Body mass, length, oxygen consumption ((M) over dot o(2)), and heart rate (f(H)) were measured in "embryos" (prior to hatching), "larvae" (days 10-20), "juveniles" (days 30-70 in 10-day intervals), and "adults" (day 100) of the zebrafish Danio rerio. Fish were chronically reared at either 25, 28, or 31 degrees C and then acutely exposed to hypoxia at different developmental stages. We hypothesized that at any given rearing and measurement temperature, D. rerio would maintain Mot at lower ambient Po-2 [i.e., have a lower critical partial pressure (P-crit)] as development progressed and that at ally given developmental stage individuals reared and measured at higher temperatures would show a more pronounced hypoxic bradycardia. (M) over dot o(2) in normoxic fish at 28 degrees C peaked at similar to 40 mu mol . g(-1) . h(-1) at day 10, thereafter falling to 4-5 mu mol . g(-1) . h(-1) at day 100. The Q(10) for Mo-2 was 4-5 in embryos, falling to 2-3 from day 10 to day 60 and rising again to 4-5 at day 100. P-crit at 28 degrees C was similar to 80 mmHg in embryos but decreased sharply to 20 mmHg at 100 days, supporting the hypothesis that more mature fish would be better able to oxygen regulate to lower ambient Po-2 levels. P-crit increased sharply with measurement temperature. Heart rate (f(H)) at 28 degrees C increased from about 125 beats/min in embryos to a peak of similar to 175 beats/min at days 10-30 and then fell to similar to 130 beats/min by day 100. Unlike for (M) over dot o(2), the Q(10) for f(H) was more constant at 1.2-2.5 throughout development. Hypoxic exposure at any temperature had no effect on f(H) until similar to day 30, after which time a hypoxic bradycardia was evident. As evident for (M) over dot o(2), the bradycardia in older larvae was more profound at higher temperatures. On the assumption that bradycardia is indicative of hypoxic stress, the increasing prevalence of a hypoxic bradycardia in older, warmer individuals supports the hypothesis that increasing hypoxic susceptibility with development would be exacerbated by increasing temperature. Collectively, these data indicate that the ability to regulate (M) over dot o(2) and f(H) in response to the compounding demands of increased temperature and/or decreased oxygen availability first develops after similar to 20 days in D. rerio and, thereafter, the ability to maintain (M) over dot o(2) in the face of ambient hypoxia progressively builds through to adulthood. Additionally, the temperature responses of metabolism and heart rate differ substantially at different phases of development, suggesting a loose coupling between the respiratory and cardiovascular systems, at least early in development.
