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Abstract
The bristlenose pleco (

Ancistrus cirrhosus

) is a species of Loricariidae armored catfish that breathes air using its highly vascularized stomach when faced with hypoxic aquatic environments. The main goals of this study were to determine the developmental onset of air breathing and air-breathing behavior in A. cirrhosus from juveniles to adults. Developing juveniles reach functional maturity within 4 to 6 months of hatching, growing to an adult length of 8 to 10 cm. To examine the timing of the developmental onset of air breathing, we tested for air-breathing behavior in juveniles beginning at 1 cm in length up through adults at 8 cm in length by exposing each fish to an acute gradual decrease in aquatic oxygen content from 100% air saturation down to 8% air saturation over a 50-min period. Juvenile fish first began to breathe air at body lengths between 2.1 and 3 cm and masses of 1.1 and 2 g. Fish with body lengths above 3 cm all breathed air when hypoxia challenged. In fish that breathed air, there was a weak negative correlation between fish length and % air saturation at which air breathing began. Fish were also exposed to an acute bout of 15% air saturation for 60 min, and breathing behavior was observed. Fish exposed to 60 min acute 15% air saturation exhibited a negative correlation between the frequency of air breathing and fish length. The developing A. cirrhosus can take advantage of the aerial environment once they reach 3 cm in length and 2 g when exposed to aquatic hypoxia. Prior to this size, these animals would potentially succumb to severe aquatic hypoxia without the ability to breathe air.