Investigation of the reproductive system in calanoid copepods: A new approach using 3D reconstructions from serial semi-thin cross-sections in Calanus glacialis and Metridia longa

The structure of the female reproductive system of the calanoid copepods Calanus glacialis and Metridia longa from the White Sea was studied using light microscopy, scanning and transmission electron microscopy, as well as confocal laser scanning microscopy. For the first time, we applied also the method of 3D reconstructions from semi-thin cross-sections to visualize the general plan of the reproductive system in both species. The application of a combination of methods provided novel and detailed information on the genital structures and muscles located in the genital double-somite (GDS) as well as structures used for the reception and storage of spermatozoa, fertilization and release of eggs. An unpaired ventral apodeme and associated muscles located in the GDS are described for the first time for calanoid copepods. The role of this structure in copepod reproduction is discussed. Stages of oogenesis and the mechanism of yolk formation in M. longa are studied using semi-thin sections for the first time. A combination of non-invasive (LM, CLSM, SEM) and invasive techniques (semi-thin sections and TEM) applied in this study substantially improves our understanding of the functioning of the genital structures in calanoid copepods and could be recommended as a standard set of methods for future research in the reproductive biology of copepods.