Adaptations and constraints of Antarctic fishes
Notothenioids are best known for two adaptations related to existence in the cold, often energy-limited waters of the area, where water temperatures average −1.87°C and total darkness prevails for 4 months each year. First, their blood contains remarkably effective antifreeze compounds that depress the freezing point of their body fluids and make it possible for them to live in water that is colder than the freezing point of most fish blood including, remarkably, their own. Second, some have evolved neutral buoyancy, which has permitted these species to move off the crowded bottom where most notothenioids live and into the water column.
No known species of fish can actually tolerate having its tissue freeze. The major threat to fishes in the Antarctic is ice, which floats at the surface in the form of bergs, sheets, and platelet ice, but also attaches to the bottom in water less than 30 m deep in a form called anchor ice. The greatest danger comes from ice crystals penetrating or propagating across the body and seeding the formation of ice inside the fish, which would cause cell rupture. Many Antarctic fishes live in water that is colder than their blood’s freezing point. Fishes from lower latitudes typically freeze when placed in water colder than −0.8°C, whereas Antarctic fishes can live in water as cold as −2.19°C. They accomplish this because their blood contains the salts normally found in fish blood and also as many as eight different glycopeptide antifreeze compounds. The glycopeptides apparently function by keeping the ice from propagating across the fish’s skin. A notothenioid can be cooled as low as −6°C
without freezing, as long as free ice is not in the water.
Several other adaptations accompany the production of antifreeze compounds. Notothenioids are relatively unusual among teleosts in that their kidneys lack glomeruli, which are the structures that remove small molecules from body fluids and transfer them to the urine for excretion. Glomeruli would remove the antifreeze glycopeptides, which would be energetically expensive to continually replace (see Coping with temperature extremes). A fairly strong correlation exists between antifreeze effectiveness and the frequency with which a species encounters free ice. For example, the shallow water bathydraconid dragonfishes frequently come in contact with ice and have the highest levels of antifreeze compounds. Within the cod icefish genus Trematomus, shallow water species that live in the coldest water and rest in ice holes or on anchor ice have freezing points of −1.98 to −2.07°C, whereas deeper living species that seldom encounter ice crystals freeze at −1.83 to −1.92°C. Even within species, shallow water populations have significantly more freezing resistance than deeper water populations (DeVries 1970). The primitive bovichtid thornfishes of New Zealand live in temperate waters and do not produce antifreeze. Bovichtids possess glomeruli, indicating that the aglomerular condition of Antarctic species evolved along with other adaptations to the colder Antarctic environment (Eastman 1993).
Neutral buoyancy has developed in at least two water column dwelling members of the family Nototheniidae, the Cod Icefish, Pleuragramma antarcticum, and its giant predator, the Antarctic Toothfish, Dissostichus mawsoni. Whereas most Antarctic fishes are 15–30 cm long, toothfish reach lengths of 1.6 m and weights of over 70 kg. Neutral buoyancy allows these fishes to occupy the comparatively underutilized water column zone, thus taking them away from threatening anchor ice crystals and into a region of seasonally abundant food sources such as fish larvae and krill. Both species have evolved from benthic ancestors and have retained what can only be viewed as a phylogenetic constraint on living in open water: they are similar to benthic notothenioids in that they lack a gas bladder. As fish muscle and bone are relatively dense, a gas bladderless fish would constantly have to fi ght gravity to stay in the water column. Neutral buoyancy in these two nototheniids is achieved via several mechanisms. Toothfish have cartilaginous skulls, caudal skeletons, and pectoral girdles, which reduces their mass because cartilage is less dense than bone. The skeleton itself is less mineralized than in benthic relatives, by a factor of six in the toothfish and 12 in Pleuragramma. Bone is also reduced in the vertebral column, which is essentially hollow except for the notochord. Additional buoyancy is achieved by lipid deposits dispersed around the body, including a blubber layer under the skin, and fat cells or sacs located between muscle fi bers or muscle bundles (Eastman & DeVries 1986; Eastman 1993). Weightlessness via analogous routes of weight reduction and replacement is also seen convergently in bathypelagic fishes, another water column dwelling group where evolution has placed a strong premium on energy-saving tactics.
A unique trait of channichthyid icefishes may represent an evolutionary adjustment to polar conditions. These fishes are sometimes referred to as “white blooded” or “bloodless” because their blood contains no hemoglobin and their muscles contain no myoglobin, giving them a very pale appearance. The highly oxygenated, cold waters of Antarctica may have been responsible for the evolutionary loss of respiratory pigments, perhaps via a “regressive” evolutionary process similar to the one that led to pigmentless, eyeless cave fishes (see below, Caves). Channichthyids possess a number of other characteristics that have evolved in conjunction with a lack of hemoglobin, including relatively low metabolic requirements (reduced protein synthesis, reduced activity, slow growth), increased vascularization of skin and fins to increase gas exchange, and an increase in cardiac size, output, and blood volume (Hemmingsen 1991). Some nototheniids have increased blood volumes and reduced hemoglobin concentrations, perhaps reflecting an intermediate stage in the response to respiratory conditions in the Antarctic that have led to the hemoglobin-free condition of the channichthyid icefishes (Wells et al. 1980).