Regional Studies in Marine Science 54 (2022) 102450 Contents lists available at ScienceDirect Regional Studies in Marine Science journal homepage: www.elsevier.com/locate/rsma Reproductive strategies and population characteristics of key Cladocera species in the Sea of Marmara Dalida Bedikoğlu a,∗ , I. Noyan Yilmaz a,b , Nazli Demirel a a Institute of Marine Sciences and Management, Istanbul University, Turkey b School of Life and Environmental Sciences, Centre for Integrative Ecology, Deakin University, Australia article info Article history: Received 23 February 2022 Received in revised form 16 May 2022 Accepted 17 May 2022 Available online 24 May 2022 Keywords: Reproductive cycle Birth rate Penilia avirostris Mucilage event Sea of Marmara abstract Zooplankton is an essential component of marine ecosystems, with Copepoda and Cladocera being the most important zooplankton groups in all the world’s seas. In the Sea of Marmara, Cladocera has the highest participation in zooplankton abundance. Here we investigated reproductive strategies and population characteristics of four Cladocera species; Penilia avirostris, Pleopis polyphemoides, Evadne nordmanni, and Pseudoevadne tergestina in the Sea of Marmara, northeastern part of Mediterranean Sea. Reproduction strategy, fecundity, and embryonic development of four Cladocera species were achieved. During our study period from 2004 to 2008, there was an extreme jellyfish bloom caused by Liriope tetraphylla and following mucilage event occurred between the years 2006 and 2008. Thus, we also compared the change in phenology and reproductive strategy of cladocerans amongst periods characterized by extreme events and against a suite of environmental parameters. The most dominant Cladocera species in the Sea of Marmara were Penilia avirostris and Pleopis polyphemoides. The population structures of Cladocera species according to their reproductive strategies, partheno- genetic female individuals provided rapid exponential reproduction, and were dominant at 60%–100%. Parthenogenetic fecundity success was found significantly different among Cladocera species. The most remarkable result during the study period was observation of increase in embryo production while populations were in decrease. © 2022 Elsevier B.V. All rights reserved. 1. Introduction In marine ecosystems, zooplankton is a critical component that affects all living groups in the pelagic food web, starting from the primary producers with its high abundance, wide dis- tribution and predator–prey relationships (Cushing, 1990; Clark, 1992). Cladocera, a prominent group of mesozooplankton like Copepoda, has more than 700 identified species on the earth (Smirnov, 2014). Contrasting its diversity, only eight Cladocera species are marine forms (Onbe, 1977), having a wide distribu- tion in the world’s seas. Although seasonal abundance peaks of marine cladocerans are well known, studies investigating that their reproductive strategies and fecundity are limited (Marazzo and Valentin, 2003a, 2004a; Atienza et al., 2008; Dvoretsky and ve Dvoretsky, 2020). Their life cycle is characterized by the alternation between gamogenesis and parthenogenesis. During parthenogenesis, the genetic characters of parthenogenetic em- bryos are the same as those of the parent individuals as genetic recombination does not occur (Hebert and Ward, 1972). This ∗ Corresponding author. E-mail address: dalidabedikoglu@gmail.com (D. Bedikoğlu). strategy is an effective biological mechanism of choice for achiev- ing and maintaining rapid population growth under favourable environmental conditions (Hebert and Ward, 1972; Frey, 1982; Egloff et al., 1997). Immediately after the peak of their pop- ulations, parthenogenetic females in the water column change their reproductive strategy to become gamogenetic females. The gamogenesis stage is the only way for the species to produce resistant individuals (Rispe and Pierre, 1998). Furthermore, it is the preferred reproductive strategy when adverse environmen- tal conditions develop (Egloff et al., 1997). Finally, gamogenetic females produce resting eggs to sink to the seafloor during the seasonal disappearance of Cladocera populations (Onbe, 1985; Egloff et al., 1997). Here we investigated reproductive strategies and population characteristics of four Cladocera species; Penilia avirostris, Pleopis polyphemoides, Evadne nordmanni, and Pseudoevadne tergestina in the Sea of Marmara, northeastern part of Mediterranean Sea. The lack of knowledge on the phenology of Cladocera species in the SoM and the reason for their exceptionally high contribution to overall zooplankton abundance led us to carry out detailed studies on their life cycles in the SoM. Thus, we have studied reproduction strategy, fecundity, and embryonic development of four Cladocera species in the highly urbanized and eutrophic https://doi.org/10.1016/j.rsma.2022.102450 2352-4855/© 2022 Elsevier B.V. All rights reserved.