We might think we know these simple creatures, but under the surface, fish are full of surprises. From species that walk to species that glide, these water-dwelling creatures are far from what we might expect. Dive into these weird and wonderful facts about fish.
Fish facts
Fish can pee
Fish are able to excrete unwanted compounds (such as salts and nitrogen-containing ammonia) via the membranes of their gills. Freshwater fish wee copiously, as they are able to flush their kidneys with large quantities of fresh water, whereas fish in the sea have to store more body water. So, in short, fish can – and do – pee! Read more fascinating pee facts from the animal kingdom here.
Some fish can walk
It might not look like the movement of legged land-dwelling mammals, but some fish can use their fins to shuffle along the ground or move between rock pools to catch prey. Bottom-dwelling frogfish (one of the world's weirdest fish) use their pectoral fins to do this, while epaulette sharks do the same – but using their pelvic fins. Find out more about other fish that can walk here.
Scientists aren’t sure if fish sleep
The question of whether fish sleep or not doesn’t have a clear answer. Most fish will enter a regular inactive state, particularly during the night, resting on the bottom or floating motionless. But it’s not clear whether this is sleep in the way we know it. They might simply be saving energy, because they can’t feed or interact in the dark, but they do seem to be less responsive to what’s going on around them.
Many fish need to swim constantly to survive
Fish can’t necessarily drown, but they will suffocate when they don’t get enough fresh water with dissolved oxygen in it. Many fish must keep constantly swimming so water is always moving over their gills. This is the case for most sharks, for instance. A shark which is unable to swim will suffocate.
Fish can do maths
A 2022 study found that cichlids and stingrays could be trained to count up to five if they are presented with a selection of shapes and are rewarded if they responded accordingly.
There’s a fish doorbell in the Netherlands that allows fish to travel through canals
In the Dutch city of Utrecht, a system called the Fish Doorbell is helping fish migrate through canals. A camera near Weerdsluis lock has been set up, which allows the public to spot fish that are waiting to move through the canal. When enough fish are spotted, viewers can press a virtual doorbell, which alerts lock operators who will then open the gate.
Freshwater fish don’t need to drink – but seawater fish do
Not all fish need to drink water. The body fluids of freshwater fish are more concentrated than their surroundings, so they take on water through their skin, gills and mouth through osmosis. Seawater, however, is more concentrated than a fish’s body fluids, so marine fish lose water and must drink to replace it.
Bluefin tuna could theoretically reach speeds of up to 144mph
It turns out, it’s quite difficult to find out what the fastest fish in the world is. Tagged sailfish have been logged accelerating at a rate that, if sustained for two seconds, would result in a speed of 78mph. Bluefin tuna are even faster, with acceleration that could help them reach 144mph.
Fish are noisy creatures
We might think of the ocean as a still and silent place, but it can actually be quite a racket down there, thanks to the various noises fish can make. Triggerfish are particularly noisy, creating a grinding noise that wards off predators, and herring expel air via their anus that creates a high-pitched chirp.
Rabbitfish team up in pairs
While most fish don’t make friends or create partnerships, rabbitfish are a peculiar exception. They team up in pairs – sometimes with members of the same sex – and help each other during feeding. They take turns to eat, while their partner looks out for predators.
Fish don’t have tongues
Instead of tongues, fish have a bony structure called the basihyal on the base of their mouth that looks a little like a tongue. It doesn’t have taste buds though, and behaves entirely differently to a tongue – it’s believed to have evolved to protect the ventral aorta (the major blood vessel from the heart), which lies close to the mouth.
Some fish can’t see colours
Some fish are able to see colours, while others live with a more limited palette. Fish active during the day tend to see a wider range of colours, while those which live at greater depths or are nocturnal have good visibility but are unable to distinguish colours.
There are 50 or so species of flying fish – but they don’t actually fly
Flying fish have what look like ‘wings’, which are elongated fins that help them travel through the air. But they’re not actually flying – they’re gliding. Find out more about how flying fish fly here.
Some fish have big lips for a reason
A study of African cichlids has found that fat-lipped species of fish have evolved facial features that help them extract prey from smaller areas with greater ease. The pointed snout can be pushed into nooks and crannies, while the big, fleshy lips create a seal with the surface, allowing the fish to suck the prey out.
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