Egg mass polymorphism in Ambystoma maculatum is not associated with larval performance or survival, or with cell density of the algal symbiont Oophila amblystomatis

Abstract

Phenotypic polymorphisms are common in nature, but the mechanism through which natural selection maintains them can be difficult to identify. Heterozygote advantage, negative frequency-dependent selection, and variation in selective environment each can explain polymorphisms in some systems. Although first observed nearly 100 years ago, the egg mass color polymorphism in the spotted salamander (Ambystoma maculatum) still is not well understood. Females of this species lay egg masses with either clear or white jelly. White egg masses are preyed upon less than clear egg masses in some environments, suggesting an advantage to white masses, whereas an advantage to clear masses has not been identified. We propose that embryos in clear masses have an advantage in some light environments, mediated by the symbiotic green alga, Oophila amblystomatis, that grows in the egg capsules. The transparent jelly of clear masses may allow for greater algal growth, which could have cascading effects on embryonic development and larval performance. We asked whether larvae from clear and white masses differ in performance or survivorship and, if so, whether this difference is mediated by differences in algal density. We paired clear and white masses in a pond during the embryonic period and quantified egg capsule algal density at hatching. A portion of the larvae were subject to swim trials and were measured for size and shape. The remaining larvae were placed in mesocosms to quantify survival in the presence and absence of newt predators. There was no difference in algal density between membranes of clear and white masses. Similarly, there were no differences in larval performance, morphology, or survivorship between morphs. Our data do not support our hypothesis that variation in predator density and light availability among microhabitats could maintain the two phenotypes within populations.