Comparison of the reproductive cycles of two cryptic soles in the Adriatic Sea

In the Adriatic Sea, two congeneric and cryptic soles coexist and share the same habitats. They are the common sole, Solea solea, one of the main Mediterranean fishery resources, and the Egyptian sole, Solea aegyptiaca. The present study investigated and compared their reproductive biology to obtain updated and original information useful for explaining their coexistence as distinct and reproductively isolated species and supporting stock assessment and management measures. The comparison of the reproductive cycle and strategy revealed similar and distinctive traits. Histological analysis indicated that both species had asynchronous ovaries and were batch-spawners. On the other hand, spawning activities occurred in late autumn or late winter-early spring in S. solea and S. aegyptiaca, respectively. Such a different reproductive timing may contribute to their sexual isolation and genetic distinction. Other pre-zygotic isolating processes have been hypothesized to act by preventing mating, such as the involvement of olfactory sense for intraspecific recognition. Fecundity estimates were similar between the two species (mean potential fecundity: 323,122 for S. aegyptiaca and 210,752 for S. solea, P > 0.05), but the oocytes produced by S. aegyptiaca were significantly smaller than those of S. solea. This study contributed to shedding light on the reproductive traits that may support the coexistence of these congeneric species and encouraged future investigations to understand their fitness in the Adriatic Sea.