Discus mucus

Fire water

Depending on which generation you belong to, the phrase ‘fire water’ will either remind you of old western movies in which native Americans are manipulated or subdued by use of liquor; or of the Bloodhound Gang’s smash hit ‘Fire Water Burn’. If neither of these cultural references rings a bell with you, consider yourself lucky: you are young! I have recently added another use of this phrase to my vocabulary: the instance when archerfish fire water at potential prey. That is right. There is a fish that fires water! And it is quite obvious that this behaviour is what gave it its name. And the choice of name is very appropriate – the fish’s abilities to use water as a weapon vastly exceed merely spitting some water around and hoping for the best. If they had been named ‘spitterfish’, that really would be an insult. They are archers, not spitters; they are the true Robin Hoods of aquatic ecosystems when it comes to their craft of water archery. That analogy ends right there, though – they are far from taking from the rich and selflessly giving to the poor. Rather, they use their skills to shoot poor insects from leaves and then try to eat them as quickly as possible. No selflessness to be detected.

Now, you may not be super impressed at first by considering a fish spitting some water at an insect. You have probably spat water at someone or something at some point in your life. And you will likely have hit your target. But may I kindly ask: did you do so from underwater, aiming at a target above water level? And was your target approximately 200 metres (650 foot) away? Because these are the conditions under which archerfish hit their prey. That distance of approximates 10 times their own body size is obviously impressive. But I would argue that hitting anything in the terrestrial realm while being submerged yourself is even more astounding. The reason is that light behaves very differently in air and in water. You may have noticed a straw looking ‘bent’ in a glass of clear liquid. Or you may have had trouble at reaching for something you dropped into a pool. I won’t bore you with the details, but the basic fact is: archerfish have to constantly correct for these physical phenomena, and these corrections are dependent on the angle at which they find themselves to their target. Move a straw around in a glass filled with water and you will see what I mean – how ‘bent’ it appears depends on the angle at which it sits in the glass.

A sketch of how light behaves when entering from air into water - it being 'bent' thus resulting in distorted visual information when eyeing items in one of the media (air or water) while being in the other. Archerfish basically face the reverse problem that kingfishers, herons and other birds hunting aquatic prey face.

In addition, the water surface is usually moving while the fish try to score their prey, the background is fuzzy and cluttered with objects, and in general it appears near to impossible for me to see anything in a situation where archerfish hunt; check out pictures and videos in the reading list below. Nevertheless, archerfish perform all the necessary corrections and calculations necessary, and they do so very very fast! While they make the necessary calculations, they also estimate how much water they need to fire, when exactly the jet of water has to collapse upon the target for maximum impact (that is, the back part of the jet catching up with the front part), and how the ballistic curve the jet will take (gravity still applies to archerfish, despite their awesomeness) will have to look to actually hit the target. All I can say is: I would not be able to recreate the archerfish’s shooting skills. And neither would you. For many reasons.

If it was just about archery, these fish would be amazing and unique, but kind of a one-trick-pony. Only, they are not. They are a Swiss-army-knife-amounts-of-tricks-pony. When conditions are right, the fish elect not to fire water at all. They simply jump at their target. The above problems of seeing and focusing on an object outside the water while being underwater oneself still apply, but by manoeuvring close to smack under their prey and being very close to the surface, the fish minimise the issues of visual distortion. What that means, though, is that they have to propel themselves with a very few fin strokes (typically less than three) out of the water – and they do so over three times their own length (60+ cm, 20+ inches; check out the videos below!). When you see a clip of a whale or dolphin jumping, that is surely very impressive. But these mammals usually need quite a bit of a ‘run up’ to gather the necessary speed. And I have yet to hear of a humpback whale jumping 60 metres (200 foot) in the air. So that is two tricks down. You may have noticed a massive difference between firing water at prey to make it fall into the water and jumping at it – yes, one of them (if performed correctly) guarantees that the prey is yours. The other does not. Because when an insect falls onto the surface of a pond or stream, it is fair game for all the hungry fish below. And there are many! So how can the archerfish assure that the spoils of its archery exploits go to itself, and that it does not become Robin Hood, giving to poor non-archery-skilled fish? Here is trick number three that archerfish have up their non-existent sleeves: they calculate where the prey will land. That’s right! Not only do they all those other calculations when firing their water jets. No! They also estimate where the prey should hit the water. And it doesn’t end here: they also estimate where prey should land that other archerfish have shot at. Again, with all the issues of light being distorted, rippling surfaces, fuzzy backgrounds etc. that they already had to deal with. After rapid calculations of where a poor spat-at critter should be available, they use a classic fish behaviour, the C-start response to get them their faster than any other fish can be. These fish do amazing things with a brain that is not quite the size of a hazelnut. I am impressed. And so should you! So next time you spit water at someone or have a water gun battle, channel your inner archerfish and impress the opposition with immaculate water firing. Fire. Water. Burn!