The gilthead seabream is a protandrous hermaphrodite seasonal breeding teleost with a bisexual gonad
that offers an interesting model for studying the two first reproductive cycles (RCs), in which the specimens develops as males. During the first RC (RC1), 11-ketotestosterone (11-KT) and testosterone (T), the
main androgens in fish, play different and specific roles in the testicular physiology as they peak at different stages of RC1. Moreover, the profiles of T serum levels during the second RC (RC2) demonstrated
that this androgen is not essential in the testicular regression process that occurs during this cycle. However, changes in serum levels suggest that 17b-estradiol (E2) orchestrates this process during both RCs.
Moreover, the E2 serum levels recorded during RC2 indicate that there is a threshold level that determines the initiation of ovarian development during this cycle without promoting complete feminization.
We analysed triglyceride, protein, glucose and lactate serum levels in order to establish a relationship
between the mobilization and transfer of nutrients, the hormonal changes that take place during the
RC and body composition, finding that in vivo serum levels of metabolites change significantly throughout the first two RCs, although the physiological relevance of such changes is still unknown. Triglyceride
levels seem to be affected by the beginning of ovary development during RC2 but not by testicular recrudescence during RC1. Moreover, glucose and lactate might be important sources of energy during the
resting and testicular involution stages.