Abnormalities in shape and size of Eurytemora affinis (Copepoda, Calanoida) and its eggs under different environmental conditions

In this study we examined the morphological anomalies of 2023 ovigerous females of the copepod Eurytemora affinis from laboratory and field samples. Field copepod collections were collected from 10 sites in Europe and North America. Laboratory cultured copepods were reared at 6 experimental conditions during several generations. The observation under the microscope of the ovigerous females’ bodies revealed the presence of a ‘Tumor Like Anomaly’ (TLA) on a few individuals, which were manifested on the urosome. The occurrence of TLA was only noticed on copepods from the field samples. This result shows that under controlled laboratory conditions, E. affinis do not manifest any abnormal morphology, even when temperature and salinity conditions are stressful, which also demonstrates the high plasticity of this copepod species and its capacity to live at a wide range of environmental parameters. In this study, we also confirmed that E. affinis is capable of producing eggs with a wide range of sizes, ranging from very small and abnormal eggs of 75.79 μm only, to very large eggs, reaching a maximal diameter of 128.71 μm. The observed large eggs are abnormal compared to the average size of the other eggs produced within the same clutch. Other abnormalities on eggs, such as deformed eggs or eggs infested by fungi, are only observed from field samples at a few sampling sites. In the Seine estuary, several abnormalities in eggs and adult females were observed during February 2010. The critical period for E. affinis’ development is sampling site-specific and needs to be accurately studied in the future to better explain the causes of the abnormalities described in this work, and their effects at population level.