Novel perspectives in sexual lability through gonadal biopsy in larval sea lampreys

Gonadal biopsies of sea lamprey Petromyzon marinus larvae (118·5–142·5 mm) were examined quantitatively and categorized as ovaries, presumptive testes, or atypical gonads. Subsets of larvae were then killed 1, 2, 4, 8, 16 and 52 weeks following surgery at which time each larva was matched with its biopsy, and its gonad re–examined for quantifiable changes in composition. During 52 weeks, oocyte diameters increased significantly while numbers of oocytes per unit area significantly declined. Germ cell numbers remained within the range of typical presumptive testes. Overall abundance of typical ovaries decreased over the course of the experiment, whereas the number of presumptive testes increased. This was a direct result of presumptive testis development from atypical gonads, oocyte atresia and sex reversal. The results suggest an argument for environmental sex determination in lampreys, while indicating that they remain sexually labile throughout most of the larval period, despite primary differentiation.