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

Gonad morphology and cytology were examined in landlocked and anadromous se a lamprey, Petromyzon marinus, larvae. Larvae examined from both population s had gonads that were easily distinguished as female or male; however, man y had gonads that upon microscopic examination appeared atypical as compare d to previous descriptions. Atypical status was confirmed for approximately 90% of the larvae from each population based on quantitative assessment. D ifferences in gonad development were observed between the anadromous and la ndlocked sea lamprey. Gonad area and oocyte diameter were greater in ovarie s from the landlocked population. In addition, gonad area, perimeter and sh ape were also significantly greater in testes from landlocked population. E arlier studies on landlocked sea lampreys and non-parasitic species of lamp reys have indicated that sex ratios fluctuate with lamprey abundance and th at in non-parasitic lampreys, sex is labile and subject to environmental in fluence. It appears that sex in landlocked and anadromous sea lampreys is l abile and the atypical gonads observed are in transition from one sex to an other.