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INFORMATION FOR

Frog Identification Techniques

Tagging Xenopus has been done for quite some time, especially to identify females that lay good eggs. For frog genetics, it is especially critical to be able to uniquely distinguish a frog. Frogs have a lifespan of over 10 years, so uniquely tagging frogs is well worth the initial effort, because two genetically different X. tropicalis are so similar in appearance that they can easily be mixed up. We have investigated a number of different strategies to tag frogs and list our experiences here.

Isolation

Particular precious frogs can be isolated in tanks (male and female can be housed in 1 tank of adequate size) in order to identify them uniquely. While this is expeditious for identifying and finding a frog, it can be expensive with regard to space.

Microchips

These chips can be implanted under the skin, allowing the frog to be uniquely identified using a chip reader. Many frogs can be held in the same tank, although identifying the frog you are looking for can be time consuming. The frogs need to be removed from the tank and scanned one by one until the frog of interest is found.

Elastomers

This is a flexible colored elastomer that fluoresces under UV light. The elastomer and hardener are mixed at time of use. It can be injected under the skin of the frog, or between toe webbing. There are a limited number of colors, but by combining colors and location, a large number of unique identifications are possible. While inexpensive, it is best to tag a large number of frogs at the same time to avoid wasting the elastomer. We have found this type of tag useful for identifying a group of frogs, such as those carrying a certain mutation. One drawback is that the tags can sometime migrate out or dissapate, and need to be replaced.

Alphanumeric tags

These are fluorescent tags that can be placed under the skin to identify frogs. They consist of a letter followed by a two digit number. The number of unique IDs is limited by the numbers, letters, and locations where the tags are placed. This tag is relatively inexpensive compared to the microchip, and works well for smaller frogs. However, it is harder to insert and read. See the table below for more information on how the tags compare to the microchips.

Following is a table comparing different aspects of the Microchip versus the Alphanumeric tag. Overall, the microchip is easier to read and has a lower rate of loss (for us), but is more expensive.
MicrochipsAlphanumeric tags
Unit Cost: about $6/ chipUnit Cost: about $1/tag
Addn’l Cost: chip reader, glue, instrumentsAddn’l Cost: glue, instruments. Injector is $124, but we don’t use it
Code Identification: Simpler
Need a reader to ID. Little manipulation of the frog is necessary beyond simple restraint.
Code Identification: More Complex
Usually need brown glasses and blue light (both supplied) to fluoresce and read tag. Dark skin pigment interferes with reading code. Often requires manipulating frog to find/read the tag. Also may need to manipulate the tag under the skin to visualize the code.
Rate of Loss: Lower
Tends not to fall out if it is placed on the side of the leg opposite the incision
Rate of Loss: Higher
Tags tend to migrate under the skin and can be lost through an unhealed incision more easily
If chip falls out, it can usually be recovered from the bottom of the tankIf tag falls out, it is usually washed out of the tank, and cannot be recovered
Size: Larger
May not work in legs of smaller (male) frogs
Size: Smaller
Easily works with large or small frogs
Lifetime: life of frog (100+ years). In fact, the chip can be recovered from a dead frog, sterilized, and used again.Lifetime: life of frog
Placement orientation: not a concernPlacement position: potential to be placed upside down or to curl/fold under the skin when inserting.

Contributed by Maura Lane