Backing up from negative stimuli: A back-thrust mechanism during escape-like response in wild sevengill sharks (Notorynchus cepedianus)

Abstract

Broadnose sevengill sharks (Notorynchus cepedianus) show great interest for bait and display arepertoire of movements while engaging with it. A novel back-thrust mechanism is described in wild sevengill sharks by which individuals back up from a negative stimulus while interacting with baited video stations. This mechanism initiates upon head contact with the device that functions as a negative stimulus eliciting a startle escape-like response. By heavily flipping pectoral fins and curving the body, sharks increase hydrodynamic resistance, backing up from the negative stimulus. Once backed up, sharks performed the common C-shaped double-bend escape maneuver described for sharks. Sharks also used the same back-thrust mechanism as a repositioning maneuver, but not as part of a startle response. The quantification of the turning rate indicated context-dependent variation in velocity and confirmed that the majority of withdrawals corresponded to slow escape- like motions. In general, an elongated body and individual flipping control of pectoral fins allowed for great maneuverability and lateral flexure. Sharks exhibited great tolerance to one another during double and triple encounters. The implications for grouping and social hunting of the species are briefly discussed based on past evidence and the movement behavior, gregarious interactions and body markings observed in the present study. This work highlights the importance of studies in the natural environment, and the use of complementary approaches to investigate the broader range of locomotor aspects of different shark species.