Setal patches influence allometric growth and sexual size dimorphism in the chelipeds of <i>Hemigrapsus</i> Dana, 1851 (Decapoda: Brachyura: Varunidae)

Abstract Some brachyuran crabs in the family Varunidae exhibit sexual dimorphism in the size and structure of the chelae. Hairy structures composed of fine setae (setal patches) on chelae show three sex-specific occurrence patterns: 1) present in both sexes, 2) present only in males, or 3) absent in both sexes. Previous allometric comparisons of species of Hemigrapsus Dana, 1851 exhibiting the first two patterns suggest that the presence and size of setal patches correlate with relative chela size, influencing the degree of sexual size dimorphism. Intra- or interspecific allometric analyses, however, have not been conducted for species in which setal patches are absent in both sexes. We examined whether setal patches affect chela allometric growth by comparing adult specimens of H. takanoi Asakura &amp; Watanabe, 2005, H. penicillatus (De Haan, 1835) (both pattern 2), and H. sanguineus (De Haan, 1835) (pattern 3). The size of male setal patches relative to chela size was smaller in H. penicillatus than in H. takanoi. Interspecific differences in chela size were evident in both sexes, although these differences were minor in females and pronounced in males. Chela size in males relative to body size was highest in H. takanoi, intermediate in H. penicillatus, and smallest in H. sanguineus, producing corresponding variation in the degree of sexual size dimorphism. These results indicate that the presence and size of setal patches influence chela allometric growth and contribute to sexual size dimorphism. Additional analyses incorporating H. sinensis Rathbun, 1931 (pattern 1) and H. sexdentatus (H. Milne Edwards, 1837) (pattern 3) from previous studies supported these patterns. The findings suggest that chelae may enlarge to provide space for maintaining hairy structures, highlighting the functional and selective roles of setal patches in agonistic and reproductive interactions in Hemigrapsus species.