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

Okaji, Ken (1996) Feeding ecology in the early life stages of the crown-of-thorns starfish, Acanthaster planci (L.). PhD thesis, James Cook University.

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Abstract

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.

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.

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.

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.

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.

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.

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.

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.

Item ID: 27235
Item Type: Thesis (PhD)
Keywords: crown-of-thorns starfish; Acanthaster planci; survivorship; food selection; larval nutrition; food availability
Date Deposited: 24 Jun 2013 23:33
FoR Codes: 06 BIOLOGICAL SCIENCES > 0608 Zoology > 060806 Animal Physiological Ecology @ 33%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 34%
06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060303 Biological Adaptation @ 33%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 49%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 51%
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