25Species of the free-burrowing amphipod genus, Urothoe, are common macrobenthos on open sandy 26 beaches. On intertidal sandflats, some species are associated with burrows or tubes of large infauna. 27How this link is formed under sheltered settings was examined. On an intertidal sandflat emersed for 28 300 m seaward in mid-western Kyushu, Japan, U. carda co-occurred with the deep burrow-dwelling 29 callianassid shrimp, Nihonotrypaea harmandi. Amphipods resided in the surface 5-cm sediment 30 outside shrimp burrows, as confirmed by sediment coring and burrow casting. In the summertime 31 during 1980 to 1981, the shrimp and amphipod populations were confined to the upper shore at 32 mean densities of 182 and 701 inds m -2 , respectively. In winter to spring, when the sediment surface 33 mixing was caused by seasonal wind-induced waves, only the amphipod extended distribution to the 34 lowest shore. By 1983, the shrimp increased mean density by 2.5 times and distribution range to the 35 lowest shore. In the summers of 1984, 2010, and 2015, the amphipod extended distribution to the 36 lowest shore, with small variations in population size. Three marked changes in substrate properties 37 were associated with the shrimp inhabitation: thicker oxidized layer (proxy for oxygenated layer) in 38 the sediment column; looser surface sediment, as evaluated with vane shear strength; and coarser 39 and better-sorted surface sediment with less mud content. At least the former two changes were 40 attributable to shrimp bioturbation, which could provide the amphipod with more permeable and 41 softer substrates, leading to the formation of facultative commensalism. 42

Species of the gammaridean amphipod genus, Urothoe, commonly occur on intertidal and shallow subtidal sandy bottoms. Sexual dimorphism is reported for some species of Urothoe, but the descriptions are semi‐quantitative, lacking allometric growth models. A distinct sexual dimorphism was detected in Urothoe carda Imbach, 1967 collected from an intertidal sandflat in western Kyushu, Japan, in March, June, August, and October, 2015. The analysis for the total length–frequency distribution and maturity in each sex showed a female‐biased sex ratio (3.6 for mature individuals). Mature males had the larger eye and longer antenna 2 versus total length, expressed as generalized linear models (gamma distribution; logarithmic link‐function). Based on other studies on Urothoe spp. and peracarids, the above differences may suggest higher mate searching activity in male U. carda, with its larger eye functioning during the nocturnal excursion into the water column and its longer antenna 2 equipped with calceoli effective in detecting receptive females in the sediment.