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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.

In callichthyid catfishes, the posterior intestine is modified to function as an air breathing organ by being air-filled, thin-walled and highly vascularized. These modifications make it unsuitable for digestive functions and digesta has to be transported quickly through this region to minimize disruption of vital respiratory functions. However, the weak muscles of the wall of the respiratory intestine make this problematic. It is hypothesized that the unidirectional ventilatory air current within the respiratory intestine is responsible for the quick transport of digesta through the respiratory intestine. To verify this, movement of digesta through the alimentary tract was examined in Hoplosternum littorale and Corydoras aeneus that were either allowed to breathe air or prevented from air breathing. When air breathing was prevented, digesta was not transported to the rectum in H. littorale and there was a 94% reduction in the amount of digesta in the rectum of C. aeneus. This study suggests that the anterior digestive intestine facilitates the passage of air although it is filled with digesta. The anterior digestive intestine packages digesta into a string of slightly compressed boluses, creating an air channel in the digestive intestine thus allowing air to pass unimpeded.

Food consumption, standard metabolism, and growth of juvenile snakehead, Channa striatus, a cannibalistic and air-breathing fish were measured at 24–26 °C under controlled laboratory condition. Snakehead weighing 3.2–29.5 g were evaluated, and were fed smaller snakehead. Based on our observations, we determined bioenergetics relationships between specific food consumption, metabolic rates, and body weight. These values, along with other published parameter values allowed us to construct a bioenergetics model for snakehead. We then verified our model with growth and food consumption measurements from an independent feeding trial. Predicted fish growth closely matched observed growth. Our model underestimated cumulative food consumption when a constant activity value was used, but consumption estimates improved when we used non-constant activity values (1-5 times of standard metabolism). Predicted fish maintenance ration was 1.7% of body weight per day. Food conversion efficiency was greatest (0.59) when fed 2% body weight daily, but declined when daily consumption exceeded 6% body weight. This model provides a useful approach for assessing food requirements of snakehead under controlled condition.

Several lines of circumstantial evidence suggest that striped mullet, Mugil cephalus, use the upper posterior portion of the pharynx for aerial respiration, the air obtained either by jumping, rolling, or holding the head above the water and moving the air into the upper pharyngeal chamber. The principle evidence is that jumping frequencies are inversely correlated with dissolved oxygen concentration, and that the pharyngobranchial organ is capable of holding gas.