Maximum size of an exoskeletal creature

Question

On a world I am building, I have a species of creatures that evolved from crustaceans. The problem is that while they have an exoskeleton, they also live on a world with the same gravity as Earth. I have thought of a few solutions, from an extra internal skeleton to internal exoskeleton partitions to hold critical organs in place, but they all seem to feel like excuses and not features.

What is the maximum size of an exoskeletal creature on a world with Earth-like gravity? What can be done to their biology to increase this size? What (other than the gravity) can be done to the planet to increase its maximum size?

Answer 1

The answer is around 2 to 3 meters on ancient Earth when the Earth's climate was hotter and there was a lot more oxygen.

I can't find the reference, but from what I remember, the limitations on exoskeletal creatures, as shown in other answers, didn't correlate with the strength needed to hold the weight. The reality is that it has to do with the cardiovascular systems of exoskeletal creatures being fairly inferior and only allowing for larger structures when they live in oxygen rich environments. Something to do with surface area and respiratory efficiency.

The basics from what I can tell and piece together is that exoskeletal animals developed in the oceans and started coming onto land. Around the same time, plants developed on land and cleaned the atmosphere up, enriching it with oxygen, which allowed exoskeletal animals to grow to relatively massive sizes compared to what they are today. Then, because plants dominated the landscape, tied up their fuel source and polluted their world they reached a point where they started to die off. The exoskeleton creatures weren't producing enough CO2 and animals that did started emerging that were a lot more efficient. This caused the exoskeleton creatures and plants to die off until stability with the new CO2 producing animals stabilized the system which ended up being much lower than what was needed for the 2-3 meter bodies of the exoskeleton creatures.

There is no reason that a creature with an exoskeleton couldn't develop a more efficient respiratory system as far as I know which would allow them to continue to evolve and dominate their world at larger sizes.

Once you get that then the limitations of exoskeleton weight come into play...

An interesting idea is that hypothetically these creatures could be massive and continue to grow throughout their lives, only stopped by when they become too big to molt and grow a new exoskeleton. This means that their brains could also grow massive compared to ours, because they aren't limited by the birth canal problem.

Answer 2

There are several limitations on the size of arthropods, partly related to the square-cube law and partly related to mechanics. All of these limitations have solutions, but the result may not qualify as true arthropods. A general overview may be read in this article.

Known records

According to this article (quote by wikipedia):

The largest arthropod known to have existed is the eurypterid (sea scorpion) Jaekelopterus, reaching up to 2.5 m (8.2 ft) in body length, followed by the millipede relative Arthropleura at around 2.1 m (6.9 ft) in length.

Problem: Circulatory systems

According to this article:

Arthropods have an open circulatory system: instead of having arteries and veins to channel the blood, arthropods possess open sinus where blood bathes the organs directly. In which ways does this imply a constrain for a giant insect? While there is no active mechanism that pumps the blood throughout the body, it would be very difficult for a giant insect to oxygenate and nourish all its cells due to the gravity effect.

On the other side, most insects breath passively through their spiracles, which connect with an internal system of branched conducts called “trachea”. Thus, they don’t develop any active system to force air to enter inside their bodies, but it enters passively throughout these “trachea” and reaches the inner of arthropod’s body to oxygenate all cells.

Diffusion of gases is effective over small distances but not over larger ones. So, giant insects would face serious problems to oxygenate their tissues if they reach big sizes. In addition, current atmospheric concentration of oxygen (21%) wouldn’t be enough to oxygenate such a big organism with such a simple breathing mechanism.

It must be said that all these constrains are attenuated in aquatic ecosystems, where the cuticle’s weight and the diffusion of oxygen posed no problem for growth. That explains why the world’s biggest arthropods (and other invertebrates) are mainly located in aquatic ecosystems.

Solution: Tracheal circulatory system

According to this article:

Maybe the animal has book lungs like a spider, or maybe the spiracles of an insect have branched inward, becoming an air-filled tracheal system intertwined with the fluid-filled cardiovascular system of blood. Each leg has its own “cardio-pulmonary complex” associated with it, plus a big one in the belly to feed the organs.

