First record of Siamese twins in Eastern Mosquitofish, Gambusia holbrooki (Girard 1859)

Siamese twins at the late eyed stage of development (sensu Reznick, 1981) have been found in a brood of young fish in a wild population of Gambusia holbrooki. The female containing the brood was collected in a temporal hypersaline watercourse located in the most arid zone of Europe, an area highly polluted by agrochemical and urban inputs. Extreme salinity and water pollution, along with other environmental factors, could be related to abnormalities in the embryo’s development.