A citizen science approach to monitoring bleaching in the zoantharian Palythoa tuberculosa

Coral reef bleaching events are expected to become more frequent and severe in the near future as climate changes. The zoantharian Palythoa tuberculosa bleaches earlier than many scleractinian corals and may serve as an indicator species. Basic monitoring of such species could help to detect and even anticipate bleaching events, especially in areas where more sophisticated approaches that rely on buoy or satellite measurements of sea surface temperature are unavailable or too coarse. One simple and inexpensive monitoring method involves training volunteers to record observations of host color as a proxy for symbiosis quality. Here, we trained university students to take the 'color fingerprint' of a reef by assessing the color of multiple randomly selected colonies of P. tuberculosa at one time point in Okinawa Island, Japan. We tested the reliability of the students' color scores and whether they matched expectations based on previous monthly monitoring of tagged colonies at the same locations. We also measured three traditional metrics of symbiosis quality for comparison: symbiont morphological condition, cell density, and chlorophyll a content. We found that P. tuberculosa color score, although highly correlated among observers, provided little predictive power for the other variables. This was likely due to inherent variation in colony color among generally healthy zoantharians in midwinter, as well as low sample size and brief training owing to the course structure. Despite certain limitations of P. tuberculosa as a focal organism, the citizen science approach to color monitoring has promise, and we outline steps that could improve similar efforts in the future.