High abundances of crustose coralline algae inside cryptic coral habitats linked to coral reef functioning.

Cryptic habitats on coral reefs consist of crevices, tunnels, and holes that are estimated to comprise 30%-75% of the total reef surface area. These large habitats are vastly understudied because they are often difficult to access. Crustose coralline algae (CCA) are thought to perform important ecological functions, including calcium accretion, both inside cryptic habitats and on the exposed (top) reef. Using GoPro cameras, we surveyed 250 coral crevices from 13 reefs on West Maui, Hawai'i. We compared relative CCA cover between crevice microhabitats and the well-studied top-reef and identified abiotic and biotic factors that correlated with CCA abundance. We found that crevices had approximately 3.1 times more CCA cover than the top-reef and that CCA cover was highest on crevice ceilings and entrances, which had low sediment and macroalgal cover, compared to the back walls of crevices, which had higher sediment abundance. These results suggest that crevice openings and ceilings are key microhabitats for CCA and therefore may also be important for reef building and stabilization. Thus, these findings help establish functional links between coral reef structural complexity, cryptic habitats, and reef resilience, and highlight the importance of studying cryptic habitats to inform approaches to coral reef conservation.