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.

珊瑚幼体对其附着的基底具有选择性，但目前对这些选择所带来的结果仍知之甚少。本研究以美属维尔京群岛圣约翰岛5米水深浅礁中的幼年石珊瑚（直径小于40毫米）为研究对象，探究其在火成岩与碳酸盐岩基底上生长的相关影响。调查结果显示，幼年珊瑚的种群密度为16个群落/平方米，其在火成岩与碳酸盐岩上的分布比例与两类基底的实际丰度相符，表明珊瑚幼体在附着阶段并未对岩性产生偏好性选择。后续重复调查显示，在1月至8月期间，所有按类群合并统计的幼年珊瑚在火成岩基底上的生长速率较碳酸盐岩基底低41%；但在8月至次年1月期间，两类基底上的珊瑚生长速率并无统计学显著差异。尽管最常见的幼年珊瑚物种——团块滨珊瑚（Porites astreoides）在两类基底上的生长速率并无显著差异，但其光生理特性会受基底岩性的影响。与碳酸盐岩基底相比，该物种在火成岩基底上的最大相对电子传递速率（rETR，光合作用替代指标）下调了30%。对团块滨珊瑚体内虫黄藻（Symbiodinium）群落序列谱的系统发育分析显示，两类基底上的虫黄藻群落存在显著差异：生长于碳酸盐岩基底上的珊瑚体内，共存的ITS-2序列多样性显著高于火成岩基底上的珊瑚。尽管幼年珊瑚的特征呈现出依赖基底岩性的模式，表明珊瑚幼体的附着选择具有选择价值，但这些趋势并未转化为存活率的差异——至少在本研究考察的时间尺度内是如此。目前仍不清楚究竟是岩石的哪些特征影响了珊瑚的生长表现，但在一年中最炎热的时段，岩石温度的差异可能是关键影响因素之一。