1791 The Journal of Experimental Biology 209, 1791-1802 Published by The Company of Biologists 2006 doi:10.1242/jeb.02091 Commentary When bad things happen to good fish: the loss of hemoglobin and myoglobin expression in Antarctic icefishes Bruce D. Sidell1,* and Kristin M. O’Brien2 1School of Marine Sciences, University of Maine, 5751 Murray Hall, Orono, ME 04469-5751, USA and 2Institute of Arctic Biology, University of Alaska, Fairbanks, PO Box 757000, Fairbanks, AK 99775, USA *Author for correspondence (e-mail: [email protected]) Accepted 12 January 2006 Summary The Antarctic icefishes (Family Channichthyidae) and their associated nitric oxide (NO)-oxygenase activities, provide excellent examples of unique traits that can arise may have accelerated the development and evolution of in a chronically cold and isolated environment. Their loss these cardiovascular modifications. The high levels of NO of hemoglobin (Hb) expression, and in some cases, loss of that should occur in the absence of Hb and Mb have been myoglobin (Mb) expression, has taught us much about the shown in other animal groups to lead to an increase in function of these proteins. Although absences of the tissue vascularization, an increase in the lumenal diameter proteins are fixed traits in icefishes, the losses do not of blood vessels, and an increase in mitochondrial appear to be of adaptive value. Contrary to some densities. These characteristics are all hallmark traits of suggestions, loss of Hb has led to higher energetic costs for Antarctic icefishes. Homeostatic feedback mechanisms circulating blood, and losses of Mb have reduced cardiac thus may have accelerated evolution of the pronounced performance. Moreover, losses of Hb and Mb have cardiovascular traits of Antarctic icefishes. resulted in extensive modifications to the cardiovascular system to ensure adequate oxygen delivery to working Key words: hemoglobin, myoglobin, Antarctic icefish, nitric oxide, muscles. Recent studies suggest that losses of Hb and Mb, heart, circulation. Introduction temperatures of –1.8°C (DeWitt, 1971). The water column Comparative physiologists instinctively are drawn to south of the Antarctic Polar Front is exceedingly well mixed organisms that display superlative physiological vertically, and all depths are close to complete oxygen characteristics. This really is a corollary to the famous ‘August saturation. Because oxygen solubility in seawater is inversely Krogh Principle’ that states: “For a large number of problems proportional to temperature, the cold Antarctic seas thus are an there will be some animal of choice or a few such animals on exceptionally oxygen-rich aquatic habitat. which it can be most conveniently studied” (Krogh, 1929). Notothenioids account for approximately 35% of fish Although we may take issue with the grammar of this sentence, species and 90% of fish biomass found south of the Antarctic the concept is unassailable. In the realm of comparative Polar Front (Ekau, 1990). Radiation of closely related cardiovascular physiology, few groups of animals can rival the notothenioid species has occurred rapidly (within the last 12 Antarctic icefishes in meeting the criterion described by Krogh. million years, MY) (Bargelloni et al., 1994) and under a very It is upon this group that we will focus our Commentary. unusual set of conditions. First, notothenioids have evolved in The Antarctic icefishes (Family Channichthyidae) are one of relative oceanographic isolation from other faunas due to eight families of the single perciform suborder, Notothenioidei, circumpolar currents and deep ocean trenches surrounding the which dominate the fish fauna surrounding Antarctica (for continent. Second, the Southern Ocean has been characterized excellent reviews, see Eastman, 2005; Kock, 2005). They by severely cold water temperatures for the last 10–14 MY occupy the coldest, most thermally stable marine environment (Kennett, 1977). Finally, evolution of these fishes has on earth. Sea temperatures near the Ross Ice Shelf at McMurdo progressed under conditions of very low levels of niche Station, Antarctica, are nearly constant at –1.9°C (Littlepage, competition because a dramatic crash in fish diversity occurred 1965) and even those in the more northerly Antarctic Peninsula in the Southern ocean sometime between the mid-Tertiary and range only between summer temperatures of +1.5°C to winter present (see Eastman, 1993; Eastman, 2005). These features THE JOURNAL OF EXPERIMENTAL BIOLOGY 1792 B. D. Sidell and K. M. O’Brien make Antarctic notothenioid fishes an uniquely attractive fourfold those of red-blooded teleosts, and the diameter of their group for the study of physiological and biochemical capillaries is unusually large (Fitch et al., 1984). These features adaptations to cold