijoa throughout the Pacific appear unfounded because our global sampling shows higher diversity in mtDNA (He = 0.87; π = 0.0023) and nDNA (D = 0.91; π = 0.042) throughout the Pacific relative to the Caribbean-Atlantic, indicating long evolutionary presence of Carijoa in the region. Our data clearly refute a Caribbean origin for Carijoa in the Hawaiian Archipelago, and indicate that the genus Carijoa is native to the Pacific and in need of further taxonomic evaluation.