Novel, complex burrow structure and burrowing behaviour of the mud-dwelling octopus (Octopus minor, Sasaki 1920)

: The strategies employed by octopuses for predator avoidance and escape, which are adapted to their structurally simple habitat, have been the subject of much research. In the pres-ent study, the shapes and structural characteristics of Octopus minor’s burrows were investigated and the burrowing behavior was also observed to reveal the dynamics of burrow formation. From 2012 to 2017, 85 plasticized cement burrow models were measured. The burrow, a complex project, includes 7 interconnected structural parts: digging holes (DH), digging channel (DC), horizontal channel (HC), lounge (LG), breathing channel (BC), breathing holes (BH), and breathing hole heap (BHH), with each part having its own cross-sections of special shape and size. The diameters of all parts were very significantly different and had extremely significant correlations with other factors, except for the weight of the occupants. The burrows have 1−2 DHs, 1−4 BHs and 1 BHH, with DHs and BHs distributed at opposite ends of the burrow. The burrows were categorized into 7 types according to the number of DHs, and burrows with 2 BHs accounted for a distinct type. The diameters of LG, DC and HC and the occupants’ weight decreased as the number of BHs increased. The process of excavating a burrow involved 5 steps: creating a DH, inserting arms into the DH, burrowing, excavating the BC, and creating the LG. Abandoned burrows could be reoccupied by other octopuses, regardless of whether they were larger or smaller than the previous occupants. As an adaptation to structurally simple environment, O. minor seems to use a particular skill in digging more complex burrows as shelters.