THE HEART OF THE ICEFISH - BIOCONSTRUCTION AND ADAPTATION

The Channichthyidae or ''icefish'' represent an intriguing example of extreme adaptation to the stable low temperature and high oxygen conte nt of the Antarctic waters. The lack of respiratory pigments (hemoglob in and myoglobin) in these teleosts is associated with relatively low oxygen consumption and relevant. cardio-circulatory adjustments which include large blood volume, increased relative heart weight (cardiomeg aly), and very high cardiac output. The heart has the ability to displ ace large systolic volumes at a low rate and relatively low pressure, with large ventricular fillings (high ventricular compliance), whereas it is incapable effacing increased afterloads. These functional aspec ts of mechanical flexibility and restrictions of the cardiac pump have been tentatively related to some constructional aspects of the icefis h cardiomegaly, particularly, at the whole ventricular level, to the t rabeculate type of myo-architecture, and, at the subcellular level, to the conflict in space economy between the exceptionally high mitochon drial densities and the consequent severe reduction in myofibrillar vo lume. On the basis of this morphodynamic approach, we suggest that the icefish may illustrate how a certain feature (i.e., an architectural cardiac design) common to the suborder and to most teleosts, and appar ently with ''irrelevant'' properties, can become useful for a speciali zed purpose (i.e., volume pump design); and how, in contrast, the inte rnal machinery construction. because of structural and ultrastructural constraints, may pre;ent these stenothermal sedentary teleosts from c onquering niches requiring more active locomotory habits.