Sexual size dimorphism in two endemic hermit crabs, Pagurus traversi and P. novizealandiae, in New Zealand

The evolutionary factors of sexual size dimorphism and reproductive characters in Pagurus traversi and P. novizealandiae, are discussed. Investigations and sample collection were conducted in the intertidal area around Kaikoura peninsula, southern island of New Zealand from 22 February to 23 March 2012. The number of crabs collected was 1198 P. traversi and 318 P. novizealandiae. The degree of size dimorphism ((mean male size)/(mean female size)) was 1.09 and 1.03 in P. traversi and P. novizealandiae, respectively. The fecundity relationship between clutch size and female body size (shield length, mm) in P. traversi was estimated as log10[clutch size] =1.010 + 0.794 × log10[female size3], indicating that clutch size approximately increase as an isometric relationship. All males guarded females smaller than themselves in nine precopulatory guarding pairs of P. traversi. However, difference in the mean size between the guarding males (4.79 mm) and other males (4.50 mm) was small (0.29), suggesting that large size advantage might be weak in mate acquisition compared with other species of the genus Pagurus. Weak evolutionary selection pressure acting on male body size may explain relatively small degree of sexual size dimorphism in P. traversi