A Comparative Analysis of the Mouthpart Structures of Two Epischura Species (Copepoda, Calanoida) from Lakes Differing in their Trophic Status

Planktonic copepods, being a connecting link between primary production and higher order consumers, are often a key component of the food net in limnetic ecosystems. Many studies indicate a strong relationship between feeding mode and the structure of the mouthparts of copepods. Early studies attributed members of the genus Epischura to different groups according to their feeding mode and assumed that the species inhabiting the territory of Asia were only adapted to feeding on phytoplankton. We compared the fine morphology of the mouthparts of the Asian species, Epischura baikalensis and E. chankensis, living in different environments. The measurements were made using electron and light microscopy. The mesh sizes of the food capture net formed by the setae and setules ranged between 3.2 μm for E. chankensis and 9.1 μm for E. baikalensis. The area presumably covered by the first maxillae was 13215 μm2 and 66470 μm2, respectively. The resistance force of one seta is ten times higher for E. baikalensis than for E. chankensis. According to the results of calculations of hydrodynamic characteristics based on Reynolds number, these species are unable to filter food particles, but they capture them by drawing them into the mouth. The significant difference in the length of the oral appendages indicates that E. baikalensis is better adapted to living in an oligotrophic lake. For feeding on small particles at low concentrations, it is more beneficial to have long oral appendages even at high energy costs