Urchin grazing of kelp gametophytes in warming oceans.

Sea urchins can cause extensive damage to kelp forests, and their overgrazing can create extensive barren areas, leading to a loss of biodiversity. Barrens may persist when the recruitment of kelp, which occurs through the microscopic haploid gametophyte stage, is suppressed. However, the ecology of kelp gametophytes is poorly understood, and here we investigate if grazing by juvenile urchins on kelp gametophytes can suppress kelp recruitment and if this is exacerbated by climate change. We compared grazing of Ecklonia radiata gametophytes by two species of juvenile urchins, the tropical Tripneustes gratilla and the temperate Centrostephanus rodgersii, at winter (19°C), summer (23°C), and ocean warming (26°C) temperatures for the low-latitude range edge of E. radiata, which is vulnerable to ocean warming. We examined the rate of recovery of gametophytes following grazing and determined whether they survived and formed sporophytes after ingestion by sea urchins. Both T. gratilla and C. rodgersii grazed E. radiata gametophytes, reducing their abundance compared to no grazing controls. Surprisingly, temperature did not influence grazing rates, but gametophytes did not recover from grazing in the ocean warming (26°C) treatment. Gametophytes survived ingestion by both species of sea urchin and formed sporophytes after ingestion by T. gratilla, but not C. rodgersii. These results suggest complex grazer-gametophyte interactions, in which both negative (reduced abundance and poor recovery with warming) and positive (facilitated recruitment) effects are possible. Small grazers may play a more important role in kelp ecosystem function than previously thought and should be considered in our understanding of alternate stable states.

Variable food alters responses of larval crown-of-thorns starfish to ocean warming but not acidification.

Phytoplankton abundance is decreasing and becoming more variable as the ocean climate changes. We examine how low, high, and variable phytoplankton food supply affected the survival, development, and growth of larval crown-of-thorns starfish, Acanthaster sp. exposed to combined warming (26, 30 °C) and acidification (pH 8.0, 7.6). Larvae fed a low food ration are smaller, and develop slower and with more abnormalities than larvae fed a high ration. Larvae fed a variable food supply (low, followed by high ration) overcome the negative effects of low food on development rate and occurrence of abnormalities, but are 16-17% smaller than larvae fed the high ration continuously. Acidification (pH 7.6) slows growth and development and increases abnormalities regardless of the food regime. Warming slows growth and development, but these effects are mitigated by high food availability. As tropical oceans warm, the success of crown-of-thorns starfish larvae may depend on the abundance of their phytoplankton prey.

Alkalinity of diverse water samples can be altered by mercury preservation and borosilicate vial storage.

We compared the effects of preservation and storage methods on total alkalinity (AT) of seawater, estuarine water, freshwater, and groundwater samples stored for 0-6 months. Water samples, untreated or treated with HgCl2, 0.45 µm filtration, or filtration plus HgCl2, were stored in polypropylene or borosilicate glass vials for 0, 1, or 6 months. Mean AT of samples treated with HgCl2 was reduced by as much as 49.1 µmol kg-1 (1.3%). Borosilicate glass elevated AT, possibly due to dissolving silicates. There was little change in AT of control and filtered samples stored in polypropylene, except for untreated groundwater (~ 4.1% reduction at 6 months). HgCl2 concentrations of 0.02-0.05% reduced the AT of fresh, estuarine, and ground water samples by as much as 35.5 µmol kg-1 after 1 month, but had little effect on the AT of seawater. Adding glucose as a carbon source for microbial growth resulted in no AT changes in 0.45 µm-filtered samples. We suggest water samples intended for AT analyses can be filtered to 0.45 µm, and stored in polypropylene vials at 4 °C for at least 6 months. Borosilicate glassware and HgCl2 can be avoided to prevent analytical uncertainties and reduce risks related to use of Hg2+.

Echidnas of the Sea: The Defensive Behavior of Juvenile and Adult Crown-of-Thorns Sea Stars.

