Selective feeding by larvae of the crown-of-thorns starfish, Acanthaster planci (L)

The effect of phytoplankton size on feeding rates of planktonic larvae of the crown-of-thorns starfish Acanthaster planci (L.) was evaluated by examining their gut contents under an epifluorescence microscope. Concentrations of coccoid cyanobacteria in natural seawater ranged between 1.73 and 5.33 x 10(5) cells ml(-1) and were three to four orders of magnitude greater than that of eukaryotes. Under these conditions, A. planci larvae ingested similar or smaller numbers of cyanobacteria than eukaryotes. Consequently, clearance rates of A. planci larvae on cyanobacteria were approximately three orders of magnitude lower than those on eukaryotes. Cyanobacteria and eukaryotes in the gut of A. planci larvae had mean equivalent spherical diameters (ESD) of 1-2 mu m and 3.6-4.6 mu m, respectively. Thus, the volume of cyanobacteria ingested was less than 10% of the volume of eukaryotes ingested. Acanthaster planci larvae were fed cultured phytoplankton Dunaliella tertiolecta and suspensions of three different sizes of plastic beads with fluorescence labelling. There was no significant difference in clearance rates on 6 and 20 mu m plastic beads. Clearance rates on 1 mu m plastic beads were, however, much lower than those on 6 and 20 mu m plastic beads. Clearance rates of A. planci larvae on D. tertiolecta (ca. 5 mu m ESD) were significantly higher than those on 6 and 20 mu m plastic beads. Apart from particle size, this result shows that feeding of A. planci larvae is influenced by other properties of potential food particles.