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Marine lifespecies group

Icefish

Channichthyidae

Icefish are the only vertebrates with no haemoglobin. On the evidence, it is not an adaptation to the cold — it is a loss they got away with.

Antarctic icefish have colourless blood and no haemoglobin — the only vertebrates known to manage without it. They compensate with enormous hearts and huge blood volumes, and the evidence suggests this is a loss they got away with rather than an improvement.

Whalers knew about the white-blooded fish long before biologists did, and when the blood was finally examined in 1954 it had essentially no red cells and no pigment. Its oxygen-carrying capacity is about a tenth of an ordinary fish’s. Everything about how an icefish lives follows from working around that. Its heart is several times the size expected for its body, its blood volume is enormous, it pumps far more blood per beat than a comparable fish, its blood vessels are unusually wide, and its skin is unusually well supplied — some oxygen is taken up directly through it. The Southern Ocean helps: cold water holds more dissolved oxygen than warm, and a cold fish needs less. The natural way to tell this story is as a triumph of adaptation, and that is how it is usually told. The current reading among people who study it is close to the opposite. There is no identified benefit to lacking haemoglobin. The compensations are expensive — a heart that size is a real metabolic cost, sustained continuously — and the most defensible account is that the loss occurred, probably in a small isolated population, and the Southern Ocean was forgiving enough that it was not fatal. Cold, oxygen-rich, and with little competition after other fish groups had largely died out of the region. That makes the icefish a good page to read alongside the adaptation page, because it is exactly the case where finding a striking trait in an extreme environment and concluding it is an adaptation to that environment gets the story backwards.

Developed coverage · 46% complete · reviewed 2026-09-03

What this page covers

A family of around sixteen species in the Southern Ocean, all of them lacking haemoglobin. They are the only known vertebrates without it as adults.

Often confused with: Other Antarctic notothenioid fishes, which do have haemoglobin and are their close relatives; Larval fish of many species, which are transparent and haemoglobin-poor for entirely different reasons

Quick facts

What they lack
Haemoglobin and red blood cells — uniquely among vertebrates
Oxygen capacity
About a tenth of an ordinary fish’s, carried in solution
How they cope
Very large hearts, high blood volume, wide vessels, low demand
Not an improvement
No benefit has been identified; the compensations are costly

A vertebrate without haemoglobin

Established in 1954, and still the only case known.

Antarctic icefish have no haemoglobin and colourless blood. The evidence points to a loss they got away with in cold, oxygen-rich water — not an improvement, and not an adaptation to the cold.

Well supported

Good evidence backs this, though some details remain open.

Channichthyid icefishes lack erythrocytes and haemoglobin, carrying oxygen in physical solution at roughly a tenth of typical teleost capacity, and compensate through greatly enlarged hearts, high blood volume, high cardiac output and low metabolic demand. Comparative and physiological evidence indicates the loss is tolerated rather than advantageous.

Who this applies to
The Antarctic icefish family specifically — the only known haemoglobin-less vertebrates.
Studied in
Channichthyidae
Why we rate it this way, and what the caveats are
Well supportedModerate confidence

The absence itself is beyond question and the compensations are measured. That the loss is disadvantageous rather than beneficial is a comparative argument rather than an experimental one, and the counterfactual cannot be tested.

Caveats

  • Whether the additional loss of myoglobin in some species carries any benefit is still debated.
  • The argument that the loss is costly rests on the size of the compensating circulatory machinery, which is indirect evidence however striking.

Still unanswered

  • How the loss became fixed at all — whether through drift in a small isolated population, or because the cost was genuinely negligible in that environment.

Last reviewed 2026-09-03

The evidence (3 studies)
An icefish against a comparable Antarctic fish that kept its haemoglobin
MeasureIcefishRed-blooded relative
Blood oxygen capacityAbout a tenthOrdinary for a fish
Heart size relative to bodySeveral times largerOrdinary
Blood volumeTwo to four times greaterOrdinary
Blood vessel diameterNotably widerOrdinary
Cost of circulationA large share of resting metabolismSmall

Reading down that table is what makes the adaptation reading hard to sustain. Each row is a compensation, and each compensation is expensive and permanent. An animal that had gained something would not need to spend this much offsetting it.

Why this is probably not an adaptation to the cold

The usual telling has the causation backwards.

The short answer

Did icefish lose haemoglobin because it helps in cold water?

There is no evidence that it helps. The suggestion has been that thinner blood is easier to pump when cold makes it viscous, but no benefit has been demonstrated, and the compensations the fish must maintain are costly.

The better-supported account is that the loss happened and was survivable rather than useful. The Southern Ocean is cold, so the fish’s oxygen demand is low; it is oxygen-rich, so what dissolves in plasma goes further than it would elsewhere; and the region’s fish fauna had been largely emptied out, so the competition that would ordinarily punish an expensive circulatory system was absent. That combination makes a costly loss tolerable. It is a less satisfying story than an adaptation and it fits the measurements better, which is the trade this site generally takes.

