Tide-related periodicity of manta rays and sharks to cleaning stations on a coral reef

Although the movements of fishes on coral reefs have been well studied, there are few data on the movement of elasmobranchs on and around cleaning stations. The visitation to cleaning stations by elasmobranchs was documented by direct observation and remote video capture at an oceanic reef in the Coral Sea and the outer Great Barrier Reef at time scales of hours to weeks. Cleaning was only observed at Osprey Reef and duration of occupancy was recorded for all elasmobranch clients. Strong tidal patterns were detected, with 49% of sharks and 59% of mantas engaging in cleaning interactions on ebb tides. Forty-four per cent of non-cleaned sharks were also observed on ebb tides. Some manta rays (n=19) were individually identified through ventral skin pigmentation to determine site fidelity; three were seen more than once with repeat observations occurring within days. This was consistent among weeks and days within weeks, regardless of time of day. Hypotheses for tidal behaviour are discussed and we argue that these observations are critical in elucidating previously unknown behaviours in elasmobranch ecology. Our study indicates that observations of large elasmobranchs at cleaning stations are another tool to elucidate elasmobranch ecology.