Non-lethal detection of the invasive American eel parasite Anguillicola crassus

The invasive species Anguillicola crassus is a nematode that infects the swimbladders of anguillid eels. Heavy, repeated infections cause the swimbladder to become thickened and scarred, which can alter swimbladder gas volume, increase energy demands of buoyancy regulation, and influence normal function. Silver-phase (sexually maturing) eels migrate up to thousands of kilometers to the Sargasso Sea to reproduce, and increased energetic requirements may be detrimental to migration and breeding success. Currently, the best practice to confirm A. crassus infection is to dissect an eel and examine the swimbladder. We used a portable digital X-ray system to determine the presence of A. crassus in American eels Anguilla rostrata. Silver-phase eels were anesthetized and radiographed. Post-imaging, individuals were dissected to compare the contents of the swimbladder to the radiographs. Infections appeared opaque on radiographs. Results showed no false positives and an accuracy of 74.8%. Out of 193 X-rayed eels, 107 contained parasites; 27 infections were undetectable on radiographs (false negatives). Detection was influenced by the intensity, size, and location of parasites within the swimbladder. This digital X-ray method is a quick and non-lethal process that could be incorporated into existing monitoring programs.