Effect of chronic ammonia exposure on locomotor activity in the fiddler crab Uca princeps upon artificial tides and light cycles

The aim of this study was to evaluate the effect of continuous exposure to sublethal ammonia concentrations upon the characteristics of the locomotor activity rhythms in Uca princeps, exposed to artificial tides and light cycles. Adult male crabs were used for standardized 24 h ammonia acute toxicity tests. Sublethal ammonia concentrations were selected considering the results obtained in the acute bioassays as well as the unusual high ammonia concentrations registered in their natural habitat. Three independent groups of eight crabs each were exposed during 1520 days to 0.125, 3.5, and 18 mg N-TA/L in artificial sea water. Locomotor activity was recorded individually by means of infrared light crossings. Artificially created tidal and lightdark cycles were used along the experiments. The 24 h-LC50 was 186.02 mg N-TA/L. No mortality was registered in the crabs exposed to the sublethal ammonia concentrations. In control groups, organisms showed tidal activity onset mainly in low tide (ebb) and no response to light cycles while ammonia exposed crabs shifted their onset of activity to high tide, and a general increasing activity was observed along the experiment. The obtained results indicate that ammonia affects the entraining features of the tidal activity rhythm and demonstrate that under ammonia sublethal exposure, tides may present relative coordination with light cycles in the tidal activity rhythm of the crabs.