Environmental effect on plasma thyroxine (T4), 3,5,3'-triido-L-thyronine (T3), prolactin and cyclic adenosine 3',5'-monophosphate (cAMP) content in the mudskippers Periophthalmus chrysospilos and Boleophthalmus boddaerti.

1. In both Periophthalmus chrysospilos and Boleophthalmus boddaerti, T4 was involved in enabling the fish to cope with terrestrial stress and not in osmoregulation in waters of different salinities. In B. boddaerti, however, 3,5,3'-triiodo-L-thyronine (T3) played a more significant role in osmoregulation under the various aquatic conditions. 2. The control of osmoregulation mechanisms in P. chrysospilos kept in waters of different salinities was taken over by prolactin instead, whereas prolactin was only involved in osmoregulation in B. boddaerti under extreme osmotic stress (100% SW). Prolactin is also involved in the terrestrial adaptations of P. chrysospilos. 3. Plasma cAMP levels in P. chrysospilos increased with increasing salinity of the external environment (Tables 4 and 5) implicating its role in the stimulation of chloride secretion and in intracellular isosmotic regulation. 4. Significant increase in the plasma cAMP level of B. boddaerti submerged in 100% SW was also observed. However, the plasma cAMP levels of B. boddaerti fully submerged in 30% and 50% SW were not significantly different from the control as these conditions simulated those of their natural habitats.