Through the Looking-Glass: Shell Morphology, Anatomy and Mating Behavior of Reversed Pomacea Canaliculata (Caenogastropoda, Ampullariidae)

Snails exhibit a primary left-right asymmetry that appears during the first cleavages of the eggs, and a secondary asymmetry, related to the coiling of the shell. Most species are constituted by either dextral or sinistral morphs (enantiomorphs) while individuals with reversed primary asymmetry are extremely rare. Freshwater snails of the family Ampullariidae are normally dextral enantiomorphs with planispiral, hyperstrophic or orthostrophic shells. Pomacea canaliculata, a well-studied species because of its invasive status, shows dextral primary asymmetry and orthostrophic growth that results in clock-wise shells. Despite the great number of studies focused on P. canaliculata, only two specimens with reversed asymmetry have been hitherto reported. Here we report the finding of two live snails and three empty shells of P. canaliculata with anti-clockwise coiling that appeared in two populations from the southern Pampas, Argentina. Both anti-clockwise live snails were males that attempted to copulate with clock-wise females in the laboratory but failed to inseminate them. The apex of anti-clockwise shells and the anatomy of the snails revealed that the reversal of coiling was due to an orthostrophic development of sinistral enantiomorphs. Morphological analysis performed through geometric morphometrics did not find other differences with clock-wise snails other than coiling direction. We conclude that these anti-clockwise snails are probably engendered, as in other snails species, when the mother is a recessive homozygote for reversing alleles that show delayed maternal inheritance. The chances of establishment of populations with dimorphic asymmetry are very low because of the reproductive disadvantages of anti-clockwise individuals.