Abstract
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.
Highlights
COTS Larval Starvation or Satiation on the Great Barrier Reef (GBR) has decreased by around 0.53% yr-1, with an estimated 42% of this negative growth attributed to predation by A. planci outbreak populations [4,6,7]
We focused on the larval stage and its response to different food levels to determine the effects of food concentration on larval development and recruitment success of A. planci
To address our hypotheses on the response of A. planci larvae to food conditions ranging from starvation to satiation, larvae were reared under carefully controlled conditions with respect to algal density, chl a levels and food ration per larva
Summary
COTS Larval Starvation or Satiation on the Great Barrier Reef (GBR) has decreased by around 0.53% yr-1, with an estimated 42% of this negative growth attributed to predation by A. planci outbreak populations [4,6,7] These predicted values are suggested to overestimate the total impact of this seastar because they are often based on selective, small-scale studies [8]. The enhanced nutrients hypothesis has received considerable traction [14,28,37], there remains a lack of empirical data on larval responses to food regimes, especially in an ecologically robust context with respect to the algal and chl a concentrations experienced by the larvae in nature [12]. As an ecologically opportunistic species that can quickly benefit from increased food supply (e. g. runoff conditions), high nutrient levels were expected to be beneficial to larval growth and support higher survival to the juvenile stage, thereby providing empirical data to support the enhanced nutrients hypothesis
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