Mesopodial and Digit Number Variation in the Limb Reduced Salamander Family Amphiumidae

Digit loss has occurred independently multiple times in the order Caudata. Within the family Amphiumidae, significant digit reduction has taken place in the three species: Amphiuma tridactylum (Three-toed Amphiuma), A . means (Two-toed Amphiuma), and A . pholeter (One-toed Amphiuma). Species within this genus present a unique system to study the evolution and development of limbs due to their elongated body, reduced limb length, and digit loss along an apparent taxonomic morphocline. Individuals within the genus Amphiuma lack limb regeneration after amputation, thus the mesopodial variation results from alterations in the developmental processes. Significant mesopodial variation was present in each species with several novel arrangements, but predominately shared mesopodial patterns to other species with the same digit number. Our results showed more variation in the mesopodial pattern of the forelimbs than the hind limbs in all three species, and more variation in A . tridactylum and A . means compared to A . pholeter . Limbs of A. pholeter had fewer elements present, which we hypothesize as a consequence of the species being in the process of complete limb loss. Lastly, specimens of A . pholeter were consistently found with two digits present bound in hard syndactyly, contrary to the previously assumed one digit.