Whalesuckers on spinner dolphins: an underwater view

Abstract

At Fernando de Noronha Archipelago in the tropical West Atlantic, spinner dolphins (Stenella longirostris) regularly congregate in a shallow bay where they engage in resting, social, and eliminative behaviours. The dolphins' congregation allowed underwater observations from close quarters of the whalesucker (Remora australis), an open-water fish species found attached to cetaceans only and whose habits remain thus poorly known. Whalesuckers were recorded year-round on the Fernando de Noronha spinners. Estimated size-range of whalesuckers in 211 records was 4–52 cm in total length (TL). Number of fish per dolphin was 1–3, the most frequently recorded attachment place being the host's belly (78.2%). Ratio of whalesuckers to spinner dolphins was 1–1.8 (mean = 1.32; SD ± 0.47) over one year. Juvenile, recently recruited individuals (less than 10 cm TL) were recorded year-round. Whalesuckers occasionally fed on dolphin faeces, and thus would also be able to feed on plankton while riding the spinners. Cleaning of the host's wounds was recorded twice during the study. Adult fish side by side or close to each other were regarded as an indication of reproductive pairs. The association of the whalesuckers with a middle-sized, fast and social cetacean likely provided this remora with ride, food, increased opportunity for reproduction and mating place, and perhaps protection from fast predators such as sharks, tuna, and larger dolphins.