It is difficult to keep a large number of cultured octopuses together in the same holding tank due to the high incidence of cannibalism. Appropriate shelters within which the octopuses can more readily avoid their peers are expected to ameliorate this problem. Shelters can be constructed from square plates assembled with appropriate spacing (gap) between them. The purpose of this study was to ascertain the optimum plate orientation and gap width between plates for shelters to maximize the accommodation of octopuses. It was found that the number of shelter occupants increased with the number of gaps available, such that more than half of the individuals in the experimental tank used the same shelter simultaneously. Octopuses tested singly to determine preferences for shelters placed vertically or horizontally preferred the vertical arrangement. Octopuses of more than 300 g tested singly in aquaria given shelters with different gap widths (20–100 mm) were found to not enter shelters with gap widths of 20 or 40 mm. It is concluded that octopus shelters constructed from square vertical plates spaced with gap widths suitable for octopuses are a useful contribution towards maximizing the number of octopuses that can be maintained in culture conditions.

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During a post-mortem inspection of common octopus Octopus sinensis, digenean parasites were found on the visceral surface. Parasitological examination revealed that 77 of 131 octopuses were infested, with maximum intensity of 975 digeneans. In all the retrieved digeneans, 92.3% were found to be attached to the renal sac. Species identification based on morphology was not possible because no fully mature digenean worms were obtained. Based on molecular analysis of 18S and 28S rDNA, the digenean was identified as Proctoeces cf. major. This is the first report of Proctoeces infection in common octopus in Japan.