Data from: Mortality and morphology in egg masses of unisexual and Jefferson Salamanders

Unisexual Ambystoma salamander egg masses have often been observed to exhibit very high rates of embryo mortality. The ecological consequences and underlying mechanisms are of great concern to researchers and managers studying these and other members of the species complex, all of which are listed as rare species throughout much of their range. Substantial embryo mortality is commonly used by field ecologists as an indicator that unisexual salamanders are present in a pond; egg masses of unisexual salamanders appear otherwise very similar to those of A. jeffersonianum (Jefferson Salamander). Early researchers suggested that elevated mortality among unisexual salamanders was due to lack of fertilization caused by sperm limitation. However, recent work has suggested that embryo failure is due to genetic errors particular to the unisexual salamander lineage. Our goals in this study were to (1) identify when during development embryonic mortality occurs in unisexual salamanders, and (2) to develop a morphological metric to distinguish egg masses of unisexual and Jefferson salamanders. Collecting from sites across western Massachusetts, we reared 356 eggs from 11 egg masses of known species identity in the laboratory, examined field photographs of 96 egg masses of known species identity (based on mitochondrial sequencing and 6 microsatellite alleles), and examined 757 field photographs of egg masses of unknown species identity. We developed a simple, scale-independent metric to distinguish Jefferson Salamander egg masses from those of co-occurring unisexual salamanders. Among developing embryos beyond the earliest stages, we found no difference in mortality rates between unisexual salamanders and Jefferson Salamanders. However, we observed a large pulse of embryo mortality in the earliest stages of development, followed by a trickle of additional mortality at later stages. Our results suggest that the primary cause of embryo mortality in Massachusetts populations of unisexual salamanders involves failure of embryos to initiate development.