Wolfe Kennedy, Graba-Landry Alexia, Dworjanyn Symon A, Byrne Maria
School of Medical Sciences, The University of Sydney, Sydney, NSW 2006, Australia.
National Marine Science Centre, Southern Cross University, Coffs Harbour, NSW 2450, Australia.
PLoS One. 2015 Mar 19;10(3):e0122010. doi: 10.1371/journal.pone.0122010. eCollection 2015.
High density populations of the crown-of-thorns seastar, Acanthaster planci, are a major contributor to the decline of coral reefs, however the causes behind periodic outbreaks of this species are not understood. The enhanced nutrients hypothesis posits that pulses of enhanced larval food in eutrophic waters facilitate metamorphic success with a flow-on effect for population growth. The larval resilience hypothesis suggests that A. planci larvae naturally thrive in tropical oligotrophic waters. Both hypotheses remain to be tested empirically. We raised A. planci larvae in a range of food regimes from starvation (no food) to satiation (excess food). Algal cell concentration and chlorophyll levels were used to reflect phytoplankton conditions in nature for oligotrophic waters (0-100 cells ml(-1); 0-0.01 μg chl a L(-1)), natural background levels of nutrients on the Great Barrier Reef (GBR) (1,000-10,000 cells ml(-1); 0.1-1.0 μg chl a L(-1)), and enhanced eutrophic conditions following runoff events (100,000 cells ml(-1); 10 μg chl a L(-1)). We determine how these food levels affected larval growth and survival, and the metamorphic link between larval experience and juvenile quality (size) in experiments where food ration per larvae was carefully controlled. Phytoplankton levels of 1 μg chl a L(-1), close to background levels for some reefs on the GBR and following flood events, were optimal for larval success. Development was less successful above and below this food treatment. Enhanced larval performance at 1 μg chl a L(-1) provides empirical support for the enhanced nutrients hypothesis, but up to a limit, and emphasizes the need for appropriate mitigation strategies to reduce eutrophication and the consequent risk of A. planci outbreaks.
棘冠海星（Acanthaster planci）的高密度种群是珊瑚礁衰退的主要原因之一，然而该物种周期性爆发背后的原因尚不清楚。营养增强假说认为，富营养化水域中幼虫食物的脉冲增加有助于变态成功，并对种群增长产生连锁反应。幼虫恢复力假说表明，棘冠海星幼虫在热带贫营养水域中自然生长良好。这两种假说都有待实证检验。我们在一系列食物条件下饲养棘冠海星幼虫，从饥饿（无食物）到饱食（过量食物）。藻类细胞浓度和叶绿素水平被用来反映自然中贫营养水域（0-100个细胞/毫升；0-0.01微克叶绿素a/升）、大堡礁（GBR）自然背景营养水平（1000-10000个细胞/毫升；0.1-1.0微克叶绿素a/升）以及径流事件后富营养化增强条件（100000个细胞/毫升；10微克叶绿素a/升）下的浮游植物状况。在精心控制每个幼虫食物定量的实验中，我们确定了这些食物水平如何影响幼虫的生长和存活，以及幼虫经历与幼体质量（大小）之间的变态联系。叶绿素a浓度为1微克/升的浮游植物水平，接近GBR上一些珊瑚礁的背景水平以及洪水事件后的水平，对幼虫的成功发育最为有利。高于和低于这种食物处理水平时，发育情况较差。在1微克叶绿素a/升时幼虫表现增强为营养增强假说提供了实证支持，但有一定限度，并强调需要采取适当的缓解策略来减少富营养化以及随之而来的棘冠海星爆发风险。