AbstractCrown-of-thorns sea stars are one of the most ecologically important tropical marine invertebrates, with boom-bust population dynamics that influence the community structure of coral reefs. Although predation is likely to influence the development of population outbreaks, little is known about the defensive behavior of crown-of-thorns sea stars. Righting behavior after being overturned, a key defensive response in echinoderms, was investigated for the newly settled herbivorous juvenile, the corallivorous juvenile, and adult stages of crown-of-thorns sea stars. The average righting time of the newly settled juveniles (0.3-1.0-mm diameter) was 2.74 minutes. For the coral-eating juveniles (15-55-mm diameter), the righting time (mean = 6.24 min) was faster in larger juveniles, and the mean righting time of the adults was 6.28 minutes. During righting and in response to being lifted off of the substrate, the juveniles and adults exhibited an arm curling response, during which their arms closed over their oral side, often forming a spine ball, a feature not known for other asteroids. The righting and curling responses of the corallivorous juveniles were influenced by the presence of a natural enemy, a coral guard crab, which caused the juveniles to spend more time with their arms curled. These behaviors indicate that crown-of-thorns sea stars use their spines to protect the soft tissue of their oral side. The highly defended morphology and behavioral adaptations of crown-of-thorns sea stars are likely to have evolved as antipredator mechanisms. This points to the potential importance of predators in regulating their populations, which may have decreased in recent times due to fishing, a factor that may contribute to outbreaks.

Knowledge Gaps in the Biology, Ecology, and Management of the Pacific Crown-of-Thorns Sea Star Acanthaster sp. on Australia's Great Barrier Reef.

AbstractCrown-of-thorns sea stars (Acanthaster sp.) are among the most studied coral reef organisms, owing to their propensity to undergo major population irruptions, which contribute to significant coral loss and reef degradation throughout the Indo-Pacific. However, there are still important knowledge gaps pertaining to the biology, ecology, and management of Acanthaster sp. Renewed efforts to advance understanding and management of Pacific crown-of-thorns sea stars (Acanthaster sp.) on Australia's Great Barrier Reef require explicit consideration of relevant and tractable knowledge gaps. Drawing on established horizon scanning methodologies, this study identified contemporary knowledge gaps by asking active and/or established crown-of-thorns sea star researchers to pose critical research questions that they believe should be addressed to improve the understanding and management of crown-of-thorns sea stars on the Great Barrier Reef. A total of 38 participants proposed 246 independent research questions, organized into 7 themes: feeding ecology, demography, distribution and abundance, predation, settlement, management, and environmental change. Questions were further assigned to 48 specific topics nested within the 7 themes. During this process, redundant questions were removed, which reduced the total number of distinct research questions to 172. Research questions posed were mostly related to themes of demography (46 questions) and management (48 questions). The dominant topics, meanwhile, were the incidence of population irruptions (16 questions), feeding ecology of larval sea stars (15 questions), effects of elevated water temperature on crown-of-thorns sea stars (13 questions), and predation on juveniles (12 questions). While the breadth of questions suggests that there is considerable research needed to improve understanding and management of crown-of-thorns sea stars on the Great Barrier Reef, the predominance of certain themes and topics suggests a major focus for new research while also providing a roadmap to guide future research efforts.

Diet flexibility and growth of the early herbivorous juvenile crown-of-thorns sea star, implications for its boom-bust population dynamics.

The ecology of the early herbivorous juvenile stage of the crown-of-thorns sea star (COTS, Acanthaster spp.) is poorly understood, yet the success of this life stage is key to generating population outbreaks that devastate coral reefs. Crustose coralline algae (CCA) has been considered to be the main diet of herbivorous juveniles. In this study, we show that COTS can avail of a range of algal food. Juveniles were reared on CCA, Amphiroa sp., and biofilm, and survived for 10 months on all three diets. The juveniles fed CCA and Amphiroa sp. reached 15-16.5 mm diameter at ~ 6 months and maintained this size for the rest the experiment (an additional ~4 months). Juveniles fed biofilm grew more slowly and to a smaller maximum size (~3 mm diameter). However, when juveniles were switched from biofilm to CCA they resumed growth to a new asymptotic size (~13.5 mm, 13-20 months). In diet choice experiments, juveniles did not show a preference between Amphiroa sp. and CCA, but generally avoided biofilm. Our results show that juvenile COTS grew equally well on CCA and Amphiroa sp. and can subsist on biofilm for months. Some juveniles, mostly from the biofilm diet treatment, decreased in size for a time and this was followed by recovery. Flexibility in diet, growth, and prolonged maintenance of asymptotic size indicates capacity for growth plasticity in herbivorous juvenile COTS. There is potential for juvenile COTS to persist for longer than anticipated and increase in number as they wait for the opportunity to avail of coral prey. These findings complicate our ability to predict recruitment to the corallivorous stage and population outbreaks following larval settlement and the ability to understand the age structure of COTS populations.

