Sound detection in situ by the larvae of a coral-reef damselfish (Pomacentridae)

Abstract

Settlement-stage pelagic larvae of the coral-reef damselfish Chromis atripectoralis consistently swam to the south at 24 to 25 cms-1 in day-time ambient conditions off Lizard Island, Great Barrier Reef. This was true on both the windward and leeward sides of the island, 100 to 1000 m from the nearest reef. Larvae released during the day 25 to 100 (mean 58) m from an underwater speaker broadcasting nocturnal reef sounds had no overall swimming direction. This was true on both windward and leeward sides of the island, 500 to 1000 m from the nearest reef. The broadcast sounds resulted in an alteration of behavior indicating that the larvae heard them. In the presence of the nocturnal reef sounds, swimming speed increased about 5 cm s-1 off the leeward side but not the windward side. Larvae released 50 to 150 (mean 78) m from a speaker broadcasting artificial sound (pure tones) at the leeward location swam to the south at 30 cm s-1. This shows that larvae of C. atripectoralis can distinguish between a sound with potential biological significance and one devoid of biological significance. Larvae did not swim in any particular direction relative to the speaker when nocturnal reef sounds were broadcast; therefore, we have no evidence that the larvae can localize these sounds. We conclude that settlement-stage larvae of this damselfish can hear reef sounds, and can distinguish between reef sounds and an artificial sound, but we have no indication that they can localize the sound. We speculate on the reasons for altered swimming behavior in the presence of reef sounds.