Who Eats Whom
Every animal in this zoo is eating something, and almost none of them can see. There are no eyes at this scale, no ears, and nothing that carries further than a few body-lengths — so finding a meal is a problem of chemistry, patience and luck, and every strategy in this room is one of those three. Below is the whole web: what eats what, and how it manages to find it.
17 organisms · 18 feeding relationships · 17 routes back to the pool
Solid threads are one organism eating another, and the light travels along them the way the carbon does. Fainter threads fall inward: what a living thing leaks, spills or bursts, returning to the pool at the centre for the bacteria to take up again. Touch any organism to light up everything it eats and everything that eats it.
Clockwise from the top, the ring runs producers, decomposers, grazers, predators, parasites — and then back to the pool it started from.
The discs are not to scale. They are sized by decade, because the span here is six orders of magnitude — at true scale a bacteriophage beside the slime mould would be a hundred thousand times smaller than a printed full stop, and there would be nothing to look at. The room that draws size honestly is Powers of Ten, and every organism on this ring is standing in it at its real depth.
Why it is a loop
The food chain you were taught at school runs in one direction and ends somewhere: grass, rabbit, fox, and the carbon exits as a fox. Drawn for the invisible world that picture is not simplified, it is wrong, and it is wrong in a way that hides the interesting part.
Start at the middle. A very large amount of the carbon down here is dissolved — leaked by living cells, spilled by clumsy grazers, released when something dies — and a dissolved molecule cannot be caught. It is too small to see, too small to grab, and too dilute to chase. To anything with a mouth it is not food at all.
Bacteria can take it up, because a bacterium is almost all surface. That single fact is the hinge: it means the only route out of the pool runs through the bacteria, and everything else in the room is eating bacteria, or eating something that ate bacteria. Flagellates graze them, ciliates filter the flagellates, and a rare few things hunt the ciliates outright. That is the microbial loop, described in 1983 and still the organising idea of the field.
And then the viruses cut across all of it. A phage does not eat its host — it rebuilds it into more phage and bursts it, and the entire contents of that cell dissolve back into the pool at the centre without ever reaching a grazer. The carbon goes round without getting anywhere. Follow the faint threads falling inward on the diagram and you are watching the largest recycling operation on the planet, running continuously, at a scale nobody can see.
- The pool
- Carbon that has come loose. Too small to catch, too dilute to chase.
- Producers
- Makes food out of light. Nothing here is eating anything.
- Decomposers
- The only mouth small enough for a dissolved molecule.
- Grazers
- Mostly does not hunt. Makes a current, and eats what arrives.
- Predators
- Finds a specific animal and takes it. Rare, and expensive.
- Parasites
- Does not eat its host. Rebuilds it, and returns it to the pool.
How they find it
This is the harder half of the question, and the better one. Nothing in this room can see. Nothing can hear. A bacterium cannot even tell which way is up, and it certainly cannot tell that the food is over there — both ends of its body are, as far as chemistry is concerned, in the same place. There are seven answers on this ring, and only one of them looks anything like hunting.
Make it yourself
Don't find food. Make it.
It eats light. A cell this small has a surface-to-volume ratio that makes hunting pointless and absorption free, so it does neither — it sits in the top of the sunlit ocean and builds sugar out of carbon dioxide and photons, using a pigment tuned to the dim blue light that reaches a hundred metres down. The trade is that it cannot move to better light and cannot wait out a bad season. It solves that by being astronomically numerous instead.
Don't find food. Go where the food is made.
Also a photosynthesiser, but at a thousand times the diameter of Prochlorococcus and with an option the small cell does not have: it can swim. Thousands of cells beat their flagella in a coordinated roll, and the colony steers itself up or down through the water column toward the light it wants. It is the same meal by a completely different method — the difference between waiting for good weather and driving to it.
Let it come to you
Heterotrophic nanoflagellates · Paramecium · Vorticella · Spirostomum · Stentor
Beat a flagellum, and the meal arrives.
Too small to chase anything and too small to be worth chasing, a nanoflagellate feeds by making a current. One or two flagella beat, water is drawn past the cell, and bacteria are intercepted and engulfed one at a time. They are the dominant bacterivores in most of the ocean, which makes this unglamorous, invisible step the main brake on bacterial numbers anywhere in the world.
