Why are Tardigrades so small and yet seemingly indestructible?

[LaurelAnnyse]

TL;DR Summary

This might be a ridiculous question, but I'm legitimately curious. What's stopping them from being huge?

They're practically indestructible, right? So is it just some kind of coincidence that they evolved to be tiny? Is the tininess a factor in their indestructibility? Here's an animal that can survive in space, the deepest part of the ocean, major radiation... But they're microscopic.

 

[jim mcnamara]

Less science-y explanation:
Tardigrades have an exoskeleton (like the hard outer layer on insects). And no lungs/gills. So there are two factors limiting size. If they get too big oxygen cannot diffuse into their bodies. They mostly live in aquatic or semi-aquatic environments. Exoskeletons get very heavy relative to the size of of muscle tissue. Think of a knight in full armor. If it gets too thick or too large the knight is more protected but less able to move. A knight underwater could move better, but there is still an upper limit on the size of armor -- or exoskeleton.

So we have two limiting factors:
1. amount of oxygen in the atmosphere (it diffuses into water)
2. mass (weight) of the exoskeleton relative to body size.

So the logical question is:
for animals with exoskeletons, what is the biggest land animal, and biggest aquatic animal that ever lived?
During Carboniferous times oxygen levels in the atmosphere were much higher than today:
The sea scorpion grew very large, probably the largest invertebrate (like a crab) in the ocean
https://sciencing.com/sea-scorpion-8584742.html

There was a huge land dweller:
https://blog.nationalgeographic.org/2011/01/15/largest-land-dwelling-bug-of-all-time/

See https://www.prehistoricwildlife.com for more informationBe glad we live in a lower oxygen level atmosphere.

 

[BillTre]

And no lungs/gills.

To elaborate on this one a bit:

In traditional comparative metabolic principles and biophysics the ratio of body volumes to surface areas are an enduring analytic approach.

The volume (or mass) of a organism often scales with the metabolic needs of an organism (in this case an increased need for more oxygen and nutrients and removal of CO₂ and other wastes).
As volume increases, there are more cells using more resources (which must be resupplied somehow) and producing more wastes (which must be removed). Yet the transfer of such materials has to occur at some point through surfaces interfacing the organism and the external world (source of resources and dump site).

A sphere would be the simplest example. As its size increases linearly, its volume (representing its metabolic requirements) would increase as a cube function of its length, while is surface area (representing its ability to transfer materials in and out of the organism) would increase only a square function). Eventually, as size increases, limitations are eventually met.

Gills and lungs (as well as kidneys, the digesting system, and other things) have elaborated their surface areas (folded, invaginated, or branched) so they are able increase their surface area beyond what it would be for simple geometric shapes.

Surface areas are the material window (or door) to the organism.

 

[LaurelAnnyse]

Ah! These are both such well-written answers! And super easy to understand, too! Thank you both so much!

 

[Azur]

Tardigrades are microscopic, and clearly not limited by physical factors such as diffusion rate. The reason they're small is because they've evolved to fit that niche: they pierce and suck the contents of individual plant cells. In fact, they have relatives - the velvet worms, onychophorans - which are predators and NOT microscopic but up to a couple of inches long. Both onychophorans and tardigrades descend from a group called lobopodians, which were fairly large marine animals which lived during the Cambrian period.
It is also a bit of a modern myth that tardigrades are indestructible. They're only highly resistant to environmental conditions when they're in their non-feeding desiccated resting stage. When they're actively moving and feeding they're fairly fragile animals, easily killed by high temperatures or low oxygen levels. Their resting stage cysts are also not really more resistant than those of other animals which form desiccated resting stage cysts, like for instance some rotifers and ciliates.