Effect of particle size on feeding rates of larvae of the crown-of-thorns starfish, Acanthaster planci, from the Great Barrier Reef

The crown-of-thorns starfish (Acanthaster planci) larvae used in feeding experiments were reared at 28°C in natural seawater, enriched with a phytoplankton culture medium, at the Lizard Island Research Station until they reached the early brachiolaria stage. Seawater used in these experiments was collected 1 km offshore from the research station and filtered through a 25 µm mesh screen. Duplicate subsamples were then filtered onto .45 µm black filters for phytoplankton counts.A series of feeding experiments using natural seawater were conducted using bipinnaria larvae (4 days old) and brachiolaria larvae (6 days old). Starved larvae (n =10-23) were added to 10 ml of 25 µm filtered seawater and incubated for 15 minutes. A buffered gluteraldehyde solution was added to the vials, which were then kept refrigerated until the number and size of phytoplankton in the gut of the larvae were measured under an epifluorescence microscope. Ingested cells were also classified as either cyanobacteria or eukaryotes.Ingestion rates (number of cells ingested per larva per unit time) and clearance rates (volume of seawater cleared per larva per unit time) were then calculated on the assumption that no defaecation occurred during incubation. Starved early brachiolaria larvae were incubated for 3 to 5 minutes in suspensions of the cultured phytplankton, Dunaliella tertiolecta or various concentrations of fluorescence labelled plastic beads of 1, 6, or 20 µm diameter. Incubations were also conducted using different concentrations of mixed suspensions of the three sizes of beads. After incubation, samples were preserved and examined using the same procedures as natural seawater samples.
This research was undertaken to investigate the effects of phytoplankton size on the feeding rates of Acanthaster planci larvae. This information was used to assess whether observed changes in phytoplankton composition to larger size classes, as a response to terrestrial runoff and upwelling, affects the nutrition of Acanthaster planci larvae.