Hematocrit, osmolality and ion concentration in fishes: consideration of circadian patterns in the experimental design

Significant circadian patterns in plasma osmolality and chloride ion concentration were documented in the sailfin molly Poecilia latipinna (Baird & Girard) and the common snook Centropomus undecimalis Bloch; however, significant patterns were not found in the sheepshead minnow Cyprinodon variegatus Lacepede. There were, however, no distinct patterns in hematocrit for any of the three species. Circadian patterns, although important in ion and osmoregulation studies are usually not considered. Furthermore, the time of day the fishes are tested are typically not mentioned. Questionable results and difficulty in making comparisons with previously published data can occur if the patterns are present and not considered in the experimental design.