| The prolonged period of sexual indeterminacy and subsequent gonadal differentiation of lampreys was investigated. Larvae of the least brook lamprey, Lampetra aepyptera, collected from 12 geographically-diverse streams, remained sexually indifferent up to length of ca. 50 mm, or for 1-2 years. Although germ cell proliferation in sea lampreys, Petromyzon marinus, began at length of ca. 60 mm, gonadal differentiation was generally not complete in this species below lengths of 90 mm, or until ca. 3 years of age.; Sex ratio of the least brook lamprey larvae varied widely among the 12 streams. The extremes were 24 and 71% male and the proportion of males increased with population density, which was estimated from the number of larvae collected per m{dollar}sp2{dollar} of substrate. The skewed sex ratios were not likely due to sex-specific differences in mortality or recruitment to the adult population, since the sex ratios were established at gonadal differentiation and remained relatively constant thereafter. Density-dependent sex determination in lampreys is consequently proposed, and its possible adaptive significance is discussed.; The sex ratio of previously undifferentiated sea lamprey larvae ({dollar}<{dollar}60 mm initial length), maintained for 39 months in outdoor tanks and fed to excess, also varied with density. The percentage of males ranged from 33 to 46% at the lowest and highest densities, respectively, and increased significantly with density. Furthermore, mortality was neither sex-specific nor related to density. The sex ratio of larger larvae, however, was unaltered by stocking density. It appears, therefore, that sometime between the onset of germ cell proliferation and the point at which sex is discernible, the gonad loses its lability with respect to density.; In contrast, gonadal steroids were ineffective in altering the sex ratio of previously undifferentiated sea lampreys: larvae were immersed in estradiol, testosterone, or methyltestosterone at 1 of 3 doses for 21 weeks, and maintained without further treatment for another 25 months. Hormone treatment, however, resulted in high mortality and histological abnormalities in the gonads of many of the larvae. Further refinement of the treatment protocol, particularly a reduction of treatment-related mortality and an extension of the treatment period, seems necessary for successful hormonal sex control in lampreys. |
