Nature Trivia
By: Renn Tumlison
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Green Eggs and Jam: Adaptations That Help Spotted Salamanders Reproduce
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The spotted salamander (Ambystoma maculatum) shown in the photo was caught in March. It is a female whose body is quite enlarged by the numerous eggs that fill her abdomen. As large as she is, she is small in comparison to the size of the egg mass she produces. How can she develop such a large mass of eggs?
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Salamander eggs do not possess outer shells like those seen protecting the eggs of reptiles and birds. Each egg is surrounded by a group of membranes. When the eggs are laid, absorption of water makes the eggs swell. The eggs are embedded in secretions of a gland, and those secretions turn into gelatin when combined with water. This works very similar to the way a small box of Jello™ gelatin added to water becomes the familiar dessert.
The fully formed jelly coat helps to defend the embryos against predators and also helps prevent the eggs from drying out if the water level drops in the pool. This means of egg protection causes a problem, however. Oxygen is needed by the developing embryos in each egg embedded in the egg-mass. The jelly does not allow the oxygen to diffuse as readily toward the embryo (and carbon dioxide out) as it would in water. If these gases are exchanged too slowly, the embryo may die. Dead eggs sometimes become evident because a white fungus often decomposes the embryo.
A common resolution to the gas exchange problem involves a relationship between the eggs of the spotted salamander and a tiny alga. The alga, called Oophila amblystomatis (which means “loves salamander eggs”) can invade the membranes of the eggs and grow there. The alga photosynthesizes and produces oxygen near the embryo where it is needed. In exchange, the carbon dioxide released by metabolism from the embryo is just what is needed by the alga. This relationship is known as a mutualism or symbiosis. Oophila is not always found in the eggs, but more embryos tend to survive when it is present. This alga does not occur anywhere else in nature but in the eggs of a few amphibians.
The left photo shows individual eggs within a salamander egg-mass, each with a population of the tiny green alga Oophila, which provides the greenish appearance. The middle photo is an enlargement of a single egg. The head and gills of the dark, developing embryo are visible to the right. The tiny, green dots are clusters of the alga, which helps the embryos to survive by producing oxygen. Far right image compares eggs with Oophila (left) to eggs without it (right).
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Renn Tumlison
Department of Biology
Box 7861
Henderson State University
Arkadelphia, AR 71999
Phone: (870) 230-5152
E-mail: tumlison@hsu.edu
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