How to Identify Tadpoles
Tadpoles are the larvae of the anurans---frogs and toads. While relatively easy to find, as they are restricted to water bodies and may amass in large numbers, tadpoles seem to confound easy identification. They are often notably smaller, less distinctive and radically different-looking than the adult forms they metamorphose into. Nonetheless, with the help of a good taxonomic key and a little understanding of tadpole morphology, the keen-eyed naturalist can make some headway. The U.S. Geological Survey's Patuxent Wildlife Research Center has provided an excellent tadpole identification resource online.
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Instructions
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Learn basic tadpole morphology. In their essence, tadpoles consist of a round body---with eyes, nostrils, mouth (oral disc), spiracle and vent---and a proportionately large tail. Developing tadpoles will begin to grow hindlegs and eventually, as metamorphs, sprout forelegs and absorb their tail.
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Conduct some measurements for starters. It is often useful to measure the following: the tadpole's total length, from mouth to the tip of the tail; the height of the tail muscle, taken at the location of the vent; the total height of the tail (including the fin) taken at its tallest point; and the length of the body alone, not counting the tail.
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Key into the tail. What is the pattern of markings? Are they banded or striped? Uniformly colored or bicolored? Are the markings different on the tail fin and the tail muscle?
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Note the position of the eyes. Are they placed dorsally toward the top of the head, so that they don't register in a top-down or bottom-up silhouette? Or, are they lateral, bulging from the sides of the head?
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Look at the spiracle, the opening in the tadpole's body from which water exits the opercular chamber. There may be two spiracles balanced in lateral or ventrolateral position. There may be a single spiracle low on the body, approaching the vent; or there may be a one lateral spiracle on the tadpole's left side.
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Observe the tadpole's mouthparts. The jaw sheaths feature bands of keratinization that manifest as a dark border to the mouth. Eyeball the relative width of these dark sheaths: narrow (less than half the width of the jaw), medium (half the width is dark) or wide (the jaw is almost entirely dark). Some complicated identification may require close examination of the oral disc, which surrounds the jaws and is typically convoluted.
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Tips & Warnings
Measurements are best taken from a living tadpole, or at least one that is well-preserved. A source of magnification will likely be useful.
Use a taxonomic key, such as that provided online by the U.S. Geological Survey (see Resources), for in-depth identification.
