BLANK, T.M.*; BURGGREN, W.W.; Univ. of North Texas; Univ. of North Texas: Early Respiratory Development of the Blue Gourami (Trichogaster trichopterus), an Air-breathing Fish

Environmental factors can alter relative developmental timing of individuals (�heterokairy�), though how this influences onset of aerial respiration in air-breathing fish is unknown. Thus we examine the effects of hypoxia on respiratory development in blue gourami. Adult gouramis are obligatory air-breathers, using labyrinth organs in epibranchial chambers to extract aerial O₂. Larvae rely on aquatic respiration until the labyrinth functions. We hypothesize that aquatic hypoxia accelerates labyrinth development, leading to earlier respiratory transition. As a prelude to hypoxic experiments, we describe development in normoxia. Embryos were raised at 27°C in air-equilibrated water (21% O₂). Heart rate (f_H), mass-specific MO₂, and survival rates of larvae denied access to air for 24h were recorded through 14 days post fertilization (dpf). f_H increased significantly from 120±6.1 beats per minute (bpm) at 1dpf (n=30) to 232±4.1 bpm at 3dpf (n=25), but did not vary significantly thereafter, remaining ~220 bpm. Prior to hatching (1dpf) and <1d after hatching, MO₂ values were 0.01±0.001 (n=8) and 0.22±0.015 (n=11) µmolO₂·mg^-1·h^-1 respectively, and differed significantly from 3-14dpf, which remained ~0.10 µmolO₂·mg^-1·h^-1. Surface denial did not result in mortalities, indicating air breathing is not obligatory before 14dpf. Preliminary observations of hypoxic groups (10% O₂) through 8dpf revealed morphological deformities and lower f_H (133±4.6 bpm) than normoxic controls. These differences were not observed in larvae reared in 15% O₂, suggesting an O₂ threshold for normal development between 10% and 15% O₂. The rapid and complex changes in f_H and MO₂ during early development, combined with a reasonable tolerance of hypoxia, reveal the blue gourami as a useful model for future studies of heterokairy.