Metabolic depression in fish measured by direct calorimetry: A review
ABSTRACT In nature under adverse conditions like low oxygen conditions or starvation fish often lower their metabolism: ‘metabolic depression’. This strategy of lowering the metabolic rate is a survival strategy and is used to save energy stores and diminish end-product accumulation. The overall metabolic rate of animals can be deduced by measuring metabolic processes such as oxygen consumption, but the ultimate method is measuring heat flow. In this review, we will summarise the available data about metabolic depression measuring heat flow, i.e. by direct calorimetry in fishes, which were carried out almost exclusively with a 1-l flow through calorimeter. Using deconvolution techniques the time constant of this calorimeter was measured that allowed to estimate the time course of metabolic depression, which was found to take place on a time scale of 20–30 min. We demonstrated that metabolic depression is species dependent. Goldfish, eel and tilapia show metabolic depression under low oxygen conditions while this is not the case for common carp. In addition it is shown that metabolic depression is flexible and increases with decreasing oxygen availability. Furthermore using a video analysing system we demonstrated that metabolic depression is not caused by a reduction of external activity. As heart rate falls dramatically during metabolic depression as shown by small wireless transmitters, we hypothesise that blood flow reduction might be the proximate cause for metabolic depression.
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ABSTRACT: During adaptive radiations, animals colonize diverse environments, which requires adaptation in multiple phenotypic traits. Because hormones mediate the dynamic regulation of suites of phenotypic traits, evolutionary changes in hormonal signaling pathways might contribute to adaptation to new environments. Here we report changes in the thyroid hormone signaling pathway in stream-resident ecotypes of threespine stickleback fish (Gasterosteus aculeatus), which have repeatedly evolved from ancestral marine ecotypes. Stream-resident fish exhibit a lower plasma concentration of thyroid hormone and a lower metabolic rate, which is likely adaptive for permanent residency in small streams. The thyroid-stimulating hormone-β2 (TSHβ2) gene exhibited significantly lower mRNA expression in pituitary glands of stream-resident sticklebacks relative to marine sticklebacks. Some of the difference in TSHβ2 transcript levels can be explained by cis-regulatory differences at the TSHβ2 gene locus. Consistent with these expression differences, a strong signature of divergent natural selection was found at the TSHβ2 genomic locus. By contrast, there were no differences between the marine and stream-resident ecotypes in mRNA levels or genomic sequence in the paralogous TSHβ1 gene. Our data indicate that evolutionary changes in hormonal signaling have played an important role in the postglacial adaptive radiation of sticklebacks.Current biology: CB 12/2010; 20(23):2124-30. · 10.99 Impact Factor