Why are there no lobsters the size of horses or horses the size of shrimp?
April 25, 2012 8:50 AM   Subscribe

Why are there no lobsters the size of horses or horses the size of shrimp?

For the most part, you can split the creatures into two broad categories: "squishy outside" and "squishy inside". Bugs, lobsters, shrimp, and clams are all squishy inside: they use some sort of hard shell to protect their squishy bits, to hang their internal bits off, and I would assume, to act as leverage points for their motive parts.

"Squishy outside" creatures are like humans, whales, birds, and so on. They generally start with a rigid internal frame, and then hang organs from it, attach muscles to it, and so on.

Of course, there are plenty of non-plant living things that this categorization doesn't cover, or is vague about: what about water-filled rigid things like starfish, what about things that are just totally squishy like sea cucumbers, what about squids, cuttlefish, and so on that might have a rigid part, but use some sort of other structure (like tentacles) for things the bony animals use bones for? Worms? But for the most part, huge amounts of creatures fit clearly into one category or the other. It's still a useful distinction, I think.

But it seems like there are tons and tons of squishy inside creatures that are relatively small. Insects. Crustaceans. Whatever those little bug type things big whales eat are. Both in terms of total amounts of them on the planet and in terms of different types. The biggest one I can think of is a really big lobster. I've also seen some terrifying videos of huge CRABS picking their way around northern fjords, or shimmying down trees in the south pacific. The smallest are super-small. I don't know what they are but definitely sub-naked-eye visible. Aren't there some little mites that live on your eyelashes?

There are also a pretty decent amount of squishy outside creatures: lots of fish, all the birds and furry animals. The biggest ones in water are whales. The biggest on land are probably elephants, right? There are lots of examples bigger than all the biggest squishy-inside creatures I can think of. Easily. On the small side, the smallest I can think of is maybe some kind of mouse.

However, there aren't very many super-small squishy-outside creatures, and there are no super-big squishy-inside creatures. Why is this?

Besides being some sort of engineering or physical constraint, I considered the fact that maybe this was just a chance of evolution, but I looked up the page for "exoskeleton" and "endoskeleton" (there's barely anything in wikipedia for endoskeleton) and it says that at least "exoskeletons" evolved independently loads of times in totally different lineages. It doesn't say anything for endoskeletons. I'm not even sure if this is the right division of creatures. For instance, the exoskeleton page talks about lizards with armor plates, which in my classification, would probably still make more sense to look at as a squishy-outside animal. An armored lizard still has a spine, and so on. I also can't figure out where else to look this up.

In olden times, there were way bigger dinosaurs (squishy outside), but there were also way bigger dragonflies (squishy inside), right? So…what gives?

Why is this? Is there something in the engineering of squishy-inside/hard-outside creatures that doesn't scale up well? Why were there no herds of armor-plated ruminants grazing across the American plains in the 19th century? Why do you never find an infestation of thumb-sized gerbils in your attic?

It's hard not to notice that the big extant squishy-inside creatures either live in the water, or are probably super-closely-related to things that do. On the other hand, the biggest everything lives in the water, so maybe that's a red herring.

Is it that the squishy-outside/bones-inside system only evolved once, and that happens to be the system that most of the big creatures evolved from? (Except squids, and weird borderline cases like those enormous jellyfish colonies or corals or something) Or is it that over a certain size something stops working? And if so, what? Is this even the kind of question that is meaningful to biology (in that a model that produces testable predictions can be made)? Or is evolution just so complicated you have to throw up your hands and say "it's a large parameter space, we ended up on various local maxima and for the same reasons there's a dune here and not there."

If you know any resources where I can figure this stuff out for myself for the future (I have a lot of other questions about plants and animals and stuff) I'd appreciate that too.
posted by jeb to Science & Nature (23 answers total) 25 users marked this as a favorite
With regard to large exoskeletal creatures, the answer lies in the square-cube law. Basically, mass increases proportional the cube but surface area increases proportional to the square. Thus, if the support structure is on the outside of the creature then its ability to support itself will be outstripped by increase in mass. By contrast, an internal skeleton can increase in size roughly in proportion to the volume (and thus mass) of the creature.
posted by jedicus at 8:55 AM on April 25, 2012 [23 favorites]

Funny you should mention lobsters because we have no idea how big they can get.
posted by griphus at 8:55 AM on April 25, 2012 [1 favorite]

Could it have something to do with bone structures (or any rigid structure) more evenly distributing the weight load of the squishy parts? I think the hard-outside ones would have a hard time scaling up, but I don't know why.

