Seeing red: Coral larvae are attracted to healthy-looking reefs

Settlement cues play an essential role in larval habitat selection and influence post-settlement survival. Recent studies have investigated the impacts of elevated temperature and partial pressure of carbon dioxide (pCO(2)) on the ability of marine larvae to locate the reef. In coral larvae, there has been a focus on chemical settlement cues, which are critical to successful habitat selection, but less is known about the role of spectral cues. In this study, we provided larvae with crustose coralline algae (CCA), their preferred settlement surface (and chemical cue), and either a white or a red synthetic settlement surface that simulated the wavelengths emitted by bleached and unbleached CCA, respectively. We performed these experiments under 4 temperature-pCO(2) regimes to determine whether elevated temperature (+3 degrees C) or pCO(2) (900 mu atm) affected how larvae respond to settlement cues. Settlement rates increased by similar to 85% on the synthetic surface if the background colour was red compared to white. Larvae preferentially settled on the CCA chips, since these provided both a chemical and a spectral cue. However, when most of the CCA was occupied, particularly the preferred spaces along edges and in depressions, larval settlement on the synthetic surface only occurred if it appeared red. Neither elevated temperature nor elevated pCO(2) directly affected settlement rates or substrate preference, but our findings indicate that larvae could be indirectly affected by these stressors. Elevated temperatures and acidification are already known to disrupt chemical cues but may also disrupt spectral cues by causing CCA bleaching, thereby inhibiting the ability of larvae to either 'smell' or 'see' the reef.