Aquaculture Research. 2020;00:1–11. wileyonlinelibrary.com/journal/are | 1 © 2020 John Wiley & Sons Ltd 1 | INTRODUCTION Probiotic bacteria have known benefits regarding the synthesis and absorption of nutrients by its host, which relates to a more effi- cient utilization of energy that is used for the animal's development (Carnevali et al., 2013). It may also stimulate reproductive processes, especially in relation to the interaction with the animal's energy metabolism, and with the sensors responsible for the regulation of energy availability, such as neuropeptides of the pituitary–hypothal- amus–gonads axis and peripheral molecular modulators (Aydin & Çek-Yalniz, 2019; Carnevali et al., 2017; Palermo et al., 2011). The use of probiotics throughout the development of males and females can improve both the reproductive performance and physio- logical responses of adult fish, especially regarding sexual hormones, sperm quality and gonadal development, which reflects in the rela- tive and absolute fecundity of animals (Mehrim et al., 2014). Ghosh et al. (2007) observed positive effects in the development, viability, growth and survival of embryos of different fish species when in- cluding Bacillus spp. as a diet additive. According to Yang et al. (2007) reported improvements in the metabolism of fish fed with B. subtilis for acting in the excretion of active compounds and in the release of antioxidant molecules, thus influencing gametes formation. Sexual maturation in fish occurs due to changes in the expression of neuropeptides responsible for the production of regulatory hor- mones (Carnevali et al., 2013). In females, the use of probiotics stim- ulated oocytes maturation and increases the number of vitellogenic follicles, thus improving ovulation (Gioacchini et al., 2011). In males, such an inclusion fostered the action of exogenous gonadotrophins, Received: 26 July 2020 | Revised: 24 October 2020 | Accepted: 8 November 2020 DOI: 10.1111/are.15020 ORIGINAL ARTICLE Reproductive physiology of Rhamdia quelen is improved by dietary inclusion of probiotics Mariana Lins Rodrigues 1 | Danielle Zanerato Damasceno 2 | Ricácio Luan Marques Gomes 3 | Bruno dos Santos Sosa 3 | Evandro Bilha Moro 3 | Eduardo Antônio Sanches 3 | Fábio Bittencourt 1 | Altevir Signor 1 1 Grupo de Estudo de Manejo na Aquicultura – GEMAq, Universidade Estadual do Oeste do Paraná, UNIOESTE, Toledo, Brazil 2 Zoetis, São Paulo, Brazil 3 Centro de Aquicultura da Universidade Estadual Paulista “Júlio de Mesquita Filho”, UNESP, Jaboticabal, Brazil Correspondence Mariana Lins Rodrigues, Grupo de Estudo de Manejo na Aquicultura – GEMAq, Universidade Estadual do Oeste do Paraná, UNIOESTE, Endereço Rua da Faculdade, 645, Jardim La Salle, Toledo, Paraná, Brazil. Email: lins.mariana@hotmail.com Funding information Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) Abstract This study evaluated the effects of a probiotic additive on reproductive variables of the silver catfish (Rhamdia quelen). Three hundred juveniles (initial mean weight 17.35 ± 0.97 g) were fed for 90 days with diets containing increasing levels of a pro- biotic additive (0.15, 0.30, 0.45 and 0.60 g/kg of feed), which consisted of a blend between Bacillus subtilis and B. cereus, both at a concentration of 4 × 10 11 CFU/g, and a control diet without the additive. In males, the presence of the probiotic had an ef- fect on relative seminal volume, sperm cells’ survival, gonadosomatic, hepatosomatic and viscerosomatic indexes, as well as transpose beat frequency, displacement veloc- ity, motility rate and straightness of sperm cells. In females, the probiotic inclusion resulted in presented better in spawning and fertilization rates, viscerosomatic fat accumulation and development of germ cells. Additionally, the hepatic integrity and hepatocyte developmental characteristics were preserved by the presence of the probiotic additive. Thus, the inclusion of 0.60 g/kg of the probiotic could be recom- mended for the improvement of the reproductive performance of R. quelen. KEYWORDS broodstock, feed additives, native species, seminal quality, viable oocytes