Training fish for restocking: refuge and predator training in the hatchery has limited benefits for a marine fish.

Large numbers of hatchery-reared fish are released in stocking programmes; however, success is limited by high mortality. Predation is seen as the main cause of deaths but might be reduced by training fish before release to avoid predators and/or use refuge. In this study on a potential restocking species, yellowfin bream Acanthopagrus australis, the effects of predator training and refuge on the behaviour of fish in the hatchery were tested. In the first experiment, juvenile bream were exposed to predatory mangrove jack (Lutjanus argentimaculatus) fed exclusively on bream flesh while housed in tanks with and without refuge. Predator training altered fish behaviour when fish were re-exposed to predators, but the effects were subtle and varied between groups of fish. In contrast, refuge created strong and consistent changes in behaviour, significantly slowing down the amount of time that fish took to consume food. A second experiment focused on the effects of refuge. Bream were trained to use artificial seagrass or house bricks as refuge and then exposed to mangrove jacks in a laboratory predation experiment. When refuge was available, fish significantly slowed down their feeding rate. There was a small, transient increase in survival for fish given seagrass refuges, but this was irrespective of whether the bream were trained to use refuge. The results of this study indicate that the use of refuge may be innate and the benefits of refuge may be available to naive hatchery-reared fish or fish trained to use refuge shortly before release. This suggests that there is potential to improve post-release survival of fingerlings without time-consuming and expensive hatchery training.

The hidden army: corallivorous crown-of-thorns seastars can spend years as herbivorous juveniles.

Crown-of-thorns seastar (COTS) outbreaks are a major threat to coral reefs. Although the herbivorous juveniles and their switch to corallivory are key to seeding outbreaks, they remain a black box in our understanding of COTS. We investigated the impact of a delay in diet transition due to coral scarcity in cohorts reared on crustose coralline algae for 10 months and 6.5 years before being offered coral. Both cohorts achieved an asymptotic size (16-18 mm diameter) on algae and had similar exponential growth on coral. After 6.5 years of herbivory, COTS were competent coral predators. This trophic and growth plasticity results in a marked age-size disconnect adding unappreciated complexity to COTS boom-bust dynamics. The potential that herbivorous juveniles accumulate in the reef infrastructure to seed outbreaks when favourable conditions arise has implications for management of COTS populations.

Building global change resilience: Concrete has the potential to ameliorate the negative effects of climate-driven ocean change on a newly-settled calcifying invertebrate.

Global climate change is driving sea level rise and increasingly frequent storm events, which are negatively impacting rapidly-growing coastal communities. To mitigate these impacts, coastal infrastructure must be further protected by upgrading hard defences. We propose that incorporating pH-buffering materials into these upgrades could safeguard marine organisms from the adverse effects of ocean acidification and ocean warming during the vulnerable transition from planktonic larvae to benthic juveniles. To test this, we examined the effects of ocean warming (24 or 27 °C), ocean acidification (pH 8.1, 7.9, 7.7), and substratum (concrete, greywacke, granite) in all combinations on the settlement success of an ecologically and commercially important sea urchin, Tripneustes gratilla. Low pH (7.9, 7.7) generally reduced the quantity and size of juveniles four weeks post-settlement, although this was partially ameliorated by increased temperature (24 vs. 27 °C). In the warmed and acidified treatments, settlement rates were lower on concrete than granite or greywacke, but two weeks post-settlement, juveniles on concrete were larger, and had longer spines and higher survival rates than on greywacke or granite, respectively. The benefits provided by concrete to newly-settled juveniles may be related to alkali chemicals leaching from concrete buffering low pH conditions in surrounding seawater and/or increased availability of bicarbonate in the boundary layers around its surface. Our results highlight the potential for pH-buffering materials to assist marine organisms in coping with the effects of changing ocean conditions, but further research is required to understand the generality and mechanism(s) driving the beneficial effects of concrete and to test pH-buffering materials in the field.

Impacts of ocean acidification on sea urchin growth across the juvenile to mature adult life-stage transition is mitigated by warming.