Rather than breathing in and out, these animals breath THROUGH, with air entering the system through spiracles near the head and exiting near the ail. Air is pumped by action of the muscular blood vessels that wrap around the tracheal tubes, or by muscular contraction of the whole abdomen (like a balloon inflating and deflating). Running also generates more flow-through.

Problem: Molting

According to this article:

In terms of strength, chitin sheaths around legs (what bugs already have) works fine. Do the math, and you’ll find that a beetle-leg scaled up to the dimensions of one of my legs ( 100cm long by 20 cm in diameter) will have a exoskeleton about 0.6cm thick, which about the same mass and a quarter of the thickness of the bones in my leg. That’s not bad, especially considering the fun you could have with air pockets, different materials, and the exact shape of the bone in question. I’m confident exoskeletal legs will work, at least for an animal of my size.

The real problem is that an exoskeleton must be shed as the animal inside it grows. Imagine a lion-sized arthropod molting and going from armored battle-demon to squishy pink lump. It might not be able to support the weight of its own organs, let alone run and pursue prey.

Solutions: buoyancy, cocoons, growing, scales

According to this article:

There are ways to solve the problem. Dig a hole and hide in it while soft. Immerse yourself in supportive water. Build a “mobile cocoon” out of the old cast-off exoskeleton and silk. Or just have the skeleton grow with you.

Sea-urchins have exoskeletons too, but theirs are made of hexagonal plates that can be separated and the interstices filled with an intermediary material (in this case collagen) that later toughens into the necessary hardness and rigidity (in this case calcium carbonate). The bones of our skull (which are exoskeletons, in a way) work the same way. The difference is that we also have specialized cells (osteoclasts) than can destroy old bone as well as create it (osteoblasts), so even once the plates have met to form a skull, the whole thing can continue to grow as old bone is subtracted from the inside and added to the outside.

Don’t like that idea? You can break the exoskeleton up into scales, which lock edge-to-edge like puzzle-pieces, and can be lost and regrown one-at-a-time like shark teeth without sacrificing structural integrity (bonus: video-game-boss weak spots!). Muscles that are anchored to areas with no shell-scale won’t have any leverage and will be useless until the new shell hardens. The animal will have to change its behavior, either getting help from its conspecifics or building a temporary crutch for itself out of found materials (wood? old scales spun into silk?). Either that, or muscle-anchoring scales remain un-shed, built into large, dead structures as the animal grows, like the rattle of a rattle-snake.

Problem: Pin joints

According to this article:

How load-bearing joints (like knees and hips) in large creatures work is by distributing the load across as large an area as possible, and by cushioning and lubricating the joint by surrounding it with living tissue.

Obviously, creatures with exoskeletons can’t surround a joint with living tissue or they wouldn’t have an exoskeleton. And without that cushion and lubrication, they’re somewhat limited in the types of joints they can have. For instance, humans have hinge joints (elbow), ball-and-socket joints (thigh to pelvis), gliding joints (wrist), and a few others. Creatures with exoskeletons have, primarily, the pin joint.

The pin joint, essentially, has a pair of protuberances on one limb of the joint fit into a pair of depressions on the other limb of the joint. You can readily see this the next time you’re eating lobster if you closely examine where the “thumb” of the claw connects to the “hand” of the claw.

Pin joints are a problem as creatures increase in size because they place all the force of the joint into a relatively small area. I want you to stand up, right now, and stand on the balls of your feet with your heels off the ground. Then, with your back straight, slowly squat. Feel the pressure in your knees? Imagine that times thirty and you’ll have an idea what exoskeletal joints would have to resist at your size.

Solution: endoskeletal joints

According to this article:

any other kind of joint (for example the ball-and-socket joint in your thumb) would require the hard surface on the inside, which is sort of the opposite of an exoskeleton.

Conclusion

Giant arthropods cannot exist without high amounts of atmospheric oxygen. Even then their exoskeletons cannot compete with endoskeletons. Overcoming these limitations would require the evolution of a new clade of pseudo-arthropods with a variety of unique strategies.

Answer 3

Without internal bones, they could have structural tendons that work in tension, not compression. Imagine stringing strong wires across the rigid surrounding shell, like a tennis racket. This can be used for internal support.

