The effects of substratum type on the growth, mortality, and photophysiology of juvenile corals in St. John, US Virgin Islands
Coral larvae are selective with regards to the surfaces upon which they settle, but little is known about the outcome of these choices. In this study, we explored the implications for juvenile scleractinians (< 40-mm diameter) of growing on igneous versus carbonate rock on the shallow reefs (5-m depth) of St. John, US Virgin Islands. Surveys revealed that juvenile corals occurred at densities of 16 colonies m? 2 and were distributed on igneous and carbonate rocks in proportion to the abundance of these surfaces, suggesting that larvae do not discriminate between rock types at settlement. Repeated surveys demonstrated that all juvenile corals (i.e., pooled among taxa) grew 41% slower on igneous versus carbonate rock between January and August, but not between August and January when the growth was statistically indistinguishable between rock types. Although the growth of the most common juvenile coral, Porites astreoides, was similar on both substrata, the photophysiology of this species was affected by the type of rock. The maximum relative electron transfer rate (rETR, a proxy for photosynthesis) of P. astreoides was down-regulated 30% on igneous compared to carbonate rock. Phylogenetic analyses of the Symbiodinium community sequence profiles within P. astreoides revealed significant differences between substrata, with a greater diversity of co-occurring ITS-2 sequences in corals growing on carbonate compared to igneous rock. While substratum-dependent patterns in the characteristics of juvenile corals suggested there is selective value to the settlement choices made by larvae, these trends did not translate into differences in survival, at least over the time scale investigated. It remains uncertain what features of the rocks affected coral performance, but differences in the temperature of the rock may be an important feature during the warmest period of the year.