Understanding how growth trajectories of calcifying invertebrates are affected by changing climate requires acclimation experiments that follow development across life-history transitions. In a long-term acclimation study, the effects of increased acidification and temperature on survival and growth of the tropical sea urchin Tripneustes gratilla from the early juvenile (5 mm test diameter-TD) through the developmental transition to the mature adult (60 mm TD) were investigated. Juveniles were reared in a combination of three temperature and three pH/pCO2 treatments, including treatments commensurate with global change projections. Elevated temperature and pCO2/pH both affected growth, but there was no interaction between these factors. The urchins grew more slowly at pH 7.6, but not at pH 7.8. Slow growth may be influenced by the inability to compensate coelomic fluid acid-base balance at pH 7.6. Growth was faster at +3 and +6°C compared to that in ambient temperature. Acidification and warming had strong and interactive effects on reproductive potential. Warming increased the gonad index, but acidification decreased it. At pH 7.6 there were virtually no gonads in any urchins regardless of temperature. The T. gratilla were larger at maturity under combined near-future warming and acidification scenarios (+3°C/pH 7.8). Although the juveniles grew and survived in near-future warming and acidification conditions, chronic exposure to these stressors from an early stage altered allocation to somatic and gonad growth. In the absence of phenotypic adjustment, the interactive effects of warming and acidification on the benthic life phases of sea urchins may compromise reproductive fitness and population maintenance as global climatic change unfolds.

Indirect effects of ocean acidification drive feeding and growth of juvenile crown-of-thorns starfish, Acanthaster planci.

The indirect effects of changing climate in modulating trophic interactions can be as important as the direct effects of climate stressors on consumers. The success of the herbivorous juvenile stage of the crown-of-thorns starfish (COTS), Acanthaster planci, may be affected by the impacts of ocean conditions on its crustose coralline algal (CCA) food. To partition the direct effects of near future ocean acidification on juvenile COTS and indirect effects through changes in their CCA food, COTS were grown in three pHT levels (7.9, 7.8, 7.6) and fed CCA grown at similar pH levels. Consumption of CCA by COTS was bolstered when the COTS were grown in low pH and when they were fed CCA grown in low pH regardless of the pH in which the COTS were reared. COTS fed CCA grown at pH 7.6 grew fastest, but the pH/pCO2 that the COTS were reared in had no direct effect on growth. Ocean acidification conditions decreased the C : N ratio and carbonate levels in the CCA. Bolstered growth in COTS may be driven by enhanced palatability, increased nutritive state and reduced defences of their CCA food. These results indicate that near future acidification will increase the success of early juvenile COTS and boost recruitment into the coral-eating life stage.

A Waterborne Pursuit-Deterrent Signal Deployed by a Sea Urchin.

Selection by consumers has led to the evolution of a vast array of defenses in animals and plants. These defenses include physical structures, behaviors, and chemical signals that mediate interactions with predators. Some of the strangest defensive structures in nature are the globiferous pedicellariae of the echinoderms. These are small venomous appendages with jaws and teeth that cover the test of many sea urchins and sea stars. In this study, we report a unique use of these defensive structures by the collector sea urchin Tripneustes gratilla. In both the laboratory and the field, globiferous pedicellariae were unpalatable to fish consumers. When subject to simulated predator attack, sea urchins released a cloud of pedicellaria heads into the water column. Flume experiments established the presence of a waterborne cue associated with this release of pedicellariae that is deterrent to predatory fish. These novel results add to our understanding of how the ecosystem-shaping sea urchin T. gratilla is able to reach high densities in many reef habitats, with subsequent impacts on algal cover.

Moderate ocean warming mitigates, but more extreme warming exacerbates the impacts of zinc from engineered nanoparticles on a marine larva.

