| Summary: | The oxygen-limited thermal tolerance (OLTT) hypothesis suggests that an organism’s upper and lower thermal tolerance limits are linked to the point at which the organism cannot extract enough oxygen from its environment to meet its metabolic demands. Therefore, one conclusion that could be drawn from this hypothesis is that an organism’s metabolic rate (Mo₂) should be inversely correlated with the maximum environmental temperature it can tolerate. This is because organisms with a higher demand for oxygen will be the first to experience a mismatch between oxygen supply and demand as the amount of dissolved oxygen in water decreases with increasing temperature. The validity of this hypothesis was tested using the common killifish, Fundulus heteroclitus, from a contact zone between the two subspecies: the Northern Fundulus heteroclitus macrolepidotus and the Southern Fundulus heteroclitus heteroclitus. I found that there was a significant inverse correlation (p=0.0192) between the critical thermal maximum (CTMax) and routine metabolic rate (RMR) after controlling for the weight of fish, which supports the OLTT hypothesis. The predictive power of this relationship, however, was low (R²=0.16). In addition, there was a significant effect of collection location on CTMax (p<0.001) with fish that were collected from a more southern marsh having higher thermal tolerance than fish from a more northern marsh within the contact zone.
I also examined the relationship between nuclear and mitochondrial genotypes and 3 physiological measurements in fish from the contact zone: RMR, CTMax, and critical oxygen tension (Pcrit). Using 7 nuclear microsatellite markers, a hybrid index was generated for each individual in order to assess nuclear genotype. I found no correlation between nuclear genotype and any of the physiological parameters measured. I did see a trend towards a significant correlation between mitochondrial genotype on RMR, however (p=0.0595). Fish with Northern mitochondrial genotypes tended to have higher RMR’s than Southern fish, as was found in previous work on fish from the Northern and Southern extremes of the range. Together these data suggest that mitochondrial genotype may have some effect on metabolic rate and that metabolic rate has some relationship with thermal tolerance, but that these linkages are subtle.
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