A very rare teratological observation in Monachoides incarnatus (O. F. Müller, 1774) (Gastropoda: Hygromiidae), a "unicorn snail"

In April 2017, a double teratological case in an adult Monachoides incarnatus (Müller, 1774) was discovered in Liège (Belgium). The specimen constitutes a “unicorn snail”, where the snail owns only one central ommatophore (eye-stalk) with two eyes at the top of the tentacle, and a supernumerary small eye close to the right eye. The unique upper stalk is probably due to a trouble in its early development, where the upper ommatophores were abnormally merged. However, the cause and explanation of the third eye remains unexplained. Except for the upper part of the head, this specimen has all the normal characteristic of its species. Even if the specimen has not lost its sense of vision entirely, the visual field could be different. We can suspect a potential handicap to detect the direction of odors. Named ‘Cosmo’, it was able to produce eggs, unfortunately none have hatched. A succinct review of teratological cases in the soft part of the head in Stylommatophora is introduced and discussed. As far as we know, only one other case of “unicorn snail” was reported from literature in Helix pomatia. Another case in Cornu aspersum was recently detected in California the same year as our specimen.