Life in the shadows of the reef: species assemblages in cryptic coral habitats

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Angela Chun, Valerie Gracia, Leslie Nguyen, Nhi Ly, Chase McCoy and Maya S. deVries, Biological Sciences, San José State University, San Jose, CA, Emily L.A. Kelly, Stanford Center for Ocean Solutions, Stanford University, Palo Alto, CA, Stuart A. Sandin, Center for Marine Biodiversity and Conservation, Scripps Institution of Oceanography, La Jolla, CA

Background/Question/Methods
Coral reefs are among the most biodiverse ecosystems in the world. These massive structures have many tunnels and crevices that create cryptic habitats for reef organisms. Cryptic habitats are estimated to comprise 30-75% of the reef structure. However, the majority of reef species and the processes driving cryptic coral ecosystems remain unexplored in situ because of the difficulty of accessing internal reef structures non-destructively. Here, we investigate species assemblages in cryptic coral reef habitats with GoPro videos.
From July to August 2017, we gathered video data on SCUBA at 5-10 m depth at 16 different locations around West Maui, Hawai’i. Specifically, we used a GoPro HERO4 on a selfie stick to record video data. The camera was placed haphazardly inside 98 different crevices, the openings of which ranged in size from 10-30 cm. The camera was positioned 20-60 cm inside crevices and left undisturbed for 6-12 minutes. Preliminary analyses of the crevices filmed consisted of identifying and quantifying mobile species as well as documenting the presence or absence of benthic species, namely sponges, cyanobacteria, and algae. Benthic algae were further classified as turf, crustose coralline algae (CCA), green, red, brown, fleshy, and calcified.
Results/Conclusions
Within cryptic habitats, 21 mobile animals were identified. The Hawaiian whitespotted toby (Canthigaster jactator) and iridescent cardinalfish (Pristiapogon kallopterus) were observed most frequently. Percent abundance of vertebrates (fish) and invertebrates was 91% and 8%, respectively, across all sites. Analysis of benthic assemblages revealed that turf, CCA, cyanobacteria, and sponges were present in 98%, 81%, 53%, and 41% of the crevices filmed, respectively. Genera of red algae, including Peyssonnelia, Jania, and Amansia, were present in 20% of crevices.
These results suggest that turf and CCA likely dominate cryptic habitats in Maui. Turf and cyanobacteria are generally productive components of the reef benthos, and CCA signals calcium carbonate accretion that ultimately helps build reefs. Sponges, while present to a lesser extent, are important for nutrient cycling inside reefs. Low light adapted red algae were also present, indicating light availability as a driver of species assemblages. Compared to extensive literature on the benthos of the top of the reef, these results suggest that cryptic coral habitats harbor unique species assemblages that likely help govern ecosystem dynamics. Future work combining cameras that better integrate with the habitat and environmental measurements in situ, will help further uncover key patterns and processes driving life inside coral reefs.