Do not look for food. Build a current, and stand in it.
Rows of cilia cover the whole cell and beat in coordinated waves, which does two jobs at once: it swims, and it drives a stream of water into a groove running down one flank. At the bottom of that groove is the oral apparatus, where bacteria are packed into a vacuole that pinches off and travels through the cell while it is digested. There is no decision anywhere in this. The animal is a pump, and food is whatever the pump happens to bring.
Pick a spot. Never leave it. Make the water come to you.
It gives up moving entirely. Anchored by a contractile stalk to a stone, a plant or another animal, the bell opens a ring of cilia that spins a vortex in the water and draws suspended bacteria down into it. When something alarming arrives the stalk coils in a few milliseconds and snaps the bell out of reach, then slowly unwinds and resumes. The whole design is a bet that staying put and filtering hard beats going to look.
The same trick as the others, run along a much longer body.
A ciliary band runs most of the length of the cell, driving water down a long oral groove to a mouth near the middle. Being enormous for a single cell means a bigger filter and a slower life; being long and thin means it can live in the low-oxygen sediment layers where a rounder cell would struggle. It contracts to a fraction of its length in milliseconds when disturbed, which is the same bargain Vorticella makes and for the same reason.
Anchor, open wide, and let the water do the work.
The wide end of the trumpet carries a spiralling band of fused cilia that drives a steady current down into the cell. Anything suspended in that water arrives at the mouth whether it intended to or not. Because the bell is large the intake is large, and a Stentor takes prey a smaller filter feeder could not handle — flagellates and small ciliates as well as bacteria. It is the same passive strategy scaled up until it starts to look like predation.
Follow the smell
Heterotrophic bacteria · Physarum · Dictyostelium
You cannot steer. You can only decide when to stop going the wrong way.
A swimming bacterium is too small to hold a course — Brownian motion turns it faster than it could ever correct, and it has no way to sense a gradient across its own body, because both ends are in effectively the same place. So it does something stranger. It swims in a straight-ish line, then tumbles at random and sets off in a new direction. The only thing it changes is the *timing*: when conditions are improving, it delays the next tumble. Nothing aims. The random walk simply gets biased, and the cell arrives.
Go everywhere at once. Keep the routes that pay.
It has no eyes, no brain and no front. What it has is a body that can be in many places simultaneously: a network of tubes that creeps outward in all directions, sampling chemically as it goes. Where a tube finds food, the protoplasm streaming through it thickens and the tube is reinforced; where a tube finds nothing, it thins and is reabsorbed. Do that for a few hours in a maze with food at both ends and what is left is the shortest path between them. The organism did not solve the maze. The maze solved the organism, and what survived was the answer.
Follow the gradient — and if that fails, take the food with you.
Alone it is an ordinary soil amoeba, crawling up chemical gradients toward bacteria and engulfing them. What makes it remarkable is what happens when the bacteria run out. The amoebae signal to each other, stream together into a slug that migrates as one body, and build a stalk with a ball of spores on top for dispersal. About a third of wild clones do something extra on the way: they stop feeding early, carry undigested bacteria up into the spore mass, and release them at the new site. Their descendants land somewhere with a starter culture already growing.
Hold still and wait
Do not go looking. Occupy more space than you are.
The rays are axopodia — stiffened by a precise internal scaffold of microtubules and coated with organelles that discharge on contact. The animal does not swim after anything. It hangs in the water with its rays extended in every direction at once, so that a volume many times its own body is effectively covered. A flagellate that blunders into a ray sticks to it, and the ray then collapses inward — the scaffold disassembles — carrying the prey down to the cell body to be engulfed.
A fungus cannot chase anything. So it builds a snare.
Nematode-trapping fungi grow three-celled rings on short stalks throughout the soil, and then do nothing. A nematode pushes its head through a ring — they are sized for exactly that — and the contact triggers the three cells to inflate to about three times their volume, closing the noose around the worm in a fraction of a second. It cannot pull free. Hyphae then grow into the body through the puncture and digest it from the inside. The trap is built in advance, at a cost, on the chance that something will walk into it.
Chase it down
The rarest strategy in the room — actually go and get it.
