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 (∼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