Calcitic valves of non-marine ostracodes are important geochemical archives. Investigations of the relationship between the ranges of oxygen and carbon isotope values of modern ostracode populations and their host water provide important information on local or regional conditions and influences. Here we present the first δ¹⁸O_ostracode and δ¹³C of the freshwater ostracode species Cytheridella ilosvayi along with the isotopic composition of the waters in which the ostracodes calcified, δD_water, δ¹⁸O_water, δ¹³C_DIC values—covering a large geographical range (Florida to Brazil). With this data we extended a newly developed approach based on the estimation of δ¹⁸O values of monthly equilibrium calcites as references for the interpretation of δ¹⁸O_ostracode values. The expected apparent oxygen isotope fractionation between CaCO₃ and H₂O is correlated with temperature with smaller values occurring at higher temperatures as valid at isotope equilibrium (δ¹⁸O_calcite__eq). Uncertainties about the expected equilibrium calcites derive from incomplete knowledge of high-frequency variations of the water bodies caused by interplay of mixing, evaporation, and temperature. Coincidence between δ¹⁸O_ostracode and δ¹⁸O_calcite__eq is restricted to few months indicating a seasonal calcification of Cytheridella. There is a characteristic pattern in its difference between mean δ¹⁸O_ostracode and δ¹⁸O_calcite__eq which implies that Cytheridella provides a synchronous life cycle in its geographical range with two calcification periods in spring (May, June) and autumn (October). This ubiquitous life cycle of Cytheridella in the entire study area is considered to be phylogenetically inherited. It might have originally been adapted to environmental conditions but has been conserved during the migration and radiation of the group over the Neotropical realm.

非海洋介介体的方解石阀门是重要的地球化学档案。对现代介形虫种群的氧和碳同位素值范围及其宿主水之间关系的调查提供了有关当地或区域条件和影响的重要信息。在这里，我们介绍了淡水介形虫物种Cytheridella ilosvayi的第一个 δ ¹⁸ O_介形虫和 δ ¹³ C，以及介形虫钙化的水域的同位素组成、δD_水、δ ¹⁸ O_水、δ ¹³ C _DIC值——覆盖很大的地理范围（佛罗里达到巴西）。利用这些数据，我们扩展了一种新开发的方法，该方法基于每月平衡方解石的 δ ¹⁸ O 值的估计，作为解释 δ ¹⁸ O _ostracode值的参考。_{CaCO 3}和 H ₂ O之间的预期表观氧同位素分馏与温度相关，在较高温度下出现较小的值，这在同位素平衡时有效 ( _δ ¹⁸ O_方解石_eq）。关于预期平衡方解石的不确定性源于对由混合、蒸发和温度相互作用引起的水体高频变化的不完全了解。δ ¹⁸ O _ostracode和 δ ¹⁸ O _{calcite_eq}之间的一致性仅限于几个月，表明 Cytheridella_的季节性钙化。^{平均 δ 18} O _ostracode和 δ ¹⁸ O_方解石_ _eq之间的差异有一个特征模式，这意味着Cytheridella在其地理范围内提供了一个同步的生命周期，在春季（5 月、6 月）和秋季（10 月）有两个钙化期。Cytheridella在整个研究区域中无处不在的生命周期被认为是系统发育遗传的。它最初可能已经适应了环境条件，但在该群体在新热带地区的迁移和辐射过程中得到了保护。