Peer Review #1 of "Long-term study of behaviors of two cohabiting sea urchin species, Mesocentrotus nudus and Strongylocentrotus intermedius, under conditions of high food quantity and predation risk in situ (v0.1)"

Abstract

Background. In the predator-sea urchin-macrophyte trophic cascade, the ecological effect of sea urchins as grazers depends both on their density and the changes in foraging activity, which are influenced by various disturbing factors. However, the complete duration of the alarm reactions of echinoids has not been studied until now. Here, we tested a hypothesis that two cohabiting sea urchins, Mesocentrotus nudus and Strongylocentrotus intermedius, which differ morphologically, might display different behavioral responses to high hydrodynamic activity and predation. Methods. We used continuous time-lapse video recording to clarify behavioral patterns of M. nudus and S. intermedius in presence of a large quantity of food (the kelp Saccharina japonica) but under different weather conditions and different types of predation threat: (1) calm weather conditions, (2) stormy weather conditions, (3) predation risk associated with the presence of several sea star species, and (4) predation risk associated with an alarm stimulus (crushed conspecifics or heterospecifics). Three separate video recording experiments (134 days in total) were conducted under field conditions. Video recording analysis was performed to determine the number of specimens of each sea urchin species in the cameras' field of view, size of sea urchins' groups, movement patterns and the duration of the alarm responses of both sea urchin species. Results. We showed that in the presence of kelp, M. nudus and S. intermedius exhibited both similar and different behavioral responses to hydrodynamics and predation threat. Under calm weather, movement patterns of both echinoids were similar but M. nudus exhibited the higher locomotion speed and distance travelled. Furthermore, S. intermedius but not M. nudus tended to group near the food substrate. The stormy weather caused a sharp decrease in movement activity followed by escape response in both echinoids. Six starfish species failed to predate on healthy sea urchins of either species, and only a few attacks on ailing