ABSTRACT Rensch’s rule attempts to explain the evolutionary processes that have resulted in the current patterns of sexual size dimorphism of several taxa. This rule predicts that sexual size dimorphism increases according to the average body size, when males are larger, decreasing when females are the largest sex. In decapod crustaceans, the general pattern of sexual size dimorphism is males with hypertrophied structures used in agonistic disputes, and females with a greater expansion of the abdominal chamber. We analyzed sexual size dimorphism of the freshwater shrimp Macrobrachium jelskii and its relationship to Rensch’s rule, as well as the relative growth between the sexes, for subpopulations from the Rosário reservoir in Ceará state, Brazil. The results of sexual size dimorphism show that females are morphometrically larger than males. Such a pattern of sexual dimorphism probably results from the selection of female fecundity, giving them greatest fecundity. This pattern indicates a female-biased SSD for this species, supported by what is proposed in Rensch’s rule. In the relative growth analysis, the males and females showed negative allometry of the cheliped. The absence of cheliped hypertrophy in males may indicate the presence of ‘pure-search’ reproductive behaviour, thus, no agonistic disputes for females.

中文翻译：

---

淡水虾 Macrobrachium jelskii (Miers, 1877) (Decapoda: Palaemonidae) 的性别大小二态性及其与 Rensch 规则的关系

摘要 Rensch 的规则试图解释导致当前几种分类群性别大小二态性模式的进化过程。该规则预测性别大小二态性根据平均体型增加，当雄性较大时，当雌性最大时减少。在十足目甲壳类动物中，性别大小二态性的一般模式是雄性具有用于争斗的肥大结构，而雌性具有更大的腹腔扩张。我们分析了巴西塞阿拉州 Rosário 水库亚种群的淡水虾 Macrobrachium jelskii 的性别大小二态性及其与 Rensch 规则的关系，以及两性之间的相对增长。性别大小二态性的结果表明，女性在形态测量上比男性大。这种两性异形的模式可能是由于对雌性生殖力的选择，赋予了它们最大的生殖力。这种模式表明该物种的 SSD 偏向女性，这得到了 Rensch 规则中提出的支持。在相对生长分析中，雄性和雌性都显示出螯足的负异速生长。雄性没有螯肢肥大可能表明存在“纯搜索”生殖行为，因此，雌性没有争斗。