Occurrence of multiple congenital anomalies in Potamotrygon amandae (Elasmobranchii, Potamotrygoninae) embryos, including the first report of bicephaly.

Reports of teratogenic embryos in elasmobranchs have been documented in multiple species, with proposed aetiologies including environmental disturbances, genetic mutations, predation, exposure to endocrine-disrupting compounds and maternal stress. Nevertheless, complex congenital malformations such as bicephaly remain exceptionally rare, particularly among freshwater stingrays of the family Potamotrygonidae. The present study constitutes the first record of bicephaly in the genus Potamotrygon, documenting multiple congenital anomalies in two embryos from a single litter. One female embryo exhibited incomplete fusion of the anterior margin of the right pectoral fin, a malformation previously reported in batoid fishes. The second case involved a male parapagus bicephalic twin; internal examination demonstrated duplicated brains and hearts, as well as partially individualized gastrointestinal tracts, whereas the reproductive and urinary systems were shared. Skeletal assessment revealed discrete chondrocrania and precaudal vertebrae, with spinal fusion occurring at the transitional caudal region. The underlying causes of these anomalies remain unresolved. Two primary hypotheses are discussed: (1) genetic alterations, including misexpression of critical developmental regulators such as Sonic hedgehog (Shh), which has been implicated in cephalic duplication and fin malformations; and (2) maternal physiological stress, potentially linked to anthropogenic factors such as recurrent capture events, which may disrupt embryogenesis. This report broadens the spectrum of known teratological conditions in freshwater elasmobranchs and underscores the importance of further research into the mechanistic basis of congenital anomalies in this group.