Individual dietary specialization and intraspecific morphological divergence of an endemic fish species: the case of <i>Glanidium ribeiroi</i> in the Iguassu ecoregion

Because of the importance of information about endemic species and global biodiversity ecoregion, we tested the hypothesis that individuals of <i>Glanidium ribeiroi</i> (endemic species in the Iguassu ecoregion) are specialists and that this specialization decreases with increasing body size, especially concerning the size of the mouth and head, which can be limiting for the quantity and size of prey consumed by smaller individuals. Fish sampling was conducted monthly using gill nets and longlines from 2013 to 2016 in the main channel of the Iguaçu River and two tributaries. We observed the consumption of aquatic insects by smaller individuals and seeds, unidentified fish, and Decapoda by larger individuals. Individual dietary specialization was high and related to head width. The morphological traits of smaller-sized individuals were related to displacement and movement (larger peduncle), and the larger-sized individuals were related to high performance in the exploitation and prey acquisition (larger head and mouth, dorsal eyes, longer intestines). Our results highlight the importance of assessing individual variations within a population, favoring an understanding of population dynamics. In addition, morphological differences may imply differences in the use of habitat and food resources between individuals of larger and smaller body sizes and influence individual specialization.鉴于特有物种相关信息与全球生物多样性生态区的重要性，我们验证了如下假说：<i>Glanidium ribeiroi</i>（伊瓜苏生态区（Iguassu ecoregion）特有物种）的个体为食性特化类群，且该特化程度会随体型增大而降低，尤其与会限制小型个体可捕食猎物的数量与体型的口部和头部尺寸密切相关。 本研究于2013年至2016年间，在伊瓜苏河主河道及两条支流中，采用刺网（gill nets）与延绳钓（longlines）开展月度鱼类采样工作。 研究观察到，小型个体的食物组成以水生昆虫为主，而大型个体则摄食植物种子、未鉴定鱼类及十足目（Decapoda）生物。个体食性特化程度较高，且与头部宽度呈显著相关。 小型个体的形态特征与移动及活动能力相关（具更宽大的尾柄），而大型个体则具备利于猎物开发与获取的高性能形态特征：头部与口部更大、眼部背位、肠道更长。 本研究结果凸显了评估种群内个体差异的重要性，有助于深化对种群动态的理解。此外，不同体型个体间存在形态差异，这可能意味着其在栖息地利用与食物资源获取上存在分化，并会对个体食性特化程度产生影响。