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There has been a debate over whether the growth and development of the larvae of the crown-of-thorns starfish Acanthaster planci are severely food-limited. This debate has raised a range of questions, including the one relating to the role of heterotrophic bacteria in the nutrition of larvae. In this study, the feeding rate of larvae on bacteria as well as on other ultraplankton (<5 {mu}m) was determined by counting the number of the fluorescence-labeled cells (FLC) in the gut after short incubation. Preliminary experiments showed no detrimental effect of the fluorescence dye (5-(4,6-dichlorotriazin-2-yl) aminofluorescein) on the development of larvae and demonstrated the usefulness of FLC in feeding experiments as food particles analogous to living cells of ultraplankton. There was no evidence that larvae ingested bacteria. Larvae did ingest two strains of photosynthetic cyanobacteria, which had equivalent spherical diameters (ESD) of 1 and 1.8 {mu}m, but these tiny cells were cleared more than 10 times slower than the larger algae Phaeodactylum tricornutum (4.7 {mu}m ESD) and Dunaliella tertiolecta (5.1 {mu}m ESD). Regardless of the size of FLC used, the clearance rate (volume of water cleared per animal per unit time) increased by 50-120%, as larvae developed from the late bipinnaria stage to the late brachiolaria stage. These results show that larvae may derive a sizable proportion of their nutrition from ultraplankton, but not from bacteria.