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 ¼ 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.

Theoretical and computational fluid dynamics of an attached remora ( Echeneis naucrates )

“…As host speed (Reynolds number) increases, the maximum difference between the drag experienced at posterior and anterior attachment sites tends toward zero. This result casts doubt on the hypothesis that remoras are discriminating in their attachment location based on hydrodynamic reasons (Silva and Sazima, 2008), at least with respect to frictional drag, especially considering the flat-plate simulation results had slightly thicker boundary layers than the flow tank measurements, which makes them an upper bound estimate for drag reduction.…”

“…Although remoras are not considered parasites in the sense that they feed off their host, they are certainly parasites in the sense that extra swimming effort must be supplied by the host to compensate for the hitchhiking remora (Fertl and Landry, 2002;Fish et al, 2006;Weihs et al, 2007;Silva and Sazima, 2008). To estimate the increased rate of energy expenditure by the host, the parasitic drag power was computed as the product of the drag force and the host swimming speed (equivalent to the free-stream velocity with respect to the remora).…”

“…The authors suggested that remoras preferentially attach to certain sites to either minimize interference with the dolphin's habitual behaviors and sensitive areas (such as those responsible for vision, respiration, swimming, mating, etc.) or for hydrodynamic reasons (Silva and Sazima, 2008). While the first conclusion is almost self-evident, the latter suggests that remoras are taking advantage of reduced drag associated with the host's fluid boundary layer.…”

“…For certain simple swimming gaits such as gliding between bouts of active swimming, this technique can deliver reasonable results (Webb, 1975;Miller et al, 2004); however, for more complex swimming gaits the body motions of the host (swimming kinematics) must be included to deliver accurate drag estimations 4 (Webb, 1975;Fish, 1993;Anderson et al, 2001). With respect to remoras, it has been observed that there is a high degree of attachment site fidelity (Strasburg, 1962), and although it has been noted that remoras may reposition themselves on the host (Ritter, 2002;Silva and Sazima, 2008), they typically cease undulatory motion once attachment is created (Sewell, 1925). Therefore, because remoras naturally glide through the water under the power of their host, treating a remora as a rigid body should provide a reasonable estimate of the drag associated with attachment.…”

“…For example, a study by Silva and Sazima (2008) observed the frequency with which remoras (Remora australis) attached to various locations on spinner dolphins (Stenella longirostris). The authors suggested that remoras preferentially attach to certain sites to either minimize interference with the dolphin's habitual behaviors and sensitive areas (such as those responsible for vision, respiration, swimming, mating, etc.)…”