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15 August 2022 Ontogenetic Expressions of Sexually Dimorphic Traits in the Skeleton Shrimp Caprella scaura (Crustacea: Amphipoda)
Yohei Otomo, Junpei Shinji, Hisanori Kohtsuka, Toru Miura
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In sexual dimorphism, males often exhibit exaggerated characters as weapons or ornaments. Among the numerous amphipod species (Amphipoda, Crustacea) showing sexual dimorphism, caprellids (Caprellidae) are characterized by considerably larger males that possess weapons, although the developmental processes underlying these sex-related differences are largely unknown. Therefore, to clarify the process of sexual differentiation during postembryonic development in caprellids, morphometric analyses of Caprella scaura were conducted. Principal component analysis using 31 morphometric traits showed drastic allometric changes occurring at two ontogenetic body length (BL) points (i.e., 3.8 and 8.8 mm). In individuals larger than 3 mm, head spines appeared in both sexes, and penises did only in males, allowing the discrimination of juveniles from larvae. Moreover, in larger males (BL > 8.8 mm), traits used in reproductive behavior, i.e., the first antenna, second gnathopod, and first to fifth pereonites, were extremely exaggerated. Observations of pre-copulatory behavior along with morphological assays revealed that sexually mature males could be identified by the size ratio between the triangular projection and palmar spine on the propodus of the second gnathopod. In contrast, female maturation could be determined by the marginal setae of oostegites forming a brood pouch. The body size distribution of sexually mature females was concentrated within a narrow range of BLs (6–9 mm), whereas that of sexually mature males showed a broader range (BL 9–18 mm), suggesting that, in C. scaura, males continue to molt and grow even after sexual maturation via indeterminate growth, to increase their lifetime reproductive success.

© 2022 Zoological Society of Japan
Yohei Otomo, Junpei Shinji, Hisanori Kohtsuka, and Toru Miura "Ontogenetic Expressions of Sexually Dimorphic Traits in the Skeleton Shrimp Caprella scaura (Crustacea: Amphipoda)," Zoological Science 39(5), 431-445, (15 August 2022).
Received: 14 April 2022; Accepted: 23 May 2022; Published: 15 August 2022
indeterminate growth
ontogenetic allometry
precopulatory behavior
sexual dimorphism
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