The Reproductive System of Two Species of the Centropagoidea Superfamily From the White Sea

We studied the gross gonad morphology, the morphology of genital structures involved in sperm reception and egg release, as well as oocyte development in female Centropages Hamatus and Temora longicornis from the White Sea. Our research employed a comprehensive morphological approach, including light microscopy, transmission electron microscopy, and three-dimensional reconstructions based on series of semithin cross-sections. This integrated methodology enabled detailed visualization of the fine-scale morphological structures and main muscle bundles associated with the genital complex and precise mapping of oocyte distribution at different stages of development within the ovaries and diverticula of mature females. In T. longicornis, we discovered glands in the last thoracic somite that were similar in structure and position to the shell glands of egg-brooding calanoid copepods. Unlike previous studies, our approach documented anatomical details of the reproductive system which are typically not detectable using two-dimensional imaging techniques alone. Our observations indicate also that during the reproductive season females of both species are characterized by continuous oocyte maturation and readiness to lay eggs. However, due to the lack of a spermatheca, these females cannot store sperm and, subsequently, require repeated matings to fertilize each batch of eggs produced during the reproductive cycle. We assume that the sex ratio skewed towards males in adult populations of C. hamatus and the equal sex ratio in adult populations of T. longicornis reflect these morphological limitations—specifically, the lack of sperm storage capacity in females coupled with sustained oocyte production in females.