It swims fast, in bands of cilia, and it searches. On contact with a Paramecium it fires toxicysts from the snout — projectile organelles that anchor into the prey and paralyse it — then hauls it in and distends its own body around a cell that can be longer than itself, closing over it like a bag. The whole capture takes seconds. Pursuit at this scale is unusual precisely because it is so expensive, and Didinium pays for it by eating almost nothing else: it is a specialist, and in the absence of Paramecium it encysts and waits.
Get inside it
Too small to eat it from outside. So get in.
It swims extremely fast for its size, collides with a Gram-negative bacterium, attaches, and drills through the outer membrane into the periplasm — the space between the prey's two membranes. Once in, it seals the hole behind it, rounds the prey off into a sealed compartment, and eats the cell from the inside while sheltering in its own victim. When the resources are gone it divides into several offspring and bursts out to find the next one. It is predation, parasitism and infection at the same time, and biology has never quite settled which word to use.
No searching. No swimming. Collide, and be the right shape.
A phage cannot move. It has no flagellum, no metabolism and no way of detecting anything at a distance; it drifts until Brownian motion happens to knock it into a cell, and then the only question is whether its tail fibres fit the receptors on that particular surface. If they do, it injects its genome and the cell begins — using its own machinery, on its own energy budget — to build more phage until it bursts. It is the purest version of the problem this room is about: finding food with no senses whatsoever, solved by being so numerous that chance is enough.
Walk into it
Crawl through the film of water on everything, and swallow what is in it.
A bacterivorous nematode moves through the thin water films between soil grains, following chemical cues toward patches where bacteria are dense, and pumps them in through a muscular pharynx. It is grazing rather than hunting — the prey is not chased and not selected individually, it is browsed through. They are the reason a spoonful of soil is a food web rather than a pile of minerals.
Walk until you meet something. Then put a hole in it.
A tardigrade has no way to search at a distance. It walks — slowly, on eight clawed legs, through the water film on moss and lichen and sediment — until it is touching something worth piercing. Then a pair of hardened stylets push out of the mouth, punch through the wall of the cell or the cuticle of the animal, and a muscular pharynx sucks the contents out. The prey is not swallowed. It is emptied.
The register
Every node on the ring, in the order the diagram runs: the pool at the centre, then round from the producers to the viruses. What each one eats is authored on the eater; what eats it is worked out by reading the web backwards, so nothing here can disagree with the picture above.
The pool
Dissolved organic matter
The largest pool of carbon in the ocean, and it is all crumbs.
It is not food you could see, or point at, or serve. It is carbon that has come loose: molecules leaked by a living cell, spilled by a clumsy grazer, or released all at once when a virus burst its host. Dissolved, diluted, drifting.
There is an enormous amount of it. The dissolved organic carbon in the ocean is roughly comparable to all the carbon in the atmosphere, and almost none of it is available to anything with a mouth.
So this pool sits at the middle of the room and everything on the ring is arranged around one question: how do you get the carbon back out?
Eaten by
Everything, eventually, and only by the bacteria directly. Nothing bigger than a bacterium can eat a dissolved molecule — it is too small to catch and too diluted to chase. That single constraint is why this room has the shape it has.
Producers · 600 nm
Prochlorococcus
The most abundant photosynthesiser on Earth, and nobody saw it until 1988.
Roughly a third of the photosynthesis in the open ocean is done by cells you could fit a thousand of across a full stop.
For this room, the relevant fact is not that it is small or that it is common. It is that Prochlorococcus is where a very large share of the carbon in the microbial world enters the story. Everything else on this ring is, at one remove or several, eating sunlight that something like this caught first.
Eats
Nothing. It builds its own food out of light.
Eaten by
Returns to the pool by
exudation
A growing photosynthetic cell leaks a fraction of the carbon it fixes back into the water as dissolved organic molecules. How large that fraction is varies with light, nutrients and stress, and the published range is wide — this is a real and central flow, not a precise number.
It holds a recordBack to the web
Sources10.1038/334340a0 · 10.3354/meps010257
Producers · 500 µm
Volvox
A green sphere that rolls toward the light, and the only thing here nothing eats.
