I have this alien race, lets call them Amorphs, to imagine just think of an amoeba mixed with a spook fish. They are surrounded by thick, clear, blue membrane with the internal organs showing, and the membrane is the only part that touches the outside, and because of this they have no mouth, and eat like fungi by releasing chemicals and digesting the food on the outside. Also to spare themselves from damage the chemicals they released are calculated by the membrane on how to digest the food on the outside to get the resources needed. Also they are detrivores but primarily eat poop.

How efficient would this be and how could it be more efficient?


How efficient would this be

Whatever the surface of the creature is made of, it'll only work well if the creature is small, since the area of a $3D$ element grows as a power of $2$, but the volume grows as the power of $3$.

For example:

The area of a cube is $$6s^2, \text{while}\space\text{the}\space volume\text{ is}\space s^3,$$ where $s$ is the length of an edge.

Here's a table:

Length   Area   Volume
 1         6        1
 2        24        8
 3        54       27
 4        96       64
 5       150      125
 6       216      216
 7       294      343
 8       384      512
 9       546      729

Note how when the length of an edge is $\lt 6$, there's more surface area with which to absorb nutrients. At exactly $6$, the area and volume are equal, and once the length gets longer than $6$, the volume gets much larger.

(If you're thinking that the mammalian food absorption surface area is pretty small, think again: the human intestines are 25 feet long. That's considered short because we cook our food, making nutrient extraction easier.)

how could it be more efficient?


  1. give it a mouth and digestive system so that it's prey doesn't escape once it feels those digestive juices eating away at it, or
  2. make it a detrivore so that it's food doesn't run away.
  • $\begingroup$ Well they are human sized and flatten out to increase surface area, and it is a detrivore, so could it work like that? $\endgroup$ – Amoeba May 10 '18 at 0:38
  • $\begingroup$ @Amoeba yes, if that's the case then it seems reasonable. (This is fiction, so maintaining Suspension of Disbelief is all that matters.) $\endgroup$ – RonJohn May 10 '18 at 6:36
  • $\begingroup$ Good point about area vs surface, but - just as the intestines became longer to increase their absorbing surface, these amoeba creatures could create folds and creases on their surface to increase their effective surface area (think about fractals, there's no real hard limit to their surface area, regardless of their volume) - take Koch's Snowflake as a 2D example. $\endgroup$ – G0BLiN May 13 '18 at 15:15

It's hard to be without a digestive pathway

The invention of the mouth wasn't necessarily about an operational method to gather food. It was a logical outcome from the very origin of animal life.

Essentially we are all based on tubes. The tube is an efficient way to evolve and grow, as essentially it is a simple genetic set of instructions that could change size and length over generations. Early genetic dead-ends, such as fractal structures, could not lead further than a plant-like fractal structure evolutionarily.

As nutrients pass through the tube (actively or passively) it is also absorbed through the edges of the tube to integrate within the rest of the structure, and we can grow ourselves.

We then evolved appendages (sexual reproduction organs, limbs, skeletal structures, lungs and so on) over time such that we are now very complex tubes.

In your case can an organism evolve without the origin of a simple digestive system? Can a fungus essentially grow organs and internal complex structures? The fact they haven't in several billion years of evolution likely means it is difficult for fungus to evolve beyond far from their current forms, much like early fractal organisms.

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But we don't know that much really

However life is diverse, and much more complex than we realise. Animal life is but a very small slice of the amazing world of life. Often there is such variety we struggle even now to make sense of it, and are continually revising our understanding the more we discover.

From the above diagram there are many other types of organisms and they all have different efficient solutions to eating and multiplying, and not necessarily like an animal.


Not very efficient, internal digestion evolved because surrounding the digesting material is faster, yields more resources, requires less digestive enzymes, and does not require you to stay put while digesting.

External digestion is a really bad choice if you are a creature that moves around. Amoeba don't actually digest things like this they envelope the material they want then digest it internally (phagocytosis).

Note there are mobile organisms the eat in the way you want, but they have to be extremely thin thinner than a sheet of paper to have enough surface are, a mouse could rip one apart with ease. the first thing organisms that want any thickness do is develop a mouth of some kind.

  • $\begingroup$ how can it be made more efficient then? $\endgroup$ – Amoeba May 10 '18 at 0:47
  • $\begingroup$ by having them form a pouch like amoeba, you are up against the physics of diffusion which you cant really change. $\endgroup$ – John May 10 '18 at 20:19

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