Check it for yourself

No advantage to lacking haemoglobin has ever been demonstrated in icefish. The suggestion that thinner blood is easier to pump in the cold is a hypothesis, and the compensations the fish maintain are expensive.

Emerging evidence

Real findings exist, but too few or too recent to be settled.

No fitness or physiological benefit of haemoglobin loss has been experimentally demonstrated in Channichthyidae. The proposed advantage of reduced blood viscosity at low temperature remains untested, while the compensatory cardiovascular apparatus imposes a substantial and continuous metabolic cost.

Who this applies to
The Antarctic icefish family specifically.
Studied in
Channichthyidae
Why we rate it this way, and what the caveats are
Emerging evidenceModerate confidence

That no benefit has been demonstrated is straightforwardly true of the literature. That none exists is a stronger claim which the absence of a testable counterfactual makes hard to establish, so this is stated as the current reading rather than as a result.

Caveats

  • Absence of demonstrated benefit is not demonstration of absent benefit, and the viscosity hypothesis has not been ruled out so much as left untested.
  • Whether the further loss of myoglobin in some species is beneficial remains actively debated.

Still unanswered

  • Whether blood viscosity at Southern Ocean temperatures imposes a cost large enough for its reduction to matter at all.

Last reviewed 2026-09-03

The evidence (2 studies)

It is worth being clear about what is and is not settled here. That the fish lack haemoglobin is a fact. That they compensate in the ways described is measured. That the loss was not beneficial is a comparative argument — the counterfactual icefish with haemoglobin does not exist to be tested — and it is the current reading rather than a demonstration. NatureHQ marks it that way rather than presenting either the adaptation story or its reversal as established.

The same ocean, the other story

Their close relatives kept their haemoglobin and evolved antifreeze instead.

The icefish belong to the notothenioids, the group that dominates Antarctic fish life, and their relatives are the fish in which antifreeze glycoproteins were discovered. So the same waters hold two well-known stories that pull in opposite directions: a protein whose evolution can be traced gene by gene and which is unambiguously doing a job, and a loss with no demonstrated benefit that has survived for millions of years anyway. Holding both is a good corrective to the assumption that everything remarkable about an animal in a hard place is there because the place is hard.

Related

The research behind this page

9 studies, newest first. Each one has a page explaining what it found and what it could not show.

2012Proceedings of the National Academy of Sciences

Ancient climate change, antifreeze, and the evolutionary diversification of Antarctic fishes

Antifreeze evolved roughly ten million years before the group diversified.

2006Journal of Experimental Biology

When bad things happen to good fish: the loss of hemoglobin and myoglobin expression in Antarctic icefishes

The evidence indicates loss without compensating advantage: icefish sustain oxygen delivery through very large hearts, high blood volume and low metabolic demand, at considerable circulatory cost.

2001Annual Review of Physiology

Antifreeze and ice nucleator proteins in terrestrial arthropods

Antifreeze proteins act by binding to the surface of small ice crystals and preventing their growth, producing a gap between the freezing and melting points; ice-nucleating proteins do the reverse.

1997Proceedings of the National Academy of Sciences

Evolution of antifreeze glycoprotein gene from a trypsinogen gene in Antarctic notothenioid fish

The antifreeze glycoprotein gene evolved from a pancreatic trypsinogen gene — a digestive enzyme — by amplification of a short repeated segment, with parts of the ancestral gene still recognisably present.

1982Paleobiology

Exaptation — a missing term in the science of form

Current utility and evolutionary origin are separate questions, and a large fraction of useful traits were co-opted rather than built for the job they now do.

1979Proceedings of the Royal Society of London B

The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme

A trait can exist because of developmental constraint, because it is a by-product of something else, because of genetic drift, or because it was built for a different purpose.

1971Science

Glycoproteins as biological antifreeze agents in Antarctic fishes

The glycoproteins lowered the freezing point without lowering the melting point by the same amount — a gap known as thermal hysteresis — showing that they act on ice crystal growth rather than by ordinary colligative depression.

1969Science

Freezing resistance in some Antarctic fishes

The fish remained unfrozen at temperatures below the freezing point predicted from their dissolved salts, indicating an additional and previously unidentified antifreeze substance in the blood.

1954Nature

Vertebrates without erythrocytes and blood pigment

These fish have essentially no red blood cells and no haemoglobin.

This page is a stop on a longer route

A guided journey reads several subjects in a deliberate order, with an argument for why one follows another. You can join in the middle.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this page is, and what it is still missing

NatureHQ publishes its own gaps. This page is at 46% completeness against what we would call a finished subject, and was last reviewed on 2026-09-03. It carries 5 claims and answers 5 mapped search questions.

  • no research from the last few years is attached — check for newer work
  • more experiments could be explained in plain English
  • no popular claim about this subject has been checked yet
  • The loss of myoglobin in some icefish species, and whether it carries any benefit, is noted as debated without being resolved.
  • How the loss became fixed — drift in a small population, or genuinely negligible cost — is an open question.
  • Icefish ecology, diet and reproduction are outside the scope of this page.