An advantage of clear over white egg mass morphs in metabolically demanding microhabitats suggests a role of symbiotic algae in the maintenance of a polymorphism in the spotted salamander (Ambystoma maculatum)

Background: Many populations of the spotted salamander, Ambystoma maculatum, have clear and white egg mass morphs. It is unclear what drives the maintenance of this polymorphism. The egg mass jelly of both morphs limits oxygen diffusion from water to interior embryos. However, an endosymbiotic alga compensates by increasing oxygen levels through photosynthesis. Jelly in white egg mass morphs may reflect the light necessary for algal photosynthesis. Question: Does the relative success of the two egg mass morphs vary with microhabitat differences in oxygen stress for embryos and, thereby, differences in need for supplemental oxygen from algal symbionts? Hypothesis: Embryos in warm water with low dissolved oxygen may require oxygen supplementation from algal photosynthesis to successfully complete development. In such conditions, the white morph may be at a disadvantage relative to the clear morph. Organism: A western North Carolina population of A. maculatum. Methods: In the laboratory, we compared the development and hatching success of clear and white egg mass morphs under light (algae present) and dark (algae absent) conditions. We also placed egg masses under a range of temperatures and dissolved oxygen conditions in the field, then compared embryo development and survival of clear and white morphs. Conclusions: When oxygen demand is high - such as in warm, shallow water - embryos from clear masses have higher survivorship than do those of white masses, supporting our hypothesis that the clear morph has a developmental advantage over the white morph that is mediated by the algae.