The Long Larval Period of the Tiny Plethodontid Salamander Urspelerpes brucei and Heterochrony in the Spelerpini.

We collected larvae of the Patch-nosed Salamander, Urspelerpes brucei, and analyzed size-frequency distributions throughout a 12-mo cycle to determine the length of the larval period and size at metamorphosis. The hatchling-size class entered the population across a wide time window, from early winter to late summer, and at a minimal size of 6-mm snout–girdle length (SGL). Examination of both pooled total sample and the samples across six seasonal periods strongly show that the typical larval period is approximately 24 mo, with metamorphosis occurring during late summer to early fall at 18–19 mm SGL. Because oviposition and hatching are extended across several months, it is possible that some animals metamorphose up to 30 mo after hatching. Larval growth in Urspelerpes is 0.5 mm/mo, which is comparable—and even slower—to the slow larval growth seen in all other stream-dwelling plethodontid larvae. Our samples of recently metamorphosed animals and mature adults indicate that males are sexually mature at metamorphosis. This is more difficult to know for females, and we suggest that females may be a mixed group that contains individuals that mature at metamorphosis and individuals that delay maturation for an additional 12 mo. There is no difference in either sex between body size at metamorphosis and at sexual maturation. As in other species in the Spelerpini, the timing of metamorphosis and the timing of sexual maturation appear to be able to evolve independently, with the remarkable result that the early sexual maturation in Urspelerpes results in a tiny adult size despite having a multiyear larval period. [ABSTRACT FROM AUTHOR]