Why do you never find an infestation of thumb-sized gerbils in your attic?

Because it would be fucking terrifying, that's why.
posted by punchtothehead at 8:57 AM on April 25, 2012 [6 favorites]

Best answer: Another reason why insects specifically don't get huge is that they respirate through their exoskeletons. This presents the same square-cube problem with regard to getting enough oxygen and getting rid of enough carbon dioxide. Lungs, by contrast, increase proportional to volume.

Still, the upper limits on exoskeleton-based creatures are apparently pretty high. See this 8 foot long fossil sea scorpion, for example. Terrestrial creatures are typically smaller than aquatic ones, however.

Why do you never find an infestation of thumb-sized gerbils in your attic?

There are some pretty dang small mammals.
posted by jedicus at 9:03 AM on April 25, 2012 [2 favorites]

Lobsters who are not otherwise eaten die once the reach a certain size from structural difficulties related to their exoskeleton once it reaches a certain thickness and they need to molt, but as griphus points out, freed from that limit keep growing every year- while there are some very small mammals- the smallest is a bat that's 30-40mm in size. So there are bats as small as some prawns, though of course not at small at critters can get.
posted by Phalene at 9:04 AM on April 25, 2012

>there's barely anything in wikipedia for endoskeleton

Because just "skeletal system" is the default concept. We don't say endoskeleton. (I didn't hear it in my 3 courses that included a lot of animal biology topics.) I'm sure Wikipedia has tons about vertebrates.
posted by Listener at 9:10 AM on April 25, 2012

Best answer: jedicus beat me to it - gas exchange is a big part of it, not just the lungs but the heart-pumping-blood circulatory system that distributes oxygen throughout our squishy systems. Back in the day, those dragonflies were able to get really big in part because there was more oxygen in the atmosphere.

Now, is there any reason (beyond the vagaries of evolution) an animal with an exoskeleton couldn't also have lungs and a pump-driven circulatory system like a vertebrate? I'm not sure... it seems like it ought to be physiologically possible.
posted by mskyle at 9:11 AM on April 25, 2012

Response by poster: With regard to large exoskeletal creatures, the answer lies in the square-cube law. Basically, mass increases proportional the cube but surface area increases proportional to the square.

Can you explain this further? I had considered this possibility but rejected it. It seems to me that it should work either opposite, or equal, depending on the relative cross-sectional strengths of bone and shell. In both cases, mass is growing as a cube of the scale factor. The strength of bone and shell are both based on their cross-section, which is growing as a square, but it's the same for both. The difference is, the bone is at the center of the bending moment, and under bending (or twisting), if both bone and shell could be made equally strong per unit mass, you'd need more mass for the bone to resist the same loads without breaking.

It's like why you make the tubes of a bike wider. Or why birds have hollow bones.

However, this would explain things if shell was just much, much weaker than bone, but I have no idea if it is. At least it would on land. In the water, in wouldn't matter, and would leave the mystery of why there are huge whales and not huge lobsters. It also seems weird, because there are squishy-inside animals that are made out of lots of different materials, so one would think they'd have wildly different strengths

Another reason why insects specifically don't get huge is that they respirate through their exoskeletons. This presents the same square-cube problem with regard to getting enough oxygen and getting rid of enough carbon dioxide. Lungs, by contrast, increase proportional to volume.