body temperature. Today’s notothenioids collectively permit a large volume of blood to circulate are arguably the end result of an extraordinary natural throughout the bodies of icefishes at high flow rate, yet, at low experiment. They provide a window into the exceptional vascular pressure because of decreased peripheral resistance. physiological characteristics that can arise in animals living at Combined with the very high oxygen content of Antarctic chronically cold body temperatures. Some of these waters and relatively low absolute metabolic rates, these characteristics clearly are adaptive (e.g. development of unusual cardiovascular attributes ensure that adequate oxygen antifreeze glycoproteins). Others would be deleterious, if not is delivered to tissues to support the obligately aerobic mode lethal, in warmer and more competitive environments. of metabolism of these animals (Hemmingsen, 1991). The intracellular oxygen-binding protein, myoglobin (Mb), is also not uniformly expressed in species of Family Patterns of oxygen-binding protein loss among the Channichthyidae. Myoglobin is widely distributed in Channichthyidae aerobically poised tissues of animals, and has been ascribed One of the most unusual and fascinating physiological critical roles in both intracellular storage and diffusion of characteristics of Antarctic notothenioids is the complete loss oxygen (Wittenberg and Wittenberg, 2003). Indeed, The of hemoglobin (Hb) in the family Channichthyidae. These Journal of Experimental Biology recently carried a peculiar looking fishes were appropriately named ice fish, by Commentary (Ordway and Garry, 2004), the title of which early British whalers and were first described physiologically indicated that myoglobin was an ‘essential hemoprotein in in 1954 (Ruud, 1954). striated muscle’. It would appear that the channichthyid Icefishes are the only known vertebrate animals to lack Hb icefishes confirm the old adage that there are exceptions to as adults (Fig.·1). Oxygen is found solely in physical solution every rule. Among the 16 known species of the family, ten in icefish blood, which has an oxygen carrying capacity of icefish species do express Mb in their heart muscle, while six <10% of that seen in red-blooded notothenioid fishes (Holeton, others do not produce the protein (Grove et al., 2004). 1970). Several fairly draconian modifications of the The very close phylogenetic relationship among families of cardiovascular system of icefishes compensate for their lack of Hb-expressing and Hb-less notothenioids and among Mb- a circulating oxygen-carrier. Icefishes possess very large hearts expressing and Mb-lacking icefishes presents an unique matrix compared to red-blooded fishes of equivalent body size, of ‘naturally occurring genetic knockouts’ that can be exploited resulting in a weight-specific cardiac output that is four- to to probe and understand the myriad of processes that regulate fivefold greater than that of red-blooded species (Hemmingsen et al., 1972; Fig.·2). The blood volumes of icefishes are up to Fig.·2. Hearts from three species of notothenioid fishes. The channichthyid icefish Chaenocephalus aceratus has a pale yellow ventricle (far left) and lacks myoglobin (Mb) protein expression. The channichthyid icefish Chionodraco rastrospinosus expresses myoglobin protein and displays a rose-colored ventricle (middle). The related notothenioid species Notothenia coriiceps has a Fig.·1. Lack of circulating hemoglobin and red cells is the signature characteristically red ventricle (far right) associated with the presence characteristic of Antarctic icefishes. These two tubes contain freshly of myoglobin protein. Note that both channichthyid hearts are drawn blood from a hemoglobin-expressing notothenioid fish considerably larger than that from the red blooded species despite all (Notothenia coriiceps) on the left and a hemoglobinless Antarctic having been dissected from animals of equivalent body mass. (Figure icefish (Chaenocephalus aceratus) on the right. is from Moylan and Sidell, 2000.) THE JOURNAL OF EXPERIMENTAL BIOLOGY Icefish cardiovascular physiology 1793 both oxygen delivery and utilization in aerobic tissues. Mb Because these ‘knockouts’ have withstood the tests of real- protein world biology, they offer advantages over experimentally Champsocephalus esox – produced genetic knockouts for Mb expression in mice (Garry Champsocephalus gunnari – et al., 1998; Gödecke et al., 1999). Pagetopsis macropterus – The pattern of Hb and Mb expression in icefishes leads us Pagetopsis maculatus – Neopagetopsis ionah + to a series of intriguing questions. How and when did the losses Pseudochaenichthys georgianus + of expression of these important oxygen-binding proteins come Dacodraco hunteri – about? Was loss of expression