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Research on Champsocephalus gunnari and other icefishes of the Teleostei class has shown that their hearts are pale due to low levels of myoglobin expression, which was once thought to be related to their polar habitats. Recently, researchers have found that some tropical teleost species (such as Piaractus brachypomus) also have pale hearts, raising the possibility that myoglobin deficiency in teleosts could be _blank habitat.