How do donut fish eat?

Question

Imagine a particular species of eel that resembles the shape of a torus or donut. It is capable of generating an alternating electric current in the water as a form of communication between other donut eels, analogous to a step up/down transformer, and it can deliver a powerful electric shock to stun/kill any unfortunate nearby creatures. It also has a swim bladder for controlling buoyancy at depth and gills for breathing. However, it needs to swim constantly like some species of sharks to draw oxygen-rich water through its gills, and this consumes lots of energy. When in peril, the donut fish quickly forms into a potato chip shape to store potential energy, mimicking the shape of muscle in a peacock mantis shrimp's hammers. This can be used to deliver powerful movement, but it also requires tons of energy.

Without a mouth, how can the donut fish feed to meet its daily energy requirements in the open sea? Also, feel free to explore the mechanisms for generating A/C current without accidental self-harm or misfires.

Answer 1

You say:

Without a mouth, how can the donut fish feed to meet daily energy requirements in the open sea?

And I say that not having a way to take in food (in the form of a mouth/anus, because in some animals it doubles as both) is rare.

So first, let's take a look at something here on earth without a mouth.

The Trichoplax.

It has no organs to do specific tasks, though it does have different types of cells.

Here's how it eats:

Cells on the underside of Trichoplax have tiny hairs called cilia (SILL-ee-uh). The animal moves by twirling these cilia like propellers. When the animal finds a patch of algae, it stops. Its flat body settles down atop the algae like a suction cup. Some special cells on the underside of this “suction cup” squirt out chemicals that break down the algae. Other cells absorb the sugars and other nutrients released from this meal. So the animal’s entire underside works as a stomach. And since its stomach is on the outside of its body, it doesn’t need a mouth. When it finds algae, a Trichoplax just plops itself onto the food and begins to digest it.

But this animal is very, very simple. What you're talking about, what with the swim bladder, gills, shape-shifting, AND electric current, is a complex animal with higher energy requirements.

So turn the Trichoplax up to 11. Instead of just eating algae, your Donut Fish can excrete something more caustic, consuming whole bodies in minutes. Fish, dead things, algae, anything that stays still for more than a moment can be eaten by the Donut Fish...

From the Trichoplax article there's this picture on stomach and mouth evolution:

This series of drawings shows how early animal shapes may have evolved 500 million to 700 million years ago. The red part shows cells that can digest food. As the body shape evolved from a flat “plate” to a bowl to a vase, those cells formed a stomach inside the animal’s body.

What I am proposing is this--your animal still has a stomach on the outside. It's just that instead of forming a "vase" like figure g, wherein there is an opening (a mouth) and inside the space there's stomach, your creature covered the opening. The inside ring of the donut is the stomach.

Even though you can see right through it, and it's not a conventional "mouth" it seems to me that evolutionarily for this creature to go from vase to donut, with the inner ring being the stomach seems not to be too far of a leap.

In this way, your donut can constrict around prey. It's far more efficient if your donut can sit on the bottom to digest.

But we have a problem. Constant movement is that problem. You want your creature to act like a shark, but anything without a mouth has to "sit" on their food. Maybe they gather prey into the donut and constrict, eating on the run. Whatever the case, that's a problem you have to solve.

That's my take.

Answer 2

Fraternity for Fast Fish

The Rolling Donut was discovered a few decades ago, during the Galtieri expedition to the Malvinas Islands. It is a curious creature, seen rolling endlessly in place in the water, with no obvious means of support. It has a rather remarkable ability to generate alternating current in a network of highly conductive fibers along its circumference, and to use these as a transformer to generate high voltage current to kill other fish, even though it cannot eat them.

It was only recently understood that the fish is in an obligate symbiotic relationship with the Flying Falk - the very fast-swimming fish, with aerodynamic surfaces allowing long leaps from the water, capable of landing inadvertently on the high decks of ships, after which the Falkland Islands are named. As it so happens, the Flying Falk produces a comparatively modest electromagnetic field about its body, from which the Donut can extract energy by induction to power a robust biochemistry. Parading through the Donut lumens in fast-moving schools, the Falks literally generate all of the energy the Donut requires, while also passing a torrent of water over its gills to allow it to conserve its own energy. In exchange, the Donut immobilizes any predators that foolishly attempt to chase the Falk, or any prey toward which it is moved by the ceaseless stream of those fish.

Despite this situation, the Falk are readily spooked by humans in garb with unusual conductive properties, and will briefly leave the Donuts to defend themselves when challenged by naturalists such as Galtieri's expedition. Divers should nonetheless be wary, as after some delay they may return with hostile intent to maintain their ongoing relationship.