Larval Starvation to Satiation: Influence of Nutrient Regime on the Success of Acanthaster planci

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.

Water Quality and River Plume Monitoring in the Great Barrier Reef: An Overview of Methods Based on Ocean Colour Satellite Data

“…For example, one of the main issues facing the GBR coral reefs is the proliferation and movement of large numbers of the coral-eating Crown of Thorns starfish (COTS) [86,[108][109][110]. It has been postulated that only in periods of nutrient enrichment (river flow) are phytoplankton likely to have sufficient biomass and the correct cell type and size to support COTS larvae to a successful settlement status [66,86,110].…”

“…For example, one of the main issues facing the GBR coral reefs is the proliferation and movement of large numbers of the coral-eating Crown of Thorns starfish (COTS) [86,[108][109][110]. It has been postulated that only in periods of nutrient enrichment (river flow) are phytoplankton likely to have sufficient biomass and the correct cell type and size to support COTS larvae to a successful settlement status [66,86,110]. Agricultural development of the GBR catchment has increased delivery of nutrients to the GBR by several times since European settlement, and this delivery has occurred in pulses during wet season runoff events [11], resulting in large phytoplankton blooms [38].…”

“…Natural levels of chlorophyll-a on the GBR were determined from remotesensed reference chlorophyll-a data extracted from eReefs over the period that COTS larvae would be expected in the plankton (five months: November-March). This remote-sensed data provided background information on chlorophyll concentrations associated with hotspots of COTS outbreaks [110]. Development of metrics which can explore the long-term changes in water quality, such as the true colour products described here, which can provide quantitative evidence in the causal relationship between COTS outbreaks and water quality, would be a valuable monitoring tool.…”

The Effects of Salinity and pH on Fertilization, Early Development, and Hatching in the Crown-of-Thorns Seastar

“…Despite decades of research, the factors behind the boom and bust population cycles of COTS are not understood. The very high fecundity (up to 200 million eggs/female) and resilient larval stage indicate that success in the plankton is a key consideration in determining the causes of recruitment pulses [14][15][16][17].…”

30 Years of Research on Crown-of-thorns Starfish (1986-2016): Scientific Advances and Emerging Opportunities

“…One of the foremost hypotheses proposed to account for outbreaks of CoTS, the larval starvation hypothesis, is predicated on a link between rates of effective larval development and availability of suitable prey (mostly unicellular phytoplankton, e.g. Lucas 1982;Okaji 1996;Uthicke et al 2015b;Wolfe et al 2015a). Early studies by Lucas (1982) suggested that rates of development and survivorship increased with increasing algal concentrations.…”