There is growing concern about the combined effects of multiple human-induced stressors on biodiversity. In particular, there are substantial knowledge gaps about the combined effects of existing stressors (e.g. pollution) and predicted environmental stress from climate change (e.g. ocean warming). We investigated the impacts of ocean warming and engineered nanoparticles (nano-zinc oxide, nZnO) on larvae of a cosmopolitan tropical sea urchin, Tripneustes gratilla. Larval T. gratilla were exposed to all combinations of three temperatures, 25, 27 and 29 °C (current SST and near-future predicted warming of +2 and + 4 °C) and six concentrations of nZnO (0, 0.001, 0.01, 0.1, 1 and 10 mg nZnO·L-1). These stressors had strong interactive effects on fertilization, gastrulation and normal development of 5 day old larvae. High concentrations of nZnO had a negative effect, but this impact was less pronounced for sea urchins reared at their preferred temperature of 27 °C compared to 25 or 29 °C. Larval growth was also impacted by combined stress of elevated temperature and nZnO. Subsequent measurement of the dissolution and aggregation of nZnO particles and the direct effect of Zn2+ ions on larvae, suggest the negative effects of nZnO on larval development and growth were most likely due to Zn2+ ions. Our results demonstrate that marine larvae may be more resilient to stressors at optimal temperatures and highlight the potential for ocean warming to exacerbate the effects of pollution on marine larvae.

Consumption of aquaculture waste affects the fatty acid metabolism of a benthic invertebrate.

Trophic subsidies can drive widespread ecological change, thus knowledge of how keystone species respond to subsidies is important. Aquaculture of large carnivorous fish generates substantial waste as faeces and lost feed, providing a food source to mobile benthic invertebrates. We used a controlled feeding study combined with a field survey to better understand the interaction between salmon aquaculture and the sea urchin, Echinus acutus, a dominant mobile invertebrate in Norwegian fjords. We tested if diets affected urchin fatty acid composition by feeding them one of three diet treatments ("aquafeed", "composite" and "natural") for 10weeks. To test if proximity to fish farms altered E. acutus fatty acid composition, populations were sampled at 10 locations in Hardangerfjord and Masfjord (Western Norway) from directly adjacent and up to 12km from farms. Fatty acids were measured in gonads and eggs in the diet experiment and in gonads and gut contents from wild animals. Urchins directly assimilated aquaculture waste at farm sites, as evidenced by elevated linoleic acid (LA), oleic acid (OA) and ∑LA, OA in their tissues. The diet experiment highlighted the biosynthetic and selective dietary sparing capacity of E. acutus in both gonads and eggs, with evidence for the elongation and desaturation of eicosapentaenoic acid (EPA) and arachidonic acid (ARA) from C18 fatty acid precursors. Elevated biosynthesis of non-methylene interrupted (NMI) fatty acids, in particular 20:3Δ7,11,14 and 20:2 Δ5,11, were also linked to a high C18 fatty acid, low ≥C20 long-chain polyunsaturated fatty acid (LC-PUFA) diet. Fatty acid composition of gonads of wild urchins indicated a highly variable diet. The study indicates that the generalist feeding ecology of E. acutus, coupled with extensive biosynthetic capacity, enables it to exploit aquaculture waste as an energy-rich trophic subsidy.

Superstars: Assessing nutrient thresholds for enhanced larval success of Acanthaster planci, a review of the evidence.

Crown-of-thorns starfish, Acanthaster planci (COTS), predation is a major cause of coral reef decline, but the factors behind their population outbreaks remain unclear. Increased phytoplankton food resulting from eutrophication is suggested to enhance larval survival. We addressed the hypothesis that larval success is associated with particular chl-a levels in tightly controlled larval:algal conditions. We used chl-a conditions found on coral reefs (0.1-5.0µgchl-aL-1), including nominal threshold levels for disproportionate larval success (≥1.0µgchl-aL-1). High success to the juvenile occurred across an order of magnitude of chl-a concentrations (0.5-5.0µgchl-aL-1), suggesting there may not be a narrow value for optimal success. Oligotrophic conditions (0.1µgchl-aL-1) appeared to be a critical limit. With a review of the evidence, we suggest that opportunistic COTS larvae may be more resilient to low food levels than previously appreciated. Initiation of outbreak populations need not require eutrophic conditions.

Climate-driven disparities among ecological interactions threaten kelp forest persistence.

