Are COTS larvae food limited in the Great Barrier Reef waters?
Ken Okaji
R & D Dept., KANSO Co., Japan
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, 1 μm 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 118-358 μL 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).
