Virgin Islands National Park: Coral Reef: Juvenile Coral

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 (less than
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.