South American characids share very similar ionoregulatory characteristics

We examined ionoregulatory characteristics of four Characids from diverse locations in South America, emperor tetras (Nematobrycon palmeri), penguin tetras (Thayeria boehlkei), serpae tetras (Hyphessobrycon eques), and rosy tetras (Hyphessobrycon rosaceous). When held in 100 μmol L^-1 Na⁺ water, tetras had J_max values over 1100 nmol g^-1 h^-1, and K_m values below 60 μmol L^-1. When held in 1 mmol L^-1 Na⁺ water kinetic parameters were unchanged. Low pH had no effect on Na⁺ uptake (J_in^Na). At pH 3.25, Na⁺ loss (J_out^Na) was stimulated 35-85% in two of the four species. To test the linkage of J_in^Na to NH₃ and H⁺ extrusion we measured J_in^Na during exposure to 1 mmol L^-1 NH₄Cl (HEA) and 100 μmol L^-1 Acetazolemide (AZ). HEA stimulated J_in^Na of emperor tetras by 40%, but inhibited J_in^Na of penguin tetras by 50%; the two remaining species were unaffected. AZ (an inhibitor of carbonic anhydrase) inhibited J_in^Na of serpae tetras by 40%, but had no effect on the others. All tetras displayed ionoregulatory characteristics that are very similar to each other, which supports the argument that these physiological traits may be ancestral for this group and pre-date colonization of the Rio Negro. The novel finding that, J_max and K_m did not change after acclimation to 1 mM Na⁺ water indicates that, unlike in other species examined uptake is not plastic. The HEA and AZ results, along with pH insensitivity suggest Na⁺ uptake is not coupled to H⁺ extrusion or NH₃ excretion and leaves the exact mechanism involved unclear.