Gonadogenesis in landlocked and anadromous forms of the sea lamprey, Petromyzon marinus

Gonad morphology and cytology were examined in landlocked and anadromous sea lamprey, Petromyzon marinus, larvae. Larvae examined from both populations had gonads that were easily distinguished as female or male; however, many had gonads that upon microscopic examination appeared atypical as compared to previous descriptions. Atypical status was confirmed for approximately 90% of the larvae from each population based on quantitative assessment. Differences in gonad development were observed between the anadromous and landlocked sea lamprey. Gonad area and oocyte diameter were greater in ovaries from the landlocked population. In addition, gonad area, perimeter and shape were also significantly greater in testes from landlocked population. Earlier studies on landlocked sea lampreys and non-parasitic species of lampreys have indicated that sex ratios fluctuate with lamprey abundance and that in non-parasitic lampreys, sex is labile and subject to environmental influence. It appears that sex in landlocked and anadromous sea lampreys is labile and the atypical gonads observed are in transition from one sex to another.