A Note of Color: Marking Tadpoles with Elastomers

elastomero 01Occasionally we need to mark some individuals in order to study different aspects of their biology or of their populations. For example, in marking and recapture studies, in which we try to estimate the size of a population, with the mark of captured individuals and the registration of their successive captures, it is possible to estimate the size of the population. Other times, in some experiments it is necessary to identify some individuals in order to study certain factors, like how they can be infected with the chytrid fungus.

A lot of animals are relatively easy to mark. In the case of birds, banding rings are not only very effective due to their strength and durability, but, when done correctly, do not aggravate the animal. Unfortunately, the delicate skin and small size of amphibians means that marking them is more complicated.

elastomero 02For adult amphibians we can use microchips. These small objects, which can be the size of a grain of rice, are implanted under the skin or in the abdominal cavity. In this case, aggravation is minimal, especially with larger species or those that have thicker skin, and it lets us identify each individual for the rest of its life. We use this marking technique in order to estimate the abundance of the common midwife toad, the Iberian frog, and also to identify common midwife toads which were born in captivity.

However, when we want to mark amphibian larvae, their small size makes it difficult to use microchips. In this case, we use elastomer implants, a material that, in principle, does not interact with the animal’s tissues. The technique consists of making a mark under the skin with a hypodermic needle, using a liquid silicon based material.  

This material is composed of two parts: a pigment and a catalyst, which mix at a ratio of 10:1 just before use. When inserted under the skin, the material becomes solid, flexible and remains in position. The result is a mark easily visible under the transparent or translucent skin of an amphibian. In addition, if we use a fluorescent pigment, illuminating it with an LED lamp can be very striking.

elastomero 03This technique involves minimal impact on the animal, especially when compared with other techniques like tail or phalange amputation. Elastomer implants do not bother or irritate the animal, and the needle used is very fine. In addition, it is an economical method and making the mark is rapid and simple, after some acquired practice. The possibility of using different colors to make various marks generates a great variety of designs which can distinguish individuals.

All of this makes elastomer marking a recommended marking technique for amphibians, especially for their larvae. Nevertheless, in some cases the marks are completely lost with the passage of time or during the process of metamorphosis. Therefore, it is best used for the identification of specimens for short periods, like during the course of an experiment.

This week we have tagged some common midwife toad tadpoles in order to launch a new experiment about the dynamics of fungi infections, which we will explain later. For the moment, you can see the photos of the tagging results

In this video you can see the marking technique:

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