The combination of ocean warming and acidification brings an uncertain future to kelp forests that occupy the warmest parts of their range. These forests are not only subject to the direct negative effects of ocean climate change, but also to a combination of unknown indirect effects associated with changing ecological landscapes. Here, we used mesocosm experiments to test the direct effects of ocean warming and acidification on kelp biomass and photosynthetic health, as well as climate-driven disparities in indirect effects involving key consumers (urchins and rock lobsters) and competitors (algal turf). Elevated water temperature directly reduced kelp biomass, while their turf-forming competitors expanded in response to ocean acidification and declining kelp canopy. Elevated temperatures also increased growth of urchins and, concurrently, the rate at which they thinned kelp canopy. Rock lobsters, which are renowned for keeping urchin populations in check, indirectly intensified negative pressures on kelp by reducing their consumption of urchins in response to elevated temperature. Overall, these results suggest that kelp forests situated towards the low-latitude margins of their distribution will need to adapt to ocean warming in order to persist in the future. What is less certain is how such adaptation in kelps can occur in the face of intensifying consumptive (via ocean warming) and competitive (via ocean acidification) pressures that affect key ecological interactions associated with their persistence. If such indirect effects counter adaptation to changing climate, they may erode the stability of kelp forests and increase the probability of regime shifts from complex habitat-forming species to more simple habitats dominated by algal turfs.

Future aquafeeds may compromise reproductive fitness in a marine invertebrate.

Aquaculture of higher trophic level species is increasingly dependent on the use of terrestrial oil products. The input of terrestrially derived n-6 polyunsaturated fatty acids (PUFA) into marine environments has subsequently increased, with unknown consequences for recipient species. We exposed a sea urchin, Heliocidaris erythrogramma to three experimental diets for 78 days: a high n-3 PUFA marine imitation treatment, a high n-6 PUFA "future aquafeed" treatment and an intermediate "current aquafeed" treatment. Female urchins fed the high n-6 PUFA diet produced larvae with lower survival rates than all other treatments. Males fed the high n-6 PUFA diet produced no viable sperm. Fatty acid composition in reproductive material revealed comprehensive biosynthetic and dietary sparing capabilities in H. erythrogramma. Despite this, the ratio of n-6 PUFA to n-3 PUFA in reproductive tissue increased significantly with diet. We suggest alterations to this ratio is the likely mechanism of negative impact on larval development.

Biogenic acidification reduces sea urchin gonad growth and increases susceptibility of aquaculture to ocean acidification.

Decreasing oceanic pH (ocean acidification) has emphasised the influence of carbonate chemistry on growth of calcifying marine organisms. However, calcifiers can also change carbonate chemistry of surrounding seawater through respiration and calcification, a potential limitation for aquaculture. This study examined how seawater exchange rate and stocking density of the sea urchin Tripneustes gratilla that were reproductively mature affected carbonate system parameters of their culture water, which in turn influenced growth, gonad production and gonad condition. Growth, relative spine length, gonad production and consumption rates were reduced by up to 67% by increased density (9-43 individuals.m(-2)) and reduced exchange rates (3.0-0.3 exchanges.hr(-1)), but survival and food conversion efficiency were unaffected. Analysis of the influence of seawater parameters indicated that reduced pH and calcite saturation state (ΩCa) were the primary factors limiting gonad production and growth. Uptake of bicarbonate and release of respiratory CO2 by T. gratilla changed the carbonate chemistry of surrounding water. Importantly total alkalinity (AT) was reduced, likely due to calcification by the urchins. Low AT limits the capacity of culture water to buffer against acidification. Direct management to counter biogenic acidification will be required to maintain productivity and reproductive output of marine calcifiers, especially as the ocean carbonate system is altered by climate driven ocean acidification.

Biochar from commercially cultivated seaweed for soil amelioration.

Seaweed cultivation is a high growth industry that is primarily targeted at human food and hydrocolloid markets. However, seaweed biomass also offers a feedstock for the production of nutrient-rich biochar for soil amelioration. We provide the first data of biochar yield and characteristics from intensively cultivated seaweeds (Saccharina, Undaria and Sargassum--brown seaweeds, and Gracilaria, Kappaphycus and Eucheuma--red seaweeds). While there is some variability in biochar properties as a function of the origin of seaweed, there are several defining and consistent characteristics of seaweed biochar, in particular a relatively low C content and surface area but high yield, essential trace elements (N, P and K) and exchangeable cations (particularly K). The pH of seaweed biochar ranges from neutral (7) to alkaline (11), allowing for broad-spectrum applications in diverse soil types. We find that seaweed biochar is a unique material for soil amelioration that is consistently different to biochar derived from ligno-cellulosic feedstock. Blending of seaweed and ligno-cellulosic biochar could provide a soil ameliorant that combines a high fixed C content with a mineral-rich substrate to enhance crop productivity.

Larval starvation to satiation: influence of nutrient regime on the success of Acanthaster planci.

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.