Ecological Complexity of Coral Recruitment Processes: Effects of Invertebrate Herbivores on Coral Recruitment and Growth Depends Upon Substratum Properties and Coral Species

BackgroundThe transition from planktonic planula to sessile adult corals occurs at low frequencies and post settlement mortality is extremely high. Herbivores promote settlement by reducing algal competition. This study investigates whether invertebrate herbivory might be modulated by other ecological factors such as substrata variations and coral species identity.Methodology/Principal FindingsThe experiment was conducted at the Flower Garden Banks, one of the few Atlantic reefs not experiencing considerable degradation. Tiles of differing texture and orientation were kept in bins surrounded by reef (24 m). Controls contained no herbivores while treatment bins contained urchins (Diadema antillarum) or herbivorous gastropods (Cerithium litteratum). Juvenile corals settling naturally were monitored by photography for 14 months to evaluate the effects of invertebrate herbivory and substratum properties. Herbivory reduced algae cover in urchin treatments. Two genera of brooding coral juveniles were observed, Agaricia and Porites, both of which are common but not dominant on adjacent reef. No broadcast spawning corals were observed on tiles. Overall, juveniles were more abundant in urchin treatments and on vertical, rough textured surfaces. Although more abundant, Agaricia juveniles were smaller in urchin treatments, presumably due to destructive overgrazing. Still, Agaricia growth increased with herbivory and substrata texture-orientation interactions were observed with reduced growth on rough tiles in control treatments and increased growth on vertical tiles in herbivore treatments. In contrast to Agaricia, Porites juveniles were larger on horizontal tiles, irrespective of herbivore treatment. Mortality was affected by substrata orientation with vertical surfaces increasing coral survival.Conclusions/SignificanceWe further substantiate that invertebrate herbivores play major roles in early settlement processes of corals and highlight the need for deeper understanding of ecological interactions modulating these effects. The absence of broadcast-spawning corals, even on a reef with consistently high coral cover, continues to expose the recruitment failure of these reef-building corals throughout the Caribbean.背景 浮游型浮浪幼虫（planktonic planula）向固着型成年珊瑚（sessile adult corals）的转变过程发生频率较低，且定居后死亡率极高。植食动物通过削弱藻类竞争来促进珊瑚定居。本研究旨在探究无脊椎动物的植食作用是否会受到其他生态因子的调控，例如基质变化与珊瑚物种属性。 研究方法与主要结果 本实验于花花园暗礁（Flower Garden Banks）开展，该区域是少数未发生显著退化的大西洋礁体之一。将不同纹理与朝向的基质板放置在环绕礁体的实验容器中（水深24米），对照组未设置植食动物，处理组容器则分别放置长刺海胆（Diadema antillarum）或食草腹足类（Cerithium litteratum）。通过摄影对自然定居的珊瑚幼体进行为期14个月的监测，以评估无脊椎动物植食作用与基质特性的影响。 植食作用在长刺海胆处理组中显著降低了藻类覆盖度。共观测到两类育幼型珊瑚幼体：石珊瑚属（Agaricia）与滨珊瑚属（Porites），二者在邻近礁体上均为常见种但并非优势类群。实验基质板上未观测到广播产卵型珊瑚的定居个体。总体而言，珊瑚幼体在长刺海胆处理组以及垂直、粗糙纹理的基质板上丰度更高。尽管石珊瑚属幼体的丰度更高，但在长刺海胆处理组中其个体尺寸更小，推测原因为过度啃食带来的破坏。不过，石珊瑚属的生长速率随植食作用增强而提升，同时观测到基质纹理与朝向的交互效应：对照组中粗糙基质板上的幼体生长速率更低，而植食动物处理组中垂直朝向基质板上的幼体生长速率更高。与石珊瑚属不同，滨珊瑚属幼体在水平基质板上个体尺寸更大，且该结果不受植食动物处理的影响。珊瑚死亡率受基质朝向影响，垂直朝向的基质板可提升珊瑚的存活率。 结论与意义 本研究进一步证实了无脊椎动物植食作用在珊瑚早期定居过程中发挥着关键作用，并强调需要深入理解调控这些效应的生态交互作用。即便在珊瑚覆盖率始终较高的礁体上，广播产卵型珊瑚仍未出现定居个体，这进一步揭示了整个加勒比海地区造礁珊瑚的补充失败问题。