eDNA to detect invasive crayfish

Globally, there are around 600 crayfish species, of which only about a half dozen have become problematic invaders in North America. These non-native crayfish prey on fish eggs and destroy aquatic plants, and can negatively affect fish through competition for food and changes to their habitat.

There are economic repercussions from invasions. One eradication of rusty crayfish in Wisconsin took years and was very costly.In that instance, success may have been due to a drought that substantially lowered the lake levels and stranded their habitat. Crayfish can walk over land so if you have them in an aquaculture pond there's nothing to prevent them from crossing over a little hill and then showing up in a national park. They're also prevalent in elementary and middle school science classrooms as live animals for behavioral studies. Teachers may not want to euthanize the crayfish at the end of the school year. Often believing that there is just one crayfish species everywhere, they have an end-of-semester release party and dump aquarium contents into a local pond or stream, or send crayfish home with students who may subsequently release them.

Researchers of the University of Illinois developed an eDNA based assay to detect the invasive rusty crayfish Orconectes rusticus in Lakes across Wisconsin and tested if the method is suitable to determine its relative abundance. They used COI DNA barcodes in combination with qPCR. 

We detected Orconectes rusticus eDNA in all lakes where this species was collected by trapping, down to low relative abundances, as well as in two lakes where trap catch was zero. Detection probability of Orconectes rusticus eDNA was well predicted by relative abundance of this species and lake water clarity. However, there was poor correspondence between eDNA copy number and Orconectes rusticus relative abundance estimated by trap catches.

As with many studies using eDNA to identify and track organisms in various aquatic ecosystems, the simple presence/absence detection works quite well and only needs calibration as the method is usually very sensitive. However, this study shows once more that a quantitative eDNA technique has yet to be established.