This smells on track to me. There's gotta be some sort of floor you can put on per-second per-milliliter tissue oxygen consumption, which, given a maximum trans-exoskeleton respiratory rate per unit surface area, would put an upper bound on the size of a squishy-outside animal that breathes through it's non-convoluted shell. I'd be curious to see if this actually fits how big squishy-inside animals get. It also has the nice property of being able to explain both the underwater and overwater case via the same mechanism with different coefficients plugged in. But that would only cover creatures that breath through their shells. And haven't evolved some kind of radiator/oxygen exchanger structure that's just a convoluted mess of shell with...blood? whatever carries oxygen around inside of bugs. Do all the squishy-inside creatures from all the different lineages breath this way? Also, it leaves open the question of why there were huge bugs and stuff in dinosaur times: was their more oxygen as a percentage of the gas the huge bugs were breathing?
posted by jeb at 9:16 AM on April 25, 2012 [1 favorite]

Now, is there any reason (beyond the vagaries of evolution) an animal with an exoskeleton couldn't also have lungs and a pump-driven circulatory system like a vertebrate? I'm not sure... it seems like it ought to be physiologically possible.

If an animal has a rigid exoskeleton, it can't expand when it inhales.

If it just has a flexible membrane holding it together, it relies on internal pressure to keep itself in one particular shape, and exhaling — losing pressure — would make it limp and floppy. It's also hard to generate the air pressure to inhale again if you have nothing rigid to push against.

These aren't unsolvable engineering problems. But they're harder to solve for invertebrates than for vertebrates, so vertebrates have an easier time occupying niches that involve breathing.
posted by nebulawindphone at 9:20 AM on April 25, 2012 [3 favorites]

Another thing to consider is ecological niche. In most of the world, certain "slots" in the food chain are consistently filled by certain phyla or orders -- for example, insects live almost everywhere, but are usually pretty small, because 50-gram mammals tend to be more successful than 50-gram insects (using "50-gram thing that eats plants" as a hugely simplified ecological niche). Since New Zealand has no native mammals, the enormous weta has evolved to fill the slot that would normally be taken by mice -- but now that Europeans have introduced rodents to New Zealand, the wetas are at risk of being outcompeted by these mammals, which fill more or less the same role but are "better" at it.
posted by theodolite at 9:22 AM on April 25, 2012 [3 favorites]

Best answer: Atmospheric oxygen ~300 million years ago was 30-35%, compared to 21% now. Insects rely on diffusion to transport oxygen to their inner tissues; they don't have an active circulatory system to pump it in. When the surrounding oxygen concentrations are higher, you sustain tissue farther from the surface than you would at lower concentrations. It's not really about surface area, it's the maximum distance of any point in the body from the nearest external surface where oxygen exchange can happen.

So there's an upper limit on the body size of anything without an active circulatory system, related to oxygen levels in the environment. Since oxygen levels in water are related to the levels in the surrounding air, the limit applies both in and out of the water.
posted by echo target at 9:27 AM on April 25, 2012 [5 favorites]

The may be an aspect of your answer which involves diet too. For example a lobster might reach a size limit beyond which the amount of calcium necessary to produce enough chitin to give them a new shell becomes impossible to get from the amount of food they are able to catch and eat.
posted by rongorongo at 9:31 AM on April 25, 2012 [1 favorite]

Now, is there any reason (beyond the vagaries of evolution) an animal with an exoskeleton couldn't also have lungs and a pump-driven circulatory system like a vertebrate? I'm not sure... it seems like it ought to be physiologically possible.

You should look at spiders. For example some have 'book lungs' which facilitate gas exchange (although these lungs don't work like ours), and improve the efficiency of oxygen transport in their 'blood' using hemocyanin (which uses copper instead of hemoglobin's iron). Even though they have an open circulatory system, they still have a heart which pumps to help circulation (as do lobsters).
posted by sevenyearlurk at 9:38 AM on April 25, 2012

If an animal has a rigid exoskeleton, it can't expand when it inhales.

I guess... but think about the soda bottle lung model. (I mean, obviously the diaphragm at the bottom is squishy, but that could be between two different segments of the animal.)
posted by mskyle at 10:16 AM on April 25, 2012

Response by poster: So, I've followed a lot of these links (and benefited from better search terms) and it seems that ultimately the answer is, "scientists don't totally know for sure, but the respiration issue brought up by jedicus is clearly is huge, but it may also be that its something more like the respiration issue interacting with the 'less successful' and 'harder to solve' issues brought up by nebulawindphone and theodolite."

