The bluntnose sixgill shark (Hexanchus griseus) is a globally distributed large-bodied predator, primarily associated with deep-sea environments. Due to the logistical constraints of its habitat, much remains unknown about the life history and behavior of H. griseus. While typically considered solitary, this species has occasionally been observed in aggregate, however, these events are almost exclusively associated with foraging or the presence of introduced bait. Here, we present a novel unbaited observation of a mixed-sex aggregation of at least seven H. griseus individuals near the Ulong Channel, Palau. These represent the first published photos of such an aggregation in Micronesia. Observed in close proximity at relatively shallow daytime depths via a Remotely Operated Vehicle (ROV), the individuals did not exhibit foraging behavior. The absence of attractants and presence of scarring on female individuals suggest potential social or reproductive drivers for this aggregation. A subsequent re-sighting of two individuals at the same location indicates short-term site fidelity. These findings provide rare in situ insight into the social structure of H. griseus and highlight the potential importance of this area as a critical habitat for this species.

All data were collected using a Boxfish Luna remotely operated vehicle (ROV). The instrument is outfitted with a camera (Sony A7Siii) and LED lights. The ROV is controlled by a pilot onboard a skiff, to which it is connected via a 2.4 mm fibre optic tether. This is a mobile, unbaited unit. Sampling methods involved exploratory video capture and standardized video transect surveillance. Once site characteristics were established at a mission’s deepest depth, the ROV navigated to standardized depths to conduct 10-minute isobath surveys across the substrate, maintaining a fixed depth and speed. In the intervals between depth-stratified transects, the team captured exploratory photos and video.