A novel field technique to assess ploidy in introduced Grass Carp (Ctenopharyngodon idella, Cyprinidae)

Invasions by Asian carps have become a high profile topic due to the threat they pose to native ecosystems and associated economies. One of four primary invasive Asian carp species, Grass Carp, Ctenopharyngodon idella (Cyprinidae), is commonly produced as triploid, sterile, individuals and stocked as a biological control for the suppression of unwanted aquatic vegetation. Illegal sale and transport of diploid fish can result in unwanted fertile populations. Current methods of determining ploidy of an individual require the use of laboratory equipment (Coulter Counter) and are not possible to perform under field conditions. Here we introduce a rapid, inexpensive technique to distinguish diploid fertile versus triploid sterile Grass Carp under field conditions using a compound microscope. We compared blood smears of known diploid and triploid Grass Carp individuals, finding that the frequency of abnormally shaped erythrocyte nuclei (dumbbell or teardrop shaped) is significantly higher in the latter. This difference is accompanied by larger cell and increased nuclear volumes, and it is significantly correlated with Coulter Counter values (the standard measure of ploidy used by the United States Fish and Wildlife Service). Additionally, we field tested our method on a managed, presumably triploid, standing stock of Grass Carp and found that all individuals tested exhibited proportions of abnormal nuclei typical of triploids. Finally, a blinded study was used to confirm diagnostic reliability of our visual assessment of ploidy. Examination of blood smears for abnormally shaped nuclei may become a powerful tool in the management of invasive Grass Carp.