Evolutionary trade-offs between opaque and clear egg masses in spotted salamanders (Ambystoma maculatum)
Polymorphisms may arise in response to competing selection pressures. Spotted Salamanders (Ambystoma maculatum) have dimorphic egg masses that appear clear or opaque based on the presence or absence of a glycoprotein. Previous studies suggest that clear egg masses are preferred by foraging Wood Frog tadpoles (Lithobates sylvaticus). With predation pressure potentially driving selection for opaque egg masses, we sought to test whether a mutualistic association between A. maculatum developing embryos and an endosymbiotic algae (Oophila amblystomatis) was driving selection for clear egg masses. Oophila is thought to contribute oxygen for developing embryos and is associated with elevated hatching success and growth rates. We conducted an enclosure experiment in a wetland where both A. maculatum and L. sylvaticus breed in Watauga Co., NC. In each of our enclosures (n=16) we placed one clear and one opaque A. maculatum egg mass and established two treatments; a predation treatment with 250 L. sylvaticus tadpoles added (n=8) and a control (n=8). We quantified tadpole egg mass preference (clear or opaque) by counting the number of tadpoles feeding on each egg mass over 21 observation days and determined algal densities by counting cells present in the inner capsular membrane of the egg.
The last day of the experiment we observed 143% more L. sylvaticus tadpoles feeding on clear egg masses than on opaque egg masses in predation treatment enclosures (t7=2.36, p=0.05). Additionally, in all enclosures, the clear egg mass was completely consumed by tadpoles before the opaque mass. Analysis of algal densities revealed that clear egg masses had significantly more algae than opaque masses (t9=3.46, p=0.007). This represents a novel finding and suggests that clear masses enable greater rates of algal photosynthesis, potentially allowing for increased embryonic developmental rates and hatching success. We propose that interactions among competing selection pressures contribute to A. maculatum egg mass dimorphism, with higher Oophila densities driving selection towards clear egg masses and tadpole predation selecting for opaque masses. Our findings provide insight into the mechanisms driving the proportion of dimorphic egg masses seen in A. maculatum breeding sites and further elucidates the complex multi-trophic interactions within these communities.