“…However, reduction in Hb concentration is common in many teleost species, possibly as a strategy for reducing blood viscosity in the cold (an energetic benefit) and/or because of relaxed selection for Hb-mediated oxygen transport in typically well-oxygenated waters where diffusion-based processes can suffice (Bargelloni et al, 1998;di Prisco et al, 2002;Beers and Sidell, 2011). Nonetheless, functional disaptations (traits whose use to the organism is discernibly inferior to that of a phylogenetically antecedent trait) of the cardiovascular system have been suggested for genetic loss of Hb and Mb (Montgomery and Clements, 2000;Sidell and O'Brien, 2006;Garofalo et al, 2009;Tota et al, 2012), which may have far-reaching implications in the face of environmental change. Also along these lines, O'Brien and colleagues revealed that hearts of icefishes lack the mitochondrial isoform of creatine kinase, an enzyme that plays a pivotal role in energy flux by catalyzing the reversible transfer of phosphate between creatine and ADP (O'Brien et al, 2014).…”