It is the largest thing on this ring apart from the slime mould, and it is a producer, which makes it an unusual shape of node: carbon comes in through it and does not leave by being eaten.
Volvox is in this exhibit to mark the boundary. Draw a food web of the invisible world and it does not close neatly — it has a rim, and past that rim are the things big enough to see, which is where most people think biology starts.
Eats
Nothing. It builds its own food out of light.
Eaten by
Nothing in this room. At half a millimetre it is out of reach of every predator on this ring — the things that actually eat Volvox are rotifers and small crustaceans, which are animals, and animals are somebody else's museum. It is worth knowing that a food web drawn at one scale always has an edge, and this is where ours is.
Returns to the pool by
exudation
Like any growing alga it releases dissolved organic carbon into the water around it. Rate not measured for this genus here; the route is the claim, not a quantity.
See its portraitBack to the web
Sources10.3354/meps010257
Decomposers · 1.5 µm · approximate
Heterotrophic bacteria
The only thing in the room that can eat a dissolved molecule. Everything upstream depends on it.
Every other consumer in this room is, directly or at one remove, eating bacteria.
That is not a statement about bacteria being important in general. It is a structural fact about this particular diagram: the carbon pool at the centre is made of molecules, and a molecule is not catchable. It has to be absorbed, and only something with an enormous surface relative to its volume can absorb enough to live on. Bacteria are the gate, and there is no way around them.
The consequence, worked out in the early 1980s and still the organising idea of the field, is that the microbial world is not a chain with an end. It is a loop that keeps handing the same carbon back to itself.
Eats
- absorbs Dissolved organic matter
- The hinge of the whole exhibit. Dissolved carbon is too small and too dilute for anything with a mouth, and bacteria take it up across the membrane — which returns it to the food web instead of losing it. This is the uptake step of the microbial loop as originally described.
Eaten by
Bacteriophages · Bdellovibrio · Dictyostelium · Heterotrophic nanoflagellates · Nematodes · Paramecium · Physarum · Spirostomum · Stentor · Vorticella
Returns to the pool by
excretion
Bacteria release dissolved organic and inorganic compounds as they grow and as they die. The pool is both their meal and their waste, which is why the arrow in this room points both ways along the same spoke.
Sources10.3354/meps010257 · 10.1038/239500a0
Grazers · 4 µm · approximate
Heterotrophic nanoflagellates
The busiest mouth in the ocean, and the reason bacteria do not simply take over.
If you had to point at one arrow in this diagram and call it the important one, it would be this: bacteria into flagellates.
It is where the carbon that had gone dissolved and unreachable comes back into something big enough for a ciliate to catch. Everything else on the right of the ring — the trumpet, the barrel, the sun-shaped ambusher — is eating a meal this step assembled.
Eats
- engulfs Heterotrophic bacteria
- The step that carries carbon out of the bacterial pool and up to everything larger. Grazing pressure varies enormously with place and season; what is not in dispute is that flagellates, not ciliates, do most of it.
- engulfs Prochlorococcus
- Small photosynthetic cells are grazed by the same flagellates that take bacteria — at this size the distinction between "plant" and "bacterium" is not one a predator can act on. Stated at the level of the size class rather than the genus pair.
Returns to the pool by
excretion
Grazers are messy. A share of what is caught is never assimilated — spilled during handling, or excreted afterwards as dissolved compounds — and goes straight back to the pool for the bacteria to take up again.
Sources10.1038/nrmicro1180 · 10.3354/meps010257
Grazers · 250 µm
Paramecium
A permanent groove down one side, funnelling bacteria into a mouth that never closes.
The classic. If you have ever seen anything through a microscope, there is a good chance it was this, and a good chance it was moving too fast to look at properly.
For a food web the interesting thing about Paramecium is how little of its behaviour is about food. It swims, it reverses when it hits something, it avoids heat and acid — and while all that is going on, a groove in its side is quietly funnelling bacteria inward at a steady rate. Feeding is not an activity. It is a condition.
Eats
- filters Heterotrophic bacteria
- Bacteria are the staple; a Paramecium will also take small flagellates and yeasts, so this edge is the main flow rather than the only one.
