Fig. 1. Changes in the urea excretion rate (µmol day^-1 g^-1 fish) of Himantura signifer (open bars; N=10) or Potamotrygon motoro (filled bars; N=5) during 4 days of exposure to environmental ammonia (10 mmol l^-1 NH₄Cl). Results represent means ± S.E.M. ^aSignificantly different from H. signifer control values (P<0.05). ^bSignificantly different from H. signifer day 1 values (P<0.05). ^*Significantly different from the corresponding H. signifer value.