Development and validation of an enzyme immunoassay for testosterone: Effects of photoperiod on plasma testosterone levels and gonadal development inmale 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 u sing an antigen BSA conjugated with T. The enzyme immunoassay (EIA) had a s ensitivity 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 accel erating photoperiod regimes, compressed photoperiod (CO; 6 months), and con stant 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 group s exhibited the highest T values in December (8.69 +/-1.03 and 10.85 +/-1.0 4 ng ml(-1), respectively). However, SLmar group showed the first significa nt decrease in T plasma levels in January, two months earlier than the NP g roup, which presented elevated T levels until February. The CO group displa yed two significant T peaks, one in October (8.90 +/-1.60 ng ml(-1)) and th e other in January (9.60 +/-1.10 ng ml(-1)). Gonadosomatic index (GSI) in t he NP and SLmar groups displayed the highest values from December to Februa ry (>2.5%). However, the SLmar group showed the first significant increase in GSI in November, one month before the controls, indicating a clear advan cement 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 N P group, the percentage of running males was about 80% from December to Mar ch 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 M arch. 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 wi th respect to the control group in the SLmar and CO groups, respectively, a nd the presence of a second reproductive surge in the CO group. Collectivel y, 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 gonada l development in prepuberal male sea bass.