Consider using muscles to form strong rigid forms. I forget what it’s called, but if the muscle weave is at right angles and aligned with the tube it makes an unbending form (as opposed to a 45 degree helix, which allows for bending without kinking). The science channel special about possible future life described cephalopods moving onto land this way.

Some animals have grown shell on the inside. So if they can’t easily evolve bone from scratch, they might more easily grow the same stuff they have been, but on the inside.

Answer 4

A great explanation of the scaling problem for exoskeletal creatures

In summary:

Exoskeletons have really terrible joint options, which are not good at bearing weight. Essentially, you can have a pin joint (think crab claw), or just soften your skeleton a bit and hope it holds together. Whereas invertebrates have many joint options, many of which are spectacularly load bearing (think about knees and hips). There's just not a way to replicate this with an exoskeleton.

Exoskeletons are really heavy. They make up a greater proportion of an animal's body weight than endoskeletons do, and this gets worse as you scale up (square-cube problem; cubing a larger number). This also means having a creature with both an endoskeleton and an exoskeleton is not going to work out well. They would weigh so much, they'd barely be able to move.

Speaking of moving, they're not going to be able to run without fracturing their leg exoskeletons.

Exoskeletons are expensive. Most large creatures with exoskeletons live in the sea. One of the main reasons for this is that they use biomineralization to "mine" the water for minerals to harden their exoskeleton. Since their exoskeleton has to be molted and regrown, this is a lot more expensive than an endoskeleton which can be continually grown and not wasted.

Speaking of molting, molting is extremely dangerous for small creatures with an exoskeleton. Exoskeletons don't grow, so they have to drop their exoskeleton and live as a blob of soft goo while they expand and a new exoskeleton grows. For very small animals, this isn't too much of a problem; they've just lost their armor, which means they're tastier for predators. For larger arthropods, they have to find a safe place to do this, since they won't even be able to move (no support for their body means their muscles can't reliably move their limbs, let alone support the weight of their body). For gigantic exoskeletal creatures, molting is a deal-breaker. Imagine a human without a skeleton. You wouldn't be able to breathe or pump your blood. Same thing for a human-sized exoskeletal creature that's molting.

So... what's the maximum size of an exoskeletal creature?

Underwater: The largest American lobster weighed 44.4 pounds.

On land: The largest coconut crab weighed 9 pounds.

Those are your upper limits.

Answer 5

The big problem is volume vs area:

Take humans:

• around 1.8m,
• Legs with a section of around 10cmx10cm (I know, very approximate).
• Around 80Kg

That mean 100cm^2 need to handle around 80kg. If you divide, it is 0.8Kg per square centimeter.

Take an animal of twice size made with similar morphology/materials:

• 3.6m height,
• Legs of 20x20, or 400cm^2
• As the weight depends on the volume, you could expect 640Kg (80*2*2*2)

That mean now you have 640/400 = 1.6Kg/cm^2

Summarizing: each time you duplicate size, you need materials 2 times stronger. That is why you could have insects with very thin feet and an Elephant require very short/strong feet.

Muscles and bones have a specific material resistance, so you are limited to a maximum size. I would say around what Dinosaurs or equivalent. Basically, Gozilla or KingKong are totally out of "real" possibilities.

If you want bigger creatures, you could imagine stronger materials (metal or carbon-fiber exoskeleton), but that will just increase the maximum size by a factor of a few times (tens of times for carbon fiber).

The other alternative is to make the creature to live in an other environment which compensate gravity (water?) so it will be not able to move fast, but it may manage higher sizes (whales).

Answer 6

What's the maximum size? A lot bigger than you would think!

As previous answers have described, the problem is firstly how the exoskeleton can support its inhabitant's insides, and secondly how it can move around on legs. If you move the action underwater, you basically resolve both these problems by its insides being neutrally buoyant.

If this is a sci-fi setting and your exoskeletal race are intelligent and running an industrial society, they can fairly easily knock up some way to survive on land. Whether that's an individual suit (powered armour?) or more like a watertank on wheels (submersible style, except supporting pressure on the inside instead of outside?), it's a solvable problem.