Fish never stop growing: True/False?

Peter Corrigan wonders whether fish keep growing throughout their lives, and — if so — whether they are the only animals to do this. The short answer is yes they do and no they aren’t, but there is a little more to it than that.

The typical fish continues growing after it reaches maturity and never stops. But it doesn’t grow at a constant rate. The larger it gets the slower it grows, giving it what fish experts call an asymptotic growth trajectory (if that seems a mouthful, remember that these people refer to themselves as “ichthyologists”). The asymptote is an ideal size that fish never quite reach. Fish do have a maximum size. It’s just that, unlike a lot of other familiar animals, they reach that size near the end of their lives rather than the beginning.

This slowing growth pattern probably has to do with prioritising energy use. Growth is a big draw on energy that might be used for something else, like reproduction. On the other side of the coin if the fish finds itself eating less then it will have less energy all round and again growth will take a hit. Nothing can keep growing forever anyway because bodies don’t just keep working if they are scaled up or down. Eventually the laws of physics assert themselves (which is why we need never worry about monster ants the size of people).

Fish aren’t the only animals that never stop growing. Crayfish and octopuses don’t either. Nor do iguanas. Not all of the creatures that keep on growing do it asymptotically, but it is very common. The octopus, perhaps unsurprisingly for a creature which sports eight legs and a beak, really breaks the rules and seem to grow faster and faster.

At the risk of disappointing fishermen everywhere, just because fish are growing all the time doesn’t mean that fish dinner for one you’ve been after will eventually turn into a fish dinner for two if you wait long enough. It’s true that fish never stop growing, but it’s also a bit of a red herring.

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