Confronting the ‘nocturnal problem’ in coral reefs: sleeping site selection and cocoon formation in parrotfishes

Habitat utilization is a major component of animal behavior and ecology, yet nighttime habitat utilization by diurnal animals remains understudied for many taxa. Sheltered sleeping sites are considered particularly important for parrotfishes (Labridae: Scarini) inhabiting coral reefs, where predation risk is most prominent at night. Parrotfishes are also known to sleep in mucus cocoons, which allegedly function as a means of predator and/or ectoparasite avoidance. In this study, we present the first systematic characterization of nocturnal habitat utilization and cocoon formation in parrotfishes, in their natural environment. We performed a total of 153 surveys and sampled 225 individual parrotfishes (of 11 species) and characterized their sleeping sites. We found that several species utilized sleeping sites with distinct characteristics, such as exposed sites that provided little to no coverage or sleeping sites well above the reef floor. However, most species exhibited substantial overlap in site characteristics and exposure. We found that the utilization of sheltered sleeping sites did not correlate with body size or life stage, suggesting that sleeping-site selection in parrotfish is unlikely to be driven by competitive exclusion. We also found that while some species either never or rarely formed cocoons, many species exhibited facultative cocoon formation that was not clearly associated with exposure or habitat. While we expected cocoons to be more frequent in exposed sites to avoid predators, or on soft substrates to avoid ectoparasites, we found no effect of either predictor. As many species exhibited a wide range of sleeping sites and behaviors, nocturnal habitat utilization does not appear to be a major axis for niche differentiation in this guild.