Eaten by
Returns to the pool by
excretion
Undigested material is expelled and dissolved compounds are excreted, both returning carbon to the pool. Named as a route, not quantified.
See its portraitBack to the web
Sources10.3354/meps010257
Grazers · 100 µm
Vorticella
A bell on a spring, filtering the same patch of water for its whole life.
Most of the animals in this room are somewhere between “food” and “hunter”. This one has opted out of the argument by bolting itself down.
The stalk is the famous part — one of the fastest contractions in biology, driven by a protein spring rather than by muscle — but the stalk is a defence, not a feeding organ. It exists because a filter feeder that cannot run is otherwise the easiest meal on the ring.
Eats
- filters Heterotrophic bacteria
- Suspended bacteria and fine detritus drawn from the water column. A sessile filter feeder eats what drifts past, so its diet is a statement about the water rather than about its preferences.
Eaten by
Nothing in this room.
Returns to the pool by
excretion
Rejected particles and excreted dissolved compounds return to the pool.
See its portraitBack to the web
Sources10.3354/meps010257
Grazers · 3 mm
Spirostomum
Three millimetres of worm-shaped cell, filtering a long groove of water at a time.
It is one of the largest single cells anybody is likely to meet, and it eats the smallest things in the room.
That combination is worth pausing on. Nothing about being big at this scale implies eating big — a three-millimetre cell filtering one-micrometre bacteria is a ratio of about three thousand to one, which is roughly a person eating grains of sand. It works because there are a very great many bacteria.
Eats
- filters Heterotrophic bacteria
- Bacteria and fine suspended matter, filtered from the water and from the surface of sediment. Diet described at the level of the group.
Eaten by
Nothing in this room.
Returns to the pool by
excretion
Undigested and dissolved material returns to the pool.
See its portraitBack to the web
Sources10.3354/meps010257
Grazers · 1.2 mm
Stentor
A trumpet anchored by the tail, pulling a column of water into its wide end.
It sits still, which is the first surprising thing — and in a food web, the consequential one.
Everything else on this ring that eats well either swims after its meal or drifts with it. Stentor stays where it is and processes the water that comes past. That only works if the water is rich, which is why it lives where it does: in the film of life around pond weed and detritus, where the bacteria are thick enough that standing still is a viable career.
Eats
- filters Heterotrophic bacteria
- Bacteria and suspended detritus, the staple of the filter.
- filters Heterotrophic nanoflagellates
- Larger heterotrich ciliates take flagellates and other small protists as well as bacteria. Where the line falls between filtering and predation is a question of size, not of intent — the current is the same either way.
Eaten by
Nothing in this room.
Returns to the pool by
excretion
Undigested material is expelled and dissolved compounds excreted, back to the pool.
See its portraitBack to the web
Sources10.3354/meps010257
Grazers · 100 mm
Physarum
One cell, metres of it, solving a maze to reach the oats at the far end.
It belongs in this room for one reason: it is the clearest demonstration anywhere that finding food does not require anything that looks like thinking.
There is no part of Physarum that knows where the food is. There is only a rule about which tubes get thicker, applied everywhere in the body at once, and the rule is enough to produce routes that a transport engineer would recognise. When researchers laid oat flakes out in the pattern of the cities around Tokyo, the network it grew was comparable to the rail network somebody had spent decades designing.
The same trick, at a millionth of the size, is what a bacterium does when it decides how long to swim before tumbling.
Eats
- engulfs Heterotrophic bacteria
- Bacteria are the main prey of a wild plasmodium, taken by flowing around them and engulfing them; it also takes yeasts, spores and fungal material. The oat flakes it is famously grown on in a laboratory are a convenience food, not a natural diet.
Eaten by
Nothing on this ring. Beetles, springtails and slugs eat plasmodia, and they are all far too large to belong in a room about the invisible. Like Volvox, it marks the edge of the diagram — but at the other end.
Returns to the pool by
excretion
Undigested material and dissolved compounds are released back to the substrate as the plasmodium migrates. Route named, not quantified.
See its portraitBack to the web
Sources10.1038/35035159 · 10.1126/science.1177894 · 10.3354/meps010257
Grazers · 10 µm
Dictyostelium
An amoeba that packs a supply of bacteria for the journey, and plants them on arrival.
