Selection of shelter shape by swimming crab (Portunus trituberculatus)

Abstract Placing shelter is an effective way to mitigate cannibalism in the pond culture of swimming crab (Portunus trituberculatus). However, it is still unclear what shape of shelter the crabs prefer. To determine their preference for shelter shapes (circular, square, semicircular, and triangular), we quantified the counts and duration of the crabs occupying the different shelters. We also calculated the probability of fighting and abandoning these shelters. The results reveal that: (1) The z-scores for circular shelters were significantly higher than others, but the differences among the z-scores for square, semicircular, and triangular shelters were not significant. These indicate that the crabs had a significant preference for circular shelter when all four shapes were present in the same area. (2) When only one shape was present, the crabs spent more time occupying circular shelters and had a higher count of occupations. The differences in the counts and durations of occupation among the non-circular shelters were also not significant. (3) When the invasive crab was present, the occupancy counts and duration of the focal crab significantly increased, but its preference for the circular did not substantially change. (4) When only circular shelter was available, the aggression of the crabs was reduced, and the probability of abandoning shelter was also the lowest. These results suggest that swimming crabs have a preference for circular shelter and exhibit a relatively low willingness to fight in the arena with the circular shelter. The structural properties of circular shelters account for the observed preference of the crabs. The circular shelters are recommended to be used to reduce cannibalism and improve survival rates in swimming crab pond cultures.