Jonah’s icefish (Neopagetopsis ionah) inhabits the frigid, ice-covered seas surrounding Antarctica, an extreme environment of cold and darkness. This creature thrives in conditions that would be lethal to nearly any other vertebrate, making it a subject of scientific fascination. It exists in the perpetually cold, stable waters of the Antarctic continental shelf.
The Fish with Clear Blood
The most notable characteristic of Jonah’s icefish is its transparent, nearly colorless blood. It is the only known vertebrate to lack hemoglobin as an adult, the protein that makes blood red and transports oxygen. Without hemoglobin, the fish also lacks functional red blood cells, resulting in a whitish appearance of its gills and internal organs.
This condition is the result of a genetic mutation within the fish family Channichthyidae. While Neopagetopsis ionah possesses remnants of a non-functional hemoglobin gene, the protein is not produced. This evolutionary path has left the icefish dependent on alternative mechanisms for oxygen transport.
Surviving in the Extreme Cold
To survive without hemoglobin, Jonah’s icefish has developed several physiological traits. The icy waters of the Southern Ocean, which remain at a stable temperature just below freezing, are rich in dissolved oxygen. This allows a sufficient amount to diffuse directly into the fish’s blood plasma, a process that would be inadequate in warmer, less oxygenated waters.
To compensate for the low oxygen-carrying capacity of its plasma, the icefish circulates a much larger volume of blood compared to red-blooded fish of a similar size. Its heart is significantly larger and more powerful, enabling it to pump this increased blood volume at a high rate throughout the body. This supercharged circulatory system ensures that its cells receive enough oxygen to function, despite the absence of hemoglobin to bind and deliver it.
The icefish enhances oxygen uptake through its skin. Lacking scales, its skin is permeable and served by a dense and extensive network of capillaries, particularly near the surface. This allows for cutaneous respiration, where oxygen is absorbed directly from the surrounding water into the bloodstream, supplementing the work of its large gills. To prevent its body fluids from freezing, the fish produces antifreeze glycoproteins, which circulate in its blood and stop ice crystals from forming.
Life in the Southern Ocean
Jonah’s icefish is a benthopelagic species, living on or near the seafloor at depths ranging from 20 to 900 meters. Its diet consists mainly of krill, crustaceans, and smaller fish that share its deep, cold habitat. As a predator, it plays a role in the Antarctic food web, and it is also prey for larger animals like Weddell seals. The fish’s life is intrinsically linked to the stability of the seafloor ecosystem where it feeds and reproduces.
A remarkable discovery in 2021 revealed the largest known fish breeding colony on the planet, consisting of an estimated 60 million active Jonah’s icefish nests in the Weddell Sea. These nests are circular depressions on the seabed, each guarded by a parent fish. The females lay a relatively small number of large, yolk-rich eggs, around 1,700 per nest, which the parent protects until they hatch.
This massive nesting aggregation, covering an area of at least 240 square kilometers, suggests a previously unknown and significant ecological influence. The sheer density of the colony indicates that this specific area of the Weddell Sea provides ideal conditions for reproduction. The discovery highlights how much is still unknown about life beneath Antarctica’s ice shelves.
Conservation and Climate Change
The specialized adaptations that allow Jonah’s icefish to thrive in the Southern Ocean also make it exceptionally vulnerable to environmental changes. Its entire physiology is calibrated for life in water that is both extremely cold and highly oxygenated. Climate change poses a direct threat to its existence by warming the ocean.
Warmer water holds less dissolved oxygen, a change that would be very difficult for a species that already operates on a thin margin of oxygen supply. A small increase in temperature could reduce the available oxygen below the minimum level the icefish needs to survive, effectively suffocating it. The fish’s reliance on a narrow temperature range makes it a sensitive indicator of polar warming.
The discovery of the massive breeding colony has underscored the need for environmental protection in the Weddell Sea. Because such a large proportion of the species may depend on this single location for reproduction, the colony is a point of vulnerability. Any disruption to this specific habitat, whether from warming or other human impacts, could have serious consequences for the entire Jonah’s icefish population, pushing this unique survivor toward an uncertain future.