Patterns of air-breathing behavior in juvenile armored catfish, Pterygoplichthys sp. (Gill 1858)

Many hypoxic freshwater springs have become havens for air-breathing, invasive, loricariid armored catfish. However, this does not seem to be the case for juvenile catfish, which are rarely seen. We wondered if hypoxia and air-breathing influenced habitat selection in juveniles, and so we designed an experiment to test the sensitivity of two size classes of juvenile catfish to varying concentrations of dissolved oxygen. Small and medium-sized juvenile Pterygoplichthys sp. were exposed to hypoxic, transitional, or normoxic conditions in the laboratory, and their air-breathing rates were measured and compared between fish size and dissolved oxygen category using a two-way ANOVA. We also measured air-breathing synchrony, which was assessed with a coefficient of dispersion and compared between fish sizes with a two-tailed, one-sample t-test, and with dissolved oxygen using linear regressions. We found that all fish air-breathed significantly more frequently under hypoxic conditions and smaller fish breathed significantly more frequently than medium fish at all dissolved oxygen levels, although there was no interaction effect. There was no air-breathing threshold oxygen tension, and only small fish exhibited synchronous air-breathing behavior. All of the behavioral changes we observed as dissolved oxygen decreased would put juveniles at greater risk of predation, and likely explains why so few juveniles are found in hypoxic waters.