Role of coral-derived chemical cues in microhabitat selection by settling Chromis viridis

Insight into the mechanisms that underlie settlement and recruitment is important for our understanding of the demography and ecology of coral reef fish and the biology of their coral host. Current knowledge of larval behaviour leading up to settlement is rather meager, and is mostly derived from controlled experiments under artificial conditions. However, it has been shown that pre-settlement juvenile fishes use acoustic and olfactory cues to locate the reef and, together with visual cues, to choose their first habitat in the reef. Chromis viridis (Pomacentridae) also use chemical and physical cues to locate the coral colonies on which they settle. Moreover, they appear to consistently and preferentially utilize some, but not other, conspecific colonies. To further evaluate the cues involved in microhabitat choice at settlement, we used in situ manipulation in which water from Acropora spp. coral colonies with positive settlement histories (SH+) was transferred to colonies with negative settlement histories (SH-) and vice versa. By closely monitoring settlement to manipulated and non-manipulated colonies, we found that at least 2 different water-borne cues are informing micro-habitat selection by C. viridis. Water transferred from SH- to SH+ A. hyacintus colonies was found to discourage settlement in the SH+ colonies. On the other hand, water transferred from SH+ to SH- A. eurystoma colonies encouraged settlement in the SH- colonies. These findings show that dissolved coral-derived cues dictate the fishes' settlement decisions, which raises an intriguing question as to the information content of these cues and their evolutionary context.