Reproductive dynamics of Diplostraca in the Black Sea: insights from Hamsilos Bay, Sinop, Türkiye

In the coastal marine ecosystem of the Black Sea, Diplostraca is the second most dominant mesozooplankton after Copepoda. Its parthenogenetic reproductive strategy enables significant population growth from spring (March) to fall (December), contributing substantially to mesozooplankton abundance. This study investigated the fecundity, embryonic development stages, and population structure of four of Diplostraca species in Hamsilos Bay: Evadne spinifera, Pleopis polyphemoides, Penilia avirostris, and Pseudevadne tergestina. Samples were taken every month from three stations between July 2015 and June 2016. Population analysis showed that parthenogenetic females were dominant, reaching up to 100% of the population. Gamogenetic females, females without embryos, and males constituted 23%, 33%, and 9% of the population, respectively. Embryonic development stages were species-specific: stage A embryos were predominant in Ps. tergestina, Pl. polyphemoides, and E. spinifera, whereas stage C embryos were most common in Pe. avirostris. The ultimate stage, stage D, was rare across all species. The number of embryos per parthenogenetic female varied from 1 to 5 in E. spinifera, 2–8 in Pe. avirostris, 1–8 in Pl. polyphemoides, and 1–6 in Ps. tergestina. Diplostraca primarily reproduced via parthenogenesis, enhancing their contribution to the mesozooplankton community in Hamsilos Bay. The number of embryos observed was lower compared to a similar study performed in the Marmara Sea, possibly due to distinct regional environmental differences such as nutrient availability and temperature. This study bridges a critical gap in understanding the reproductive dynamics of Diplostraca in semienclosed seas and factors affecting them, establishing a valuable reference for future research.