Diurnal variations in the behaviour of the hagfish Eptatretus burgeri

The Japanese hagfish Eptatretus burgeri may be found in relatively shallow water, and is thus accessible to field observations by SCUBA diving. A population of hagfish living in a bay at a depth of about 10 m in Koajiro Bay, Kanagawa Prefecture, Japan has been observed periodically throughout a year by day and night diving. During the day, with rare exceptions, the hagfish remains within its burrow in the soft, muddy substrate; its anterior tip is sometimes just visible within the single opening of the burrow. There are no heaps of mud associated with the burrows, which seem to be relatively impermanent structures. During the night, many hagfish emerge from their burrows, either partially or completely. Partially emerged individuals protrude as much as a third of their length beyond the burrow opening, while completely emerged individuals either rest uncovered on the substrate surface or swim actively near the bottom. An emerged hagfish prefers to reenter a preexisting burrow; when one is not available, however, it quickly excavates a new burrow by swimming vigorously down into the mud. No hagfish were found in the investigated locality from the middle of June to the middle of October.