The Neotropics are considered to represent one of the most biologically diverse areas on Earth. Nonetheless, many species are assumed to have widespread distributions and occur in the entire Neotropical range. However, many freshwater invertebrates such as ostracods challenge this contradiction since they live in discrete habitats and possess no active dispersal abilities. The freshwater ostracod Cytheridella is a prime example for this paradigm. From three extant species, only one is described to occur ubiquitously within the Neotropics. Examination of morphological variability is the prerequisite for identification of environmentally induced variations, estimation of inter- and intraspecific variability, and, ultimately, the distinction of species. This study focuses on the quantitative investigations of the appendages of Cytheridella from several living populations sampled in Florida, Mexico, Colombia, and Brazil. The morphological traits including podomere ratios and limb-limb ratios, showed that the largest variability occurs at the antennae, and proved a geographical structure. Soft parts reflect the morphological divergence of regional Cytheridella priorly demonstrated by valves shape variability. Additionally, allometric coefficients, limb dimensions and body proportions revealed sexual and female biased size dimorphism. Large variability ranges of A-1 juveniles with reproductive anlagen could be the result of temporary deformations due to imminent moulting. The geographical structure in the morphological variability allows the conjecture how widespread (freshwater invertebrate) species in the Neotropics may have arisen. Passive dispersal via e.g., birds can constitute the maximum ranges of species. If a population has adapted to local ecological conditions and may have occupied all available niches it may impede colonisation through (occasionally) other species. Relatively recent speciation(s) could explain why morphological divergence is not recognizable in qualitative investigations.

中文翻译：

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新热带淡水被捕食龙虾Cytheridella ilosvayi的有性，成因和地理变异

新热带被认为是地球上生物多样性最丰富的地区之一。尽管如此，许多物种仍被认为具有广泛的分布，并出现在整个新热带地区。但是，由于它们生活在离散的生境中，并且没有主动的扩散能力，因此许多诸如淡水龙等淡水无脊椎动物挑战了这一矛盾。淡水蛇足类小球藻是这种范例的主要例子。在三种现存物种中，仅一种物种被描述为在新热带地区普遍存在。形态变异性的检查是识别环境引起的变异，估计种间和种内变异性以及最终区分物种的前提。这项研究的重点是从佛罗里达，墨西哥，哥伦比亚和巴西采样的几个活种群中对小体附睾的定量研究。形态特征包括足突比和肢肢比，表明最大的变异发生在触角，并证明了其地理结构。软部件反映了先前通过瓣膜形状变异性证明的区域小球藻的形态差异。此外，异速测量系数，肢体尺寸和身体比例显示出性别和女性偏向的大小二态性。带有生殖anlagen的A-1幼虫的较大变异范围可能是由于即将换羽而引起的暂时性变形的结果。形态变异性的地理结构使人们推测，新热带地区广泛出现（淡水无脊椎动物）物种。通过鸟类的被动扩散可以构成物种的最大范围。如果人口适应了当地的生态条件，并且可能已经占据了所有可用的生态位，那么它可能会阻止（偶尔）通过其他物种进行定居。相对较新的物种可以解释为什么定性研究无法识别形态差异。