. The Canadian field-naturalist. Natural history; Sciences naturelles. 2005 Harvey-Clark, Gallant, and Hart: Vision in St Lawrence River Greenland Sharks 357 head-down attitude with the front of the animal m above the substrate, the trunk slanted upward and the tail held higher off the bottom, the dorsum curved upward, the axis of the body being held approximately 10 degrees off horizontal, and the mouth held slightly open (Figure 4). This posture was maintained from one to several seconds, after which the shark would accel- erate and swim away from the diver or change course to avoid
. The Canadian field-naturalist. Natural history; Sciences naturelles. 2005 Harvey-Clark, Gallant, and Hart: Vision in St Lawrence River Greenland Sharks 357 head-down attitude with the front of the animal m above the substrate, the trunk slanted upward and the tail held higher off the bottom, the dorsum curved upward, the axis of the body being held approximately 10 degrees off horizontal, and the mouth held slightly open (Figure 4). This posture was maintained from one to several seconds, after which the shark would accel- erate and swim away from the diver or change course to avoid the diver. This motor display was repeatedly exhibited by the four sharks we observed when close- ly approached. Discussion Arctic Greenland Shark populations have high infection rates with the ocular copepod parasite Om- matokoita elongata (Grant, 1827), which has been found in up to percent of individuals in surveyed populations (Berland 1961), and in 100 percent of individuals in a recent tracking study off Baffin Island (Skomal and Benz 2004). Based on histopathological findings in copepod-parasitized eyes, Borucinska et al. (1998) postulated that sharks infected with ocular Ommatokoita were likely to be blind. However, the sharks we observed were free bilaterally of external signs and lesions of this parasite, had clear corneal epithelium, showed definite visual orientation to divers, tracked divers with horizontal and vertical eye move- ment, repeatedly demonstrated visual avoidance of divers and objects on the bottom, and repeatedly exhi- bited the described behaviour when closely approached. Vision is thought to be a major sense for investigative and social behaviour, prey recognition and predation motor patterns in the majority of shark species (Gruber 1977). The visual acuity of the non-parasitized sharks we observed in the St. Lawrence River may have pro- found implications for the social behaviour and pat- terns of predation of this population. The reason thes
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