Skip to content
NatureHQ

A guided journey · 4 stops

Ice, on purpose

Two opposite solutions to freezing, one of which involves manufacturing the thing the other spends all autumn removing.

The route

  1. Freeze tolerance
  2. Wood frog
  3. Antifreeze proteins
  4. Icefish

Below zero, an animal has two options, and they are not degrees of the same toughness. It can keep ice out, or it can let ice in.

Keeping it out means staying liquid below the freezing point — clearing the gut of anything a crystal could start from, and carrying proteins that stop small crystals growing. It works perfectly until it doesn’t, and then it fails all at once. Letting it in means doing something that sounds like a mistake: producing proteins whose entire job is to start ice, as early and as high above the danger zone as possible.

This route follows the second option, because it is the stranger one and because it makes the first one legible by contrast. It ends with the molecule, and with where that molecule came from — which turns out to be a digestive enzyme.

  1. Stop 1 of 4. Ecology

    Start with the two strategies, and why they are opposites

    The whole route depends on seeing that avoidance and tolerance are not points on a scale of cold-hardiness. One group manufactures ice nucleators; the other spends the autumn eliminating them. The difference that matters is the failure mode: avoidance has a cliff, and tolerance does not, because the thing that would trigger the cliff has already happened under supervision.

    Read Freeze toleranceA freeze-tolerant animal makes itself freeze on purpose. A slow freeze starting at −1 °C is survivable; the sudden one that follows deep supercooling is not.

  2. Stop 2 of 4. Reptiles and amphibians

    Watch it happen in a vertebrate

    A frozen wood frog has no heartbeat, no breathing, and ice through two-thirds of its body water. By any bedside test it is dead, and in spring it thaws and goes to breed. Reading it after the mechanism page means the sequence makes sense rather than merely astonishing: the liver’s glucose flood in the first minutes, the ice confined to the spaces between cells, the thaw running inside-out so the heart restarts first.

    Read Wood frogA frozen wood frog has no heartbeat, no breathing and ice through two-thirds of its body. In spring it thaws out and goes to breed.

  3. Stop 3 of 4. Marine life

    Get to the molecule, and to where it came from

    Antifreeze proteins are not antifreeze. They do not lower the freezing point the way glycol does; they bind to ice crystals and block their growth, which leaves a signature no dissolved solute can produce — a freezing point that no longer matches the melting point. And the fish version was traced back to a pancreatic trypsinogen gene, with parts of the original still sitting there doing nothing. Survival machinery is more often recruited than built, and this is the clearest molecular case of it in the corpus.

    Read Antifreeze proteinsThe freezing point stopped matching the melting point, and that gap was the fingerprint of a protein that stops ice crystals growing rather than stopping water freezing.

  4. Stop 4 of 4. Marine life

    End in the same ocean, with a trait that is not an adaptation

    The icefish are the counterweight, and they belong at the end rather than the beginning. Same waters, close relatives of the antifreeze fish, and a feature far more spectacular — no haemoglobin at all, the only vertebrates without it. The natural conclusion is that it must be an adaptation to the cold. On current evidence it is close to the opposite: a loss with no demonstrated benefit, tolerated because the Southern Ocean was forgiving enough. Coming to it after a page about an unambiguous adaptation is what makes the contrast do its work.

    Read IcefishIcefish 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.

Where this leaves you

You end knowing why a freeze-tolerant animal freezes on purpose: a slow freeze starting at −1 °C is survivable, and the sudden one that follows deep supercooling is not, because ice that forms all at once forms inside cells as well as between them.

And you end with two of the best-documented cases in the corpus of survival machinery being recruited rather than invented — an antifreeze protein that used to be a digestive enzyme, with the ancestry still legible in its sequence, and the same protein arrived at independently by Arctic cod from an unrelated gene.

What the route does not settle: how ice is kept out of cells over many hours while filling the spaces between them, which is stated as the mechanism and is not fully explained.

Last reviewed 2026-09-03. All guided journeys