Biological rhythms are widely known in terrestrial and marine systems, where the behavior or function of organisms may be tuned to environmental variation over periods from minutes to seasons or longer. Although well characterized in coastal environments, phenology remains poorly understood in the deep sea. Here we characterized intra-annual dynamics of feeding activity for the deep-sea octocoral Paragorgia arborea. Hourly changes in polyp activity were quantified using a time-lapse camera deployed for a year on Sur Ridge (1230 m depth; Northeast Pacific). The relationship between feeding and environmental variables, including surface primary production, temperature, acoustic backscatter, current speed and direction, was evaluated. Feeding activity was highly seasonal, with a dormancy period identified between January and early April, reflecting seasonal changes in food availability as suggested by primary production and acoustic backscatter data. Moreover, feeding varied with tides, which likely affected food delivery through cyclic oscillation in current speed and direction. This study provides the first evidence of behavioral rhythms in a coral species at depth greater than 1km. Information on the feeding biology of this cosmopolitan deep-sea octocoral will contribute to a better understanding of how future environmental change may affect deep-sea coral communities and the ecosystem services they provide.

The number of Paragorgia arborea coral colonies with open, intermediate and closed polyps was estimated from images collected every two hours by a time lapse-camera deployed between March 6, 2020, and March 16, 2021, at a depth of 1,230 m on Sur Ridge (Monterey Bay National Marine Sanctuary). In total, 4,439 images were annotated using the PAPARA(ZZ)I version 2.8 software application (Marcon & Purser, 2017). 5 different coral colonies were monitored and classified into one of three categories: closed (retracted polyps), intermediate (polyps in intermediate state) and open (more than half of the colony with extruded polyps with fully extended tentacles).

Environmental variables (bottom pressure, bottom temperature, acoustic backscatter, current speed and direction, northward, eastward and upward flow speeds) were measured by two ADCPs (300 kHz and 1 MHz ADCPs) deployed next to the time-lapse camera between March and December 2020. Measurements recorded over a distance of 1 m (12.5 to 13.5 and 1.5 to 2.5 m above bottom for the 300 kHz and 1MHz ADCPs, respectively) were used in the analyses. Only time points matching image acquisition times are represented. ADCP data were processed with WinADCP version 1.14 (300 kHz ADCP) and Surge version 1.15.03 (1 MHz ADCP) software programs.

8-day average/standard deviation surface primary production values were downloaded from the Ocean Productivity Website (http://sites.science.oregonstate.edu/ocean.productivity/).