Feeding ecology in the early life stages of the crown-of-thorns starfish, Acanthaster planci (L.)

Evaluating factors affecting survivorship during early life stages of the crown-of-thorns starfish, Acanthaster planci (L.), is essential to understand mechanisms of the population outbreaks. The objective of this study was to determine whether food availability is a crucial factor controlling the growth, development and survival of the larvae and the juveniles of A. planci. This was done through field and laboratory studies on their feeding ecology.\ud \ud Larvae were reared in an in situ rearing apparatus to assess the relative importance of different natural foods. This apparatus was designed to create different food environments inside its chambers by progressive filtration: 100 µm mesh filtered seawater (100 µm FSW), 1 and 0.2 µm filtered seawater (1 µm FSW and 0.2 µm FSW), and activated charcoal filtered seawater (ACF). In a preliminary rearing experiment, larvae successfully developed through to metamorphosis in 100 µm FSW, 1µm FSW and 0.2 µm FSW, but not in ACF. These results suggested that adequate amount of particulate and dissolved foods was available in ambient seawater to support larval development. However, subsequent deployment of the apparatus revealed that chlorophyll a concentrations inside the rearing chambers, except for ACF, were significantly higher than in ambient seawater. The initial objective was not achieved, because this apparatus created enriched food environments. The importance of food limitation needed to be assessed by means other than in situ larval rearing.\ud \ud To test food limitation, larvae were reared in freshly collected and coarse filtered seawater, and the same seawater treated by further filtration or food enrichment. Larvae were also reared in nutrient enriched seawater (NES), where the concentration of natural phytoplankton was elevated by adding nutrient solution and incubating for a few days. Larvae reared in 2 µm filtered seawater consistently failed to develop. Larval development and survival were nil or significantly slower in the coarse filtered seawater than in the same seawater enriched with cultured microalgae, dissolved free amino acids (DFAA), or in NES. When larvae were reared in NES having different fixed phytoplankton (chlorophyll a) concentrations, a change from modest larval survival to optimal survival through rapid development occurred between 0.5 and 0.8 µg ^(L-1). This range is slightly higher than typical chlorophyll a levels found in the Great Barrier Reef (GBR) waters.\ud \ud These results altogether suggest that, if total chlorophyll a concentration is used as the sole index of food availability, larvae are usually food limited. However, there are also the factors of particle size that the larvae can filter and ingest, and the contribution of dissolved organic matter (DOM) to the larval nutrition.\ud \ud The feeding rates of larvae on naturally-occurring particles, plastic beads of different sizes or cultured microalgae were measured to determine the dependence of larval feeding on particle size. The clearance rate of larvae feeding on eukaryotes (3.6-4.6 µm mean equivalent spherical diameter) was 118-358µL larva^(-1 h-1), while the rate when feeding on cyanobacteria (1-2 µm) was 0.035-0.349 µL larva^(-1 h-1). This was despite ambient conditions where cyanobacteria were magnitudes more abundant, showing that larvae selectively feed on nanoplankton (> 2 µm). Clearance rates of cultured microalgae were significantly higher than those of 6 and 20 µm beads, suggesting that feeding is also dependent on chemical properties of food particles.\ud \ud The net uptake rates of three DFAA, alanine, glutamic acid and arginine, by larvae were measured to estimate their potential contribution on the larval nutrition. Larvae selectively took up alanine (neutral amino acid) over glutamic acid (acidic) and arginine (basic). The net uptake rates of alanine from the initial concentrations of 148, 150 and 465 nM were 8.9, 15.4 and 36.9 pmol larva(-1 h-1), respectively. The alanine uptake at these substrate concentrations could account for 13.1, 23.1 and 55.3 % of the basic metabolic demand of a larva. Neutral DFAA are a potentially important food source, however, their ambient concentrations (trace - ca. 200 nM) generally seem too low to make a significant contribution to the larval nutrition.\ud \ud Post-metamorphic juveniles were reared in the laboratory and deployed in the field to test the effect of different coralline algae on growth rates. Juveniles feeding in the laboratory on the coralline alga, Lithophyllum insipidium, grew significantly faster than those feeding on the algae, Neogoniolithon clavacymosum and Lithothamnium pseudosorum. The estimated growth rate of juveniles in the field was relatively high, compared to rates in the laboratory, and was similar to the previous growth data obtained with animals feeding on multi-specific natural food. Because a wide range of algae is accessible to juveniles, food quality may not influence growth rate and hence survivorship in relatively shallow reef environments.\ud \ud Phytoplankton and DFAA were identified as potential food sources for larvae. Comparison between potential contributions of these food sources to the larval nutrition, in terms of ambient concentrations and the ability of larvae to utilise these food sources, indicates that nanoplankton are the major food. Because the availability of phytoplankton (chlorophyll a concentration) in the GBR waters is lower or marginal to the critical range, larvae are usually food limited. Food limitation is likely to have a significant impact not only on larval survival by extending the vulnerable planktonic period, but also on juvenile survival by reducing juvenile size at metamorphosis. This study confirms that food is a crucial environment factor controlling the growth, development and survival of A. planci larvae in the GBR waters

