The Florida Keys experienced some of the most drastic transitions from coral to macroalgae dominated states, known as phase-or regime-shifts, of any reefs in the Caribbean. Macroalgae on coral reefs lower coral recruitment by deterring coral settlement either directly through competition or indirectly by changing the chemical environment near the benthos. With evidence of species-specific interactions to coral-macroalgae competition, the type of macroalgae on a phase-shifted coral reef might be more important than just identifying a reef transition. To answer this question, I tested the effect of Laurencia intricata (a macroalgae related to the settlement inducing crustose coralline algae) and Dictyotaceae (known for its toxic or allelopathic compounds) on Porites astreoides planulae behavior, settlement and choice settlement preference, and post-settlement survival. I found that P. astreoides planulae show a positive response to chemical cues released from L. intricata, crustose coralline algae, and species in the Dictyotaceae family. However, the positive chemical cue response becomes algal-specific as larvae start probing for settlement substrate. Providing P. astreoides larvae with a choice between settlement substrates, revealed that the algal structure caused higher settlement next to L. intricata, while Dictyotaceae deterred larval settlement. It may be beneficial for larvae to settle next to L. intricata over Dictyotaceae algae. I identified that post-settlement survival was enhanced when P. astreoides larvae settled next to L. intricata while Dictyotaceae species did not enhance or deter post-settlement survival. These results indicate that coral larvae may be responding differently to a variety of chemical cues. Any chemical or physical cue from a reef may be used by coral larvae to identify and locate settlement substrate on a reef. Once they identify a reef’s location, they express a more selective behavior during settlement by avoiding Dictyotaceae macroalgae and favoring L. intricata. This suggests that the composition of a phase-shifted reef matters to coral recovery, not only that it has shifted to a dominated macroalgal state.

The global stressors of ocean warming and acidification, as well as local stressors such as eutrophication, overfishing, and coastal construction, have all contributed to the severe decline in coral populations worldwide. Recovery of coral reefs depends partly on recruitment, which relies on the response of larvae to settlement cues indicative of habitat quality; however, it remains unclear whether recruitment in disturbed areas will be compromised. Specifically, as reefs become more disturbed and dominated by macroalgae, it is important to understand larval behavior in response to changes in habitat quality. In this study, we first assessed the settlement success of newly released Agaricia agariciteslarvae in response to several settlement cues, including temperature and water movement. Then, to test the Desperate Larva Hypothesis, the ability of larvae of different ages (0-7 days) to settle and discriminate between inducing and inhibitory settlement cues was assessed. Newly released larvae displayed a stronger preference for settling on crustose coralline algae (93%) than on ceramic plates in macroalgae- (30%) or conspecific-treated seawater (5%), or filtered seawater (13%) (control). Older larvae became progressively less discriminatory of settlement cues, settling even in response to inhibitory cues. This confirms that, although the absence of good settlement cues initially deters settlement, larvae become desperate as they become older and settle even on unfavorable substrates, thus contributing to recruitment in poor quality habitats.