For spotted salamander embryos nearing hatching, they get by with a little help 鈥� not from friends, but from animals that are actually their predators. Voracious frog tadpoles chew on the egg masses, which helps them hatch, possibly because trimming away layers of the eggs鈥� protective covering increases their ability to absorb oxygen from the water.
Spotted salamanders (Ambystoma maculatum) lay their eggs in seasonal springtime pools where they are at risk of being eaten by aquatic insects and the tadpoles of some amphibians, such as wood frogs (Lithobates sylvaticus). For protection, the egg masses are cloaked in thick layers of jelly. However, this gelatinous shield makes it harder for the developing embryos to get enough oxygen, so the salamanders benefit from a symbiotic photosynthetic alga for extra oxygen.
Previous field research showed that some egg masses with intact jelly layers could die off in large numbers in low-oxygen conditions, so and at Bucknell University in Pennsylvania wondered if the inherent costs of the jelly coat could be pared back by the gnawing of predators.
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The researchers gathered 32 salamander egg masses from pools in central Pennsylvania, dividing them into eight experimental treatments. Keeping the eggs in containers, they either removed or kept the thick jelly layers, and added either zero, 50, 100 or 200 wood frog tadpoles.
The team tracked the eggs鈥� development and noted how many salamanders successfully hatched. Eggs stripped entirely of their jelly shields were totally consumed in the medium and high levels of hungry tadpoles. But without any predators around, more eggs survived when their jelly coats were removed 鈥� over 90 per cent compared with the intact eggs鈥� 72 per cent.
When intact eggs were challenged with the highest tadpole levels, they survived better than without any tadpoles.聽 at the University of Newcastle, Australia is intrigued by the lack of a fail-safe process to increase oxygen exposure for the maturing salamander embryos. 鈥淚t would make sense for the thick jelly layers to naturally thin over time to improve oxygen supply for the embryos as they mature, yet this role seems to be played by the predators to assist in thinning the jelly layers,鈥� he says.
He notes that some amphibians use a frothy nest of bubbles to aerate the eggs. The bubbles naturally break down over time, spreading the eggs out and increasing their oxygen uptake.聽But in the case of the spotted salamanders, predation is crucial to the eggs accessing this key resource.
鈥淭hese findings show how intimately linked predators and prey are. We often think that there is a reliance of predators on prey, but the reliance goes in both directions,鈥� says Gould.
Ecology and Evolution
