Are Chloride Cells Involved in Low pH Tolerance and Sensitivity? The Mayfly Possibility

Abstract

Sensitivity to low pH in freshwater fish appears to be correlated with density and replacement rates of gill chloride cells, but these cells are difficult to examine. Mayfly nymphs possess cells of similar appearance and probable function and are easily observed in dense accumulations on gill surfaces. We examined chloride cell density on the gills of nymphs of seven species of mayflies (two Heptageniidae, two Leptophlebidae, one Siphlonuridae, and two Baetidae). Although tolerance of low pH exposure varied considerably among the seven species, it was not correlated with gill area or chloride cell density. Gills of nymphs from soft water also had no greater chloride cell density than gills of nymphs of the same species from hard water. Finally, nymphs of the two species able to withstand 2 wk exposure to low pH did not show an increase in chloride cell density. It remains possible that the prolonged ionoregulatory stress of low pH results in cell damage rather than cell proliferation in sensitive species.