Sexual dimorphism in the gammaridean amphipod, Urothoe carda Imbach, 1967, from an intertidal sandflat in southern Japan

Abstract

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.