Agriculture is usually presented as the thing that separated humans from everything else. It is a nice story and it is about ten thousand years old.
Some clones of this amoeba have been doing a recognisable version of it for considerably longer: they harvest less than they could, carry the survivors, and seed them where they land. They pay for it — a farming clone produces fewer spores than a clone that simply eats everything — and they get, in return, food waiting at the other end.
It is in this room because it answers “how do you find food” in the only way nobody expects: by not needing to.
Eats
- engulfs Heterotrophic bacteria
- Soil bacteria, engulfed individually. The farming behaviour is a property of some wild clones and not of the species as a whole — roughly a third in the study that described it — and carries a real cost, so it is a strategy with a trade-off rather than a universal trick.
Eaten by
Nothing in this room.
Returns to the pool by
excretion
Undigested bacteria and dissolved compounds are released into the soil.
Sources10.1038/nature09668 · 10.3354/meps010257
Grazers · 1 mm · approximate
Nematodes
Four in five animals on Earth are one of these, and most of them are eating bacteria.
They are on this ring as prey as much as predator, and that is the point of including them: this is where the room’s food web starts touching the one you already know about.
A nematode eats bacteria, a fungus eats the nematode, and a bird eats something that ate the fungus. Somewhere along that sequence the organisms stop being invisible, but nothing about the eating changes.
Eats
- engulfs Heterotrophic bacteria
- Bacterivorous species only. Nematodes as a group include fungal feeders, plant parasites, animal parasites and predators of other nematodes — the node here stands for the bacteria-eating soil and sediment species, which is what makes it part of this loop.
Eaten by
Returns to the pool by
excretion
Excretion by bacterivorous nematodes releases nitrogen and carbon that bacteria immobilised, which is a large part of why they matter to soil.
Sources10.3354/meps010257
Predators · 400 µm
Heliozoa
A sphere of rigid rays held out in every direction, waiting for something to touch one.
It looks like a sun and it hunts like a spider’s web, which is a strange thing for a single cell to be.
The economy of it is worth stating plainly. Swimming is expensive and, at this scale, nearly useless for catching anything — the water is thick as syrup and prey that can also swim will simply be pushed aside by your own bow wave. So the heliozoan spends its energy on reach instead of on pursuit. It builds a body that is mostly empty space, and lets the prey do the travelling.
Eats
- engulfs Heterotrophic nanoflagellates
- Flagellates and other small motile protists, caught on contact. Larger heliozoans will take prey up to and including small ciliates; the edge drawn here is the well-attested staple rather than the upper limit.
Eaten by
Nothing in this room.
Returns to the pool by
excretion
Undigested remains are shed and dissolved compounds excreted.
See its portraitBack to the web
Sources10.3354/meps010257
Predators · 150 µm
Didinium
A barrel with a snout that hunts down and swallows a cell larger than itself.
Almost everything else in this room feeds by making a current and waiting. This one hunts, in the way the word normally means: it searches, it closes, it subdues, and the outcome is genuinely in doubt.
Paramecium is not defenceless. It reverses violently on contact and it can discharge defensive threads of its own, and a good proportion of attacks fail. The reason this pairing is in every ecology textbook is that in a plain culture with nowhere for the prey to hide, the predator wins completely, eats everything, and then dies of it.
Eats
- engulfs Paramecium
- The best-studied predator–prey pair in the microbial world, and a textbook case in population ecology since the 1930s — in a simple culture the predator eats the prey out and then starves, which is why the pair became the standard test of whether coexistence needs refuges.
Eaten by
Nothing on this ring. It is at the top of this particular diagram — which says more about where we drew the boundary than about Didinium, since in a real pond it is food for rotifers and small crustaceans.
Returns to the pool by
excretion
Undigested remains and dissolved compounds return to the pool.
Sources10.3354/meps010257
Predators · 400 nm
Bdellovibrio
A bacterium that hunts other bacteria by climbing inside them.
The chord this draws across the diagram is the longest one in the room, and it is worth following with a finger: it runs from the predators, all the way back past the grazers, to the bacteria.
That shortcut is the thing to understand. Every other predator here works its way up the ring — bacteria to flagellates to ciliates to something that eats ciliates — and each step costs most of the energy. Bdellovibrio skips the entire sequence by being small enough to live inside the first rung.
