EFFECTS OF SALINITY ON DISTRIBUTION, GROWTH AND SURVIVAL OF 3 NEOTROPICAL ARMORED CATFISHES (SILURIFORMES, CALLICHTHYIDAE)

Three species of closely related armoured catfishes, Hoplosternum litt orale, H. thoracatum and Callichthys callichthys, are widely distribut ed throughout (sub)tropical South America, but a detailed inspection o f collection localities in both Suriname and the South American contin ent reveals a clear pattern. Hoplosternum littorale is not found in ra inforest creeks and clear-water rivers draining the Precambrium Guiana and Brazilian Shields. Here, waters are extremely poor in dissolved m inerals. In salinity tolerance experiments, 1-week-old larvae of H. li ttorale were found to be incapable of surviving in rainwater that is a lso very poor in electrolytes. Probably for the same reason, H. littor ale is not collected in black-water rivers like the Rio Negro. Within the Amazon river-system H. littorale is found in sediment-laden white- water rivers originating in the Andes. Larvae of H. thoracatum and C. callichthys are able to survive and develop further in rainwater and c onsequently have a less restricted distribution than H. littorale. Bot h species are found in clear water and black water. All three species tolerate salinities up to 2000 mg Cl/l, and show a marked decrease in growth at low salinities.