Population structure of Macrobrachium amazonicum (Heller, 1862) (Decapoda: Palaemonidae) in Miranda Hydroelectric Plant Reservoir, Araguari river, Minas Gerais, Brazil

Abstract Aim The present study aimed to analyze a M. amazonicum population structure in a reservoir of the “Triângulo Mineiro”, Brazil. Methods Monthly surveys were carried out from April/2012 to May/2013 at the Miranda Hydroelectric Plant Reservoir located in Araguari river, state of Minas Gerais. A sampling section of 100 m long, 5 m wide and depth not exceeding 1 m was defined on the banks of the reservoir. Sampling consisted on two persons quickly passing a 2 mm sieves in partially submerged marginal vegetation, for one hour. The sampled specimens were identified, sexed and measured. Results A total of 2,584 specimens were analyzed, of which 64.62% were females (3.93% ovigerous, 5.94% non-ovigerous and 54.75% juveniles), 29.16% males (21.41% adults and 7.76% juveniles) and 6.22% juveniles of undetermined sex. The sex ratio (1:2.21) indicated a clear deviation for females, for the whole sampling period, monthly and for each size class. Males had an average carapace length (CL = 4.50 ± 0.71 mm) significantly lower than females (CL = 4.64 ± 1.47 mm) and the size frequency distribution revealed a unimodal pattern, with peaks occurring in 4.0 - 4.9 mm size class for males and in 3.0 - 3.9 mm size class for females. The relation between total and carapace length was significant, indicating a negative allometric growth. No males’ morphotypes were found. The population presented a continuous reproduction with a gradual increase between April and June/2012. Female sexual maturity was determined for 6.3 mm of CL, while for males was estimated for 4.0 mm. Conclusion The Miranda reservoir population had a body size variation similar to the continental populations of Pantanal. The sexual dimorphism observed may be related to the absence of male morphotypes and to the “pure search” reproduction strategy. The deviated sex ratio for females may increase the chance of fertilization and the reproductive success. The presence of juveniles and ovigerous females throughout the year indicates a continuous reproduction pattern of the population.

摘要：研究目标
本研究旨在分析巴西“米纳斯吉拉斯三角地带（Triângulo Mineiro）”内某水库的亚马逊沼虾（M. amazonicum）种群结构。
研究方法
于2012年4月至2013年5月，在位于米纳斯吉拉斯州阿拉瓜里河沿岸的米兰达水电站水库开展月度调查。在水库岸带划定长100 m、宽5 m、水深不超过1 m的采样断面。采样流程为2名人员以孔径2 mm的筛网在半淹没的岸生植被间快速扫掠，持续1小时。对采集所得标本进行种类鉴定、性别判定及体长测量。
研究结果
共分析2584份标本，其中雌性占比64.62%（抱卵个体3.93%、非抱卵个体5.94%、幼体54.75%），雄性占比29.16%（成体21.41%、幼体7.76%），性别未确定的幼体占比6.22%。全采样周期、月度采样及各体长组的性别比（1:2.21）均显著偏向雌性。雄性平均头胸甲长（carapace length, CL）为4.50 ± 0.71 mm，显著低于雌性的4.64 ± 1.47 mm；体长频率分布呈单峰模式，雄性峰值位于4.0~4.9 mm体长组，雌性峰值位于3.0~3.9 mm体长组。总长度与头胸甲长的回归关系显著，呈现负异速生长特征。未发现雄性形态型。种群呈现连续繁殖模式，2012年4月至6月繁殖活动逐步增强。雌性性成熟临界头胸甲长为6.3 mm，雄性则为4.0 mm。
研究结论
米兰达水库种群的体长变异特征与潘塔纳尔湿地的大陆种群相似。本次观测到的雌雄二态性可能与雄性形态型缺失及“纯搜索”繁殖策略相关。雌性占优的性别比可提升受精概率与繁殖成功率。全年均有幼体与抱卵雌性出现，表明该种群具有连续繁殖模式。