There were a number of different studies that came up along these lines (but these are all in the last five or six years) like this one:
Recent research published in the journal Proceedings of the National Academy of Science helps confirm the hypothesis that the tracheal system actually limits how big insects can be. The research provides a specific explanation for what limits size in beetles: the constriction leading to the legs.

A collaborative team of researchers from Argonne's Advanced Photon Source (APS), Midwestern University and Arizona State University wanted to study how beetles' tracheal systems change as their body sizes increase. The team took advantage of richly detailed X-ray images they produced at the APS to examine the dimensions of tracheal tubes in four beetle species, ranging in body mass by a factor of 1,000.

Overall, they found that larger beetle species devote a disproportionately greater fraction of their body to tracheal tubes than do smaller species. [emphasis mine, but shows there is definitely a respiration structure that is growing substantially faster than body size which has to have a limit.]

The team focused in particular on the passageways that lead from the body core to the head and to the legs. They reasoned that these orifices may be bottlenecks for tracheal tubes, limiting how much oxygen can be delivered to the extremities.
“We were surprised to find that the effect is most pronounced in the orifices leading to the legs, where more and more of the space is taken up by tracheal tubes in larger species,” said Alex Kaiser, biologist at Midwestern University."
One would think it would be pretty easy to find examples of this in any place where the input and output of similarly-functioning animals hits the square-cube law: for example, if you take two monkeys about 2x different in size, the required input of food and oxygen and output of of heat and waste should increase as cube, but the structures that handle this are at least sometimes increasing as a square: e.g. extracting nutrients from food follows the surface area of the intestine, sweating follows the surface area of the skin, i don't really know how excretion works. As mentioned above, lungs probably scale in throughput with volume, because they are little bags of bags, right? But anyway, you should be able to check if the double-size monkey has twice the small intestine length, or twice the density of intestinal cilia. I don't know about the heat-ditching part. I mean, humans seem to be at the exact perilous limit of their ability to shed heat now. We get seizures and heat stroke and fever-related brain damage all the time, but there are huge hairy gorillas that live in hotter places. I wonder what that's about. Of course, there must be complicating factors, like chimps are smaller than humans but stronger, so they would need thicker bones, or sexual selection factors or environmental factors for things like vocal tract length. But still I wonder if biologists can check this crap all the time, that would be cool.

It's not strictly accurate that none of the squishy-inside creatures have pumping functions. There's the book lung brought up above, some bugs do some tricks involving vents in their bodies and wings and tubes and muscles and push gasses around. Also some things push the oxygen-containing fluid around (hemolymph, although insects don't generally use it to carry oxygen, other squishy-inside things do) using mechanisms varying from hearts to combinations of one-way valves and squeezing with their locomotion parts, which sounds crazy until you remember that humans kind of work the same way, and that's one of the reasons post-operative patients need to wear compression socks and some people get deep-vein thrombosis on airplanes: foot squishing is part of the blood-, and thus respiratory, circulating system.

The lobster, actually, provides an interesting counter-example: it has gills, which means it can probably exchange gas with the water more efficiently (I mean, that's the same technology that works for big-ass sharks and stuff), and it has hemolymph, which means it's not restricted by little air tubes' diffusion of gas to tissues. and it has a heart, so it can circulate oxygen-carrying fluid effectively, and under pressure. There are no veins and arteries, but still, it's a lot better than a bug. Maybe this is why lobsters seem to be able to get a lot bigger than the other squishy-inside things? Maybe they are not subject to the same limits as insects and so on, but are one of the rare cases where these things have evolved together, but other pressures have held the lobster family from getting bigger. Or maybe there are feathery hairy lobsters the size of city busses swarming over blue whale carcasses deep in the oceans where light and man never reach, their claws as big as hospital cribs, their hideous mouthparts unceasingly grinding on putrid whale meat in the cold dark, who knows.

Anyway, for future readers of this question, yeah, Listener's point about "skeleton" was useful: there's way more stuff if you start there in Wikipedia, it's just that a lot of its about different structural systems. The other things you want to read about include: spiracles, trachea, hemolymph, discontinuous gas exchange.

