Who laid these eggs?

The Question: I came upon these while at a waterfall in Tennessee. Any help finding out what they are would be appreciated. Submitted by: Starlon, Tennessee, USA

The Short Answer: These are the eggs of a toad in the genus Anaxyrus. According to amphibian egg expert Ronald Altig, who studies the development of amphibians at Mississippi State University, they are most likely the eggs of the American Toad (Anaxyrus americanus). The jellylike structure, with prominent black centers, is a clear indication of amphibian eggs, and the long string of eggs makes it a good bet to be one of North America’s toads.

More Information: An interesting question, and one that Dr. Altig feels is under-researched, is why some amphibians lay their eggs in strings, others lay their eggs in sheets, and yet others lay eggs in clumps. According to Dr. Altig, “John Moore in 1940 is about the only one to ever comment on the significance of egg membrane configurations. People ignore eggs like they were either dangerous or totally inconsequential. Also, people tend not to collect things they cannot identify easily and eggs are paramount in this situation.”

Seventy years ago, John Moore suggested that laying eggs in clumps should provide protection from predators and from cold for at least some of the eggs, since a predator has to get through the entire mass of jelly to get at the very interior eggs. And the outer eggs and jelly provide some insulation to keep the innermost eggs from freezing. So he predicted that clumps of eggs would be laid by amphibians that lay early in the season, when there is a danger of extreme cold. One problem with a clump is that getting oxygen to the innermost eggs becomes a problem. But cold water holds more oxygen than warm water, so that is less of a problem for amphibians that lay in cold water. Laying eggs in a sheet on the surface of the water, by contrast, might make sense for amphibians that lay in warm, oxygen-poor water.

For the most part, these criteria seem to work, although not perfectly. And Moore never presented a theory that would account for strings of eggs. Dr. Altig’s suggestion is that a string affords the most surface area for oxygen exchange in stagnant water that has very little oxygen. This would apply to most North American toads. Of course, the American toad eggs you found were laid in a highly oxygenated waterfall pool, but it’s possible that the eggs are really adapted for warm water and low oxygen, because that’s the most challenging environment where American toads lay their eggs.

Sources: Thank you to Dr. Catherine J. Hanna and Dr. Altig for help in identifying these eggs.