Pituitary follicle-stimulating hormone (FSH) and luteinizing hormone (LH) levels in maturing female flounder Platichthys flesus under hydrostatic pressure simulating vertical migrations

Rhythmic vertical migration is a widespread feature among teleost species and the ultimate reasons for this behaviour could be as diverse as transport, feeding, predator avoidance or reproduction strategies. A rhythmic vertical migration implies that a fish will experience associated cyclic pressure changes. We examined the effect of 14-day exposure to cyclic variations in hydrostatic pressure (HP) and to elevated constant HP on pituitary levels of two types of gonadotropin, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), in maturing but not ovulating female European flounder (Platichthys flesus). Fish were exposed to cyclic HP with a tidal period of 12.4 h and an amplitude of 600 kPa, between 200 and 800 kPa (environmentally realistic vertical migration), to constant absolute HP of 800 kPa, and to a maximum 1 m height water column. Flounder pituitary FSH content was remarkably higher under cyclic HP conditions than under constant normal or elevated HP conditions. In fact, pituitary FSH levels were undetectable, irrespective of the constant HP regimen. By contrast, pituitary LH levels showed no significant differences between the different groups at the end of the experimental period. This study describes an important alteration of the brain-pituitary-gonad (BPG) axis provoked by exposure to HP fluctuations, which mimic conditions in nature. The results indicate that levels of FSH in female European flounder are influenced by rhythmic vertical migrations.