How do fish swim?

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Nathan Hill reveals the many methods fish adopt in order to move and what they use to propel themselves.

An eel slithers between rocks, a flick-frenzy of movement giving it the final drive into a tight crevice. A tang hangs static in the centre column as brazen cleaner wrasse dart over, nibbling parasite snacks. From a distance, a clownfish waddles around its anemone home, watching with lazy disinterest.

Collectively, we use the word 'swim' to explain the movements of fish through water, but there’s so much more to their propulsion than that.

Yes, fish swim, but to say that they only swim is an uninspiring oversimplification. They do so in many different ways.

Think about it. There are the snake-like wrigglers, powerful, ocean cruising tuna, the delicate and rapidly moving seahorses. All are borne out of evolutionary uniqueness and each is attuned for a lifestyle.

Then there’s the enviable hydrodynamic element that allows the fishes to cut through their dense universe, to cleave through that thick gloop that is water. It’s all been millions of years in the making.

Surely we can do it more justice than simply call it swimming? In many cases fish dedicate most of their mass to swimming muscles, with internal organs like the heart and liver almost stuck to one side as an afterthought.

When we think of a swimming fish, most of us associate with the majestic, sweeping side-to-side movement of sharks and pelagic fast movers; herring, mackerel and marlins (pictured above). There’s nothing wrong with that. Such fish are iconic.

That classic swimming technique is given one of two names, depending just how much of the body is involved. If a large amount of the body is used for propulsion, anywhere between a half and two-thirds of body length, then such a fish is known as a subcarangiform. Take that driving muscle down to just the last third of the body length and it becomes a carangiform.

A trout or salmon is a subcarangiform. Its body design allows for long periods of swimming straight across lakes or up rivers, but also offers the opportunity for fast evasive movements — as when needing to escape predators, catch prey or coping with ever changing currents.

True carangiforms are better geared for long periods of fast swimming. Ocean dwellers like Barracuda face different water flows to those met by riverine fish. They also tend to be hooked up with thousands of identical fish all going the same way — ruling out the need for impulsive precision movements in favour of all out speed.

Many, if not all, carangiforms are alarmingly fast yet still remarkably agile when they need to be. It might be unlikely that you’ve seen a mackerel in action, but it’s impressive and if you’ve ever had two or three medium-sized herring on the end of a fishing line, you’ll understand just how strong they are.

A technique we rarely encounter is that of the humble seahorse. It is classed as an amiiform and this is a very unique swimming style.

Movements are restricted to incredibly fast fluctuations just in the dorsal fin. For most fish, the dorsal fin is the one at the top — the characteristic ‘Jaws’ fin in films that breaks water and terrorises teenagers in flimsy sailboats.

For the seahorse, given its unusual upright posture, the dorsal fin is located centrally along the back, pointing in the opposite direction to the head. As such, it is used as the primary driver.

Yet, even given its fins’ high speeds and convenient location, the seahorse is a pathetic swimmer, often displaced by light currents created by faster tank mates.

The largest ocean farers are usually thunniforms. The classic example is the tuna, after which the type of locomotion is named. Tuna are the crème de la creme of fast, long distance swimmers, using rather small movements of the body but driving with a massive, crescent-shaped tail.

They do so with strikingly awesome muscles connected to the tail with wire-like tendons.

But it’s not just about the tail. Caudal action accounts for around 85% of forward drive in the carangiform, subcarangiform and thunniform swimming styles we see, but there’s so much more.

The other fins have their roles too. The dorsals, those on the top, account for some rapid turns and braking, but those fish with a second dorsal fin will use that to provide forward thrust too — in some cases up to 15%.

Anal fins act as brakes, ventral fins provide both braking and lifting, and other fins assist fast turns. Each fin has a role and often more than just one.

Wrasses, or labrids, typify the swimming technique of the labriforms, which use their pectoral fins in the rotating style of an Olympic swimmer performing the butterfly stroke.

Labrids are not the only fish to use this technique and many fish in your aquarium may exhibit this type of movement. It’s important not to confuse this style with that of rays.

Rays and, to a lesser extent, knifefish have their own designated swimming technique known as the rajiform. One benefit rays have over most fish is their flexible, cartilage skeleton, which lends favourably to this style of locomotion.

Unlike the rigid fin rays of the labrids, a ray can flex and bend its ‘wings’, allowing for an elaborate oscillating movement. This optimises the pushing of water over the very large surface area of the fins.

In your aquarium you’ll likely see the pectoral, or side fins, used most — unless you have a tank full of eels! The two pectorals are for ‘precise’ movements and the hardest workers in most fish. Sharks tend to use them in steering, especially up or down, but many other fish even use them as main propulsion.

Classic pectoral swimmers are the  ostraciiforms. We all love ostraciis and, with a strong dose of anthropomorphism, usually imagine they love us back.

Pufferfish and boxfish are typical members of this club and the key feature of an ostraciiform is that the tail is used primarily as a rudder, as on a ship, steering as the frantic pectoral fins waft and wave, pulling the fish along.

Undulating paired fins don’t necessarily have to be either side of the fish. Triggerfishes, those sharp beaked, inquisitive, tourist favourites, use their dorsal and anal fins to propel themselves, rather than pectorals.

This technique, known as balistiform swimming is rare and, aside those we may come across, such as the Picasso trigger, an example possibly even known to non-fishkeepers would be the impressive and gigantic Ocean sunfish (Mola mola). It drifts around the world primarily using these fins — so much so that it has evolved to do away with a tailfin (or caudal fin) altogether.

One hypnotic movement has my mind performing all manner of geometric calculations when I see it — that of the eels, or the anguilliforms. The word is derived from either Anguis or Anguilla (snake and eel respectively), and these creatures have decided to do away completely with the need for finnage, at least where swimming is concerned.

Concerning themselves with long, bodily undulations, these fish have the kind of muscles that bring tears to the eye of the most hardened bodybuilder. At best, in such as the Spiny eels (Mastacemblidae), the pectoral fins are still present and play at least a token role in movement. However, the main driver, the thrust, is generated by that long, powerful body.

That tail can be as prehensile as a monkey’s, with divers in particular wary of just how strong a moray eel can be, both emerging from its lair and returning with a human arm in tow. It uses its body to grip static objects and increase its pulling power.

I’ve always admired these fish for being as hydrodynamic backwards as they are forwards. Anguilliforms tend to be either naked, or the very small-scaled end of fishy armoury.

Those scales provide little assistance when moving backwards, which goes some way to explaining why eels feel the slippery way that they do.

So much more to explain…

How fish swim is a huge and complex subject, and there’s still a lot to cover, from fish with legs to those that use jets of expelled water to push themselves along.

This feature has been at least an insight into what’s out there, making us realise how easy it is to take such varied methods of movement for granted.

Now have a look in your aquarium. Have a stab at working out which belongs to what group. I’ll wager that you’ll not look at your fish in quite the same way from now on.

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