Evolution of Spinal Nerve Number in Anuran Larvae

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Abstract
We examined the number of segmental spinal nerves in 60 premetamorphic tadpoles, representing 43 species, 33 genera and 12 families of anurans. The number of spinal nerves shows a clear phylogenetic pattern, in which the primitive condition is variable, ranging from 23 to 29 pairs. Reduction of caudal spinal nerve number has occurred independently at least 7 times during anuran phylogeny. Reduction events have occurred among species within genera, among subfamilies within families, as well as at familial and higher taxonomic levels. In general, archaeobatrachian larvae have significantly more spinal nerves than neobatrachian larvae. While 2 of the 7 reduction events have occurred within the Archaeobatrachia, both of these have occurred at low taxonomic levels. In the Neobatrachia, 3 of the 5 reductions have occurred at subfamilial or higher taxonomic levels. Overall, reduced spinal nerve number correlates with tadpole body size, but not with relative tail length or with developmental stage, at least within the range of stages we examined. There is a positive correlation between number of spinal nerves and minimum time from hatching to metamorphosis, at least within the Archaeobatrachia. Species with more pairs of nerves take longer to reach metamorphosis, possibly because they have more neural tissue to produce and to resorb. Spinal nerve number per se does not appear to be associated with larval microhabitat ecology or swimming behavior. Tadpoles exhibit greater variation in spinal nerve number than adult frogs. This greater variation is tolerated presumably because the tail is an expendable organ, which is lost at metamorphosis.