Development and validation of an enzyme immunoassay for testosterone:: Effects of photoperiod on plasma testosterone levels and gonadal development in male sea bass (Dicentrarchus labrax, L.) at puberty

A specific immunoassay was developed for the quantification of testosterone (T) in sea bass plasma. Specific primary antibody against T was produced using an antigen BSA conjugated with T. The enzyme immunoassay (EIA) had a sensitivity of 5-0.009 ng ml(-1) and 6.2% intra-assay variation; inter-assay variation was 9.5% for sea bass plasma. The effects of two different accelerating photoperiod regimes, compressed photoperiod (CO; 6 months), and constant short photoperiod (9L:15D) with a long photoperiod (15L:9D) in March (SLmar), on T plasma levels and sexual maturation were examined during the onset of puberty in male sea bass. Natural photoperiod (NP) and SLmar groups exhibited the highest T values in December (8.69 +/-1.03 and 10.85 +/-1.04 ng ml(-1), respectively). However, SLmar group showed the first significant decrease in T plasma levels in January, two months earlier than the NP group, which presented elevated T levels until February. The CO group displayed two significant T peaks, one in October (8.90 +/-1.60 ng ml(-1)) and the other in January (9.60 +/-1.10 ng ml(-1)). Gonadosomatic index (GSI) in the NP and SLmar groups displayed the highest values from December to February (>2.5%). However, the SLmar group showed the first significant increase in GSI in November, one month before the controls, indicating a clear advancement of gonadal development with respect to the NP group. In the CO group, a bimodal pattern was observed with two peaks, one in October-November (1.30 +/-0.25%) and the second in March-April (0.97 +/-0.33%) (P <0.05). In NP group, the percentage of running males was about 80% from December to March while the percentage of running males in the SLmar group (similar to 70%) lasted only three months (December to February) decreasing (P <0.05) in March. In the CO group, spermiation began in October (60%), decreased during the next months, and increased again in March-April (30%) (P <0.05). These results indicate the advancement of puberty by either one or two months with respect to the control group in the SLmar and CO groups, respectively, and the presence of a second reproductive surge in the CO group. Collectively, these results suggest that exposure of fish to these photoperiod regimes may affect both the time of the onset of puberty and the pattern of gonadal development in prepuberal male sea bass.