Eats
- invades Heterotrophic bacteria
- Gram-negative bacteria only — the strategy depends on there being a periplasm to occupy, so Gram-positive cells are simply not on the menu. A predator with an architectural preference rather than a taxonomic one.
Eaten by
Nothing in this room.
Returns to the pool by
lysis
The prey cell is destroyed when the offspring leave, releasing whatever was not consumed into the surrounding water.
Predators · 25 µm
Drechslerella
A fungus that grows nooses, and closes them in a tenth of a second.
Every hunting strategy in this room is a way of solving one problem: you have no eyes, and the food will not hold still.
This is the most patient answer to it. The fungus is not mobile in any useful sense, so it converts the problem into a construction project — build the trap where the prey walks, pay for it up front, and wait. It is carnivory by a kingdom that has no muscles, no nerves and no way of knowing whether the investment will ever pay off.
Eats
- traps Nematodes
- Trap-forming behaviour is induced rather than constant — the fungus grows rings when nematodes are present and food is scarce, so this is a facultative predator that also lives as an ordinary decomposer. Ring closure is frequently quoted at about a tenth of a second.
Eaten by
Nothing on this ring. Fungus-feeding nematodes graze mycelium, which makes the relationship with nematodes mutual and slightly comic, but the species here is drawn only as the predator.
Returns to the pool by
excretion
What the fungus does not assimilate re-enters the soil as dissolved organic matter, which is the ordinary fate of anything a fungus digests externally.
Predators · 500 µm
Tardigrades
Two stylets, a suction pharynx, and a slow walk through the film of water on moss.
They are famous for surviving vacuum, radiation and desiccation, and almost never described as something that eats.
The feeding apparatus is the reason they can afford the rest of it. Piercing and sucking works on a plant cell, on an alga, on a rotifer and on another tardigrade — one set of tools for an enormous range of meals — and it works in a film of water thin enough to dry out by lunchtime. The famous toughness is what makes the habitat survivable. The stylets are what make it worth living in.
Eats
- pierces Nematodes
- Predatory tardigrades — Milnesium is the usual example — take nematodes and rotifers. Most tardigrades do not: they pierce plant and algal cells instead, and a few graze bacteria. This node sits with the predators for the group's most striking habit, not because every tardigrade hunts.
Eaten by
Nothing on this ring. Other tardigrades, predatory nematodes, mites and springtails eat them; only the first of those is in this room, and pointing the arrow at their own node would be a claim about cannibalism we are not making.
Returns to the pool by
excretion
Emptied prey and excreted compounds return to the water film and the substrate.
See its portraitBack to the web
Sources10.3354/meps010257
Parasites · 100 nm
Bacteriophages
The most numerous biological entity on the planet, and it does not eat anything at all.
There are more of these than there are of anything else, by a wide margin, and they are the reason this diagram is a loop and not a ring.
Every other arrow here passes carbon outward: from the pool to the bacteria, from the bacteria to the flagellates, up through the ciliates to the things that eat them. A phage does the opposite. It kills a cell without eating it, and the whole of that cell’s contents dissolve back into the pool at the centre for the bacteria to absorb again.
The carbon goes round without ever getting anywhere. It is called the viral shunt, and at any given moment it is happening to a very large fraction of the bacteria in the sea.
Eats
- lyses Heterotrophic bacteria
- Not eating in any ordinary sense — the host is not consumed, it is redirected. It is drawn as a feeding edge because the carbon moves and the host dies, which is what a food web is recording.
- lyses Prochlorococcus
- Cyanophages infect marine picocyanobacteria, so the same shunt operates on the photosynthesisers as on the heterotrophs. Rates are debated; the existence and importance of the route is not.
Eaten by
Nothing. A virus is not food for anything on this ring — which is exactly why it draws its arrow to the centre instead of to a neighbour.
Returns to the pool by
lysis
The viral shunt. A lysed cell spills its entire contents as dissolved organic matter, which bacteria take straight back up — so the carbon never reaches a grazer and never sinks. Estimates of the share of bacterial production diverted this way are large and are still argued over.
Sources10.1038/nrmicro1750