A simultaneous hermaphrodite dwarf common carp (Cyprinus carpio carpio morpha hungaricus) from a natural thermal lake, Lake Hévíz, Hungary.

The fauna of Lake Hévíz is unique due to its status as the world's only natural warm-water medicinal lake with a peat bed. A population of dwarf common carp (Cyprinus carpio carpio morpha hungaricus) forms an isolated, self-sustaining community in the lake, adapted to extreme thermal and chemical conditions. During continuous research conducted since 2007, we identified a hermaphroditic individual for the first-time following sampling during the winter period in 2024. Macroscopic examination revealed testicular tissue distinguishable from ovarian tissue. Histological analysis indicated that the small-bodied fish were in a pre-spawning phase, with gonads displaying traits of simultaneous hermaphroditism. In addition was capable of producing mature eggs and sperm concurrently. According to both literature and our experimental findings, this individual may have been capable of self-fertilization. Further investigation is required to ascertain whether specific or combined suboptimal environmental factors and/or endocrine-disrupting chemicals contributed to the emergence of hermaphroditism in this individual.