Barricade building in Ilyoplax pusillus (De Haan) (Crustacea : Brachyura)

Ilyoplax pusillus (De Haan, 1835) was sometimes found to build an earthen structure, termed a barricade, close to its neighbour's burrow. Most barricaders were males > 6.4 mm in carapace width. Either males or females were barricaded and almost all of them were smaller than their barricaders. Burrow to burrow distance between a barricader and its barricaded neighbour ranged from 1.6 to 7.2 cm. The home range of a barricaded crab was biased toward the areas devoid of the barricade, i.e. to other directions than the barricader's burrow, in either the presence or absence of the barricader. When a barricade was removed, the crab freed from it extended its home range towards the barricader's burrow. If the crab freed from a barricade moved towards the barricader, the former was frequently repelled by the latter. These findings suggest that barricades function as an aid to territorial defence by deterring invasion by a neighbour.