The contribution of ciliates as a food source to survival of first-feeding surgeonfish larvae, Paracanthurus hepatus, was examined in rearing experiments. The larvae were exposed to eight treatments; i.e. a tintinnid, Amphorellopsis acuta (1.0 × 104, 5.1 × 103 and 2.2 × 103 cells 1-1) and a naked ciliate, Euplotes sp. (1.3 × 104, 8.0 × 103 and 5.0 × 103 cells 1-1), plus two controls without ciliates. Highest survival of the larvae over the first 4-8 days was observed in the highest density of A. acuta. Rearing experiments also showed that the survivals of larvae fed with A. acuta were higher than those fed with Euplotes sp. Gut content analyses revealed loricae of A. acuta in the larvae. Although Euplotes sp. (lacking loricae) was never recognized in those larval guts, feeding on Euplotes sp. by larvae was confirmed using the ciliate labeled with fluorescent microspheres, implying that the feeding on naked ciliates by fish larvae has been overlooked. The results strongly suggested that both tintinnid and naked ciliates play important roles as alternative food sources to copepod nauplii by enhancing the survivability of fish larvae, especially those with a smaller mouth.
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