Are increased nutrient inputs responsible for more outbreaks of crown-of-thorns starfish? an appraisal of the evidence

The cause(s) of primary outbreaks of the coral-eating crown-of-thorns starfish (Acanthaster planci) are still subject to scientific controversy. The possibility of primary outbreaks being linked to terrestrial runoff has been postulated a number of times, suggesting that enhanced nutrient supply is critical for enhanced A. planci larval development. This paper examines the evidence for such a\ud cause, focusing particularly on the Great Barrier Reef (GBR). Nutrient discharges from rivers have increased at least four-fold in the central GBR over the last century, and concentrations of large phyto-plankton (>2um) of the inshore central GBR shelf in the wet season when A. planci larvae develop, is double that of other places and times. Larval development, growth and survival increase almost ten-fold with doubled concentrations of large phyto-plankton. This and other lines of evidence suggest that frequent A. planci outbreaks on the GBR may indeed be a result of increased nutrient delivery from the land

Using Long-Term Removal Data to Manage a Crown-of-Thorns Starfish Population

Background: Removal programs are effective strategies for short-term management of Crown-of-Thorns Starfish (Acanthaster spp.) populations, especially on a small scale. However, management programs are costly, and, in order to be effective, they must be based on local Acanthaster spp. population dynamics. We have developed simple models to predict the annual number of removable A. cf. solaris along the Onna coast of western central Okinawa Island, where chronic outbreaks have continued for several decades. Methods: The Onna coastal area was divided into five sectors, and annual abundance of small A. cf. solaris individuals was used to predict the total number of removable individuals of a cohort in each sector. Three models were developed, based on size class data collected by the Onna Village Fisheries Cooperative (OVFC) for 2003–2015, according to possible patterns of recruitment and adult occurrence. Using the best-fit models selected for each of the five sectors, the number of individuals that potentially escaped removal was calculated. Results: Best-fit models were likely to differ among the five sectors instead of small differences in the coefficients of determination. The models predict differences in the number of residual starfish among sectors; the northernmost sector was predicted to have a high number of residuals and the potential density of A. cf. solaris in the sector exceeded the outbreak criterion. Conclusions: These results suggest how to allocate resources to reduce the population of A. cf. solaris along the Onna coast in 2016. The OVFC implemented a control program for A. cf. solaris based on three model predictions

Selective Feeding and Microalgal Consumption Rates by Crown-Of-Thorns Seastar (Acanthaster cf. solaris) Larvae

Outbreaks of the crown-of-thorns seastar (CoTS) represent a major cause of coral loss on the Great Barrier Reef. Outbreaks can be explained by enhanced larval survival supported by higher phytoplankton availability after flood events, yet little is known about CoTS larvae feeding behaviour, in particular their potential for selective feeding. Here, single- and mixed-species feeding experiment were conducted on CoTS larvae using five algae (Phaeodactylum tricornutum, Pavlova lutheri, Tisochrysis lutea, Dunaliella sp. and Chaetoceros sp.) and two algal concentrations (1000 and 2500 algae·mL−1). Cell counts using flow-cytometry at the beginning and end of each incubation experiment allowed us to calculate the filtration and ingestion rates of each species by CoTS larvae. In line with previous studies, CoTS larvae ingested more algae when the initial algal concentration was higher. We found evidence for the selective ingestion of some species (Chaetoceros sp., Dunaliella sp.) over others (P. lutheri, P. tricornutum). The preferred algal species had the highest energy content, suggesting that CoTS selectively ingested the most energetic algae. Ultimately, combining these results with spatio-temporal patterns in phytoplankton communities will help elucidate the role of larval feeding behaviour in determining the frequency and magnitude of CoTS outbreaks

Crown-of-Thorns Starfish Larvae Can Feed on Organic Matter Released from Corals

Previous studies have suggested that Crown-of-Thorns starfish (COTS) larvae may be able to survive in the absence of abundant phytoplankton resources suggesting that they may be able to utilize alternative food sources. Here, we tested the hypothesis that COTS larvae are able to feed on coral-derived organic matter using labeled stable isotope tracers (13C and 15N). Our results show that coral-derived organic matter (coral mucus and associated microorganisms) can be assimilated by COTS larvae and may be an important alternative or additional food resource for COTS larvae through periods of low phytoplankton biomass. This additional food resource could potentially facilitate COTS outbreaks by reducing resource limitation