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

Settlement cues play an essential role in larval habitat selection and influence postsettlemen survival. Recent studies have investigated the impacts of elevated temperature an partial pressure of carbon dioxide (pCO2) on the ability of marine larvae to locate the reef. In cora larvae, there has been a focus on chemical settlement cues, which are critical to successful habita selection, but less is known about the role of spectral cues. In this study, we provided larvae wit crustose coralline algae (CCA), their preferred settlement surface (and chemical cue), and eithe a white or a red synthetic settlement surface that simulated the wavelengths emitted by bleache and unbleached CCA, respectively. We performed these experiments under 4 temperature-pCO regimes to determine whether elevated temperature (+ 3°C) or pCO2 (900 ?atm) affected how larva respond to settlement cues. Settlement rates increased by ∼85% on the synthetic surface if th 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 wa occupied, particularly the preferred spaces along edges and in depressions, larval settlement o the synthetic surface only occurred if it appeared red. Neither elevated temperature nor elevate pCO2 directly affected settlement rates or substrate preference, but our findings indicate that larva could be indirectly affected by these stressors. Elevated temperatures and acidification ar already known to disrupt chemical cues but may also disrupt spectral cues by causing CC bleaching, thereby inhibiting the ability of larvae to either 'smell' or 'see' the reef.