Use of ultrasonography to determine sex in sexually immature European river lamprey Lampetra fluviatilis (L.)

•Ultrasonography can be successfully used for sex determination in immature river lamprey.•Use of ultrasonography resulted in100% success for sex determination in immature river lamprey.•Ultrasonography is safe for sexing river lamprey. In this study, there was the first attempt to sex immature European river lampreys Lampetra fluviatilis, classified as Agnatha using ultrasonography. This species starts a spawning migration from seas to rivers in the autumn and reproduction is initiated in the late spring. It is recommended to collect breeders soon after the beginning of the spawning migration, however, to date no method has been developed for distinguishing the sex of individuals during this developmental period. The lampreys for the present study were caught in autumn (November) in the Vistula River (northern Poland) during the period of spawning migration and transported to the laboratory. The lampreys were anaesthetised (MS–222, dose: 0.1 g/dm3 prior to sex determinations (n = 100) using ultrasonography. The images obtained using ultrasonography were verified with post–mortem and histological examinations. The findings with this study confirmed that the sex of the European river lamprey can be effectively ascertained much earlier than can occur with assessment of external secondary sexual characteristics (e.g., sexual papilla, ovipositor, skinfold). The advantages of the method include: 100% effectiveness, survival of the fish after examination, non–invasiveness, rapid verification of the lamprey sex and the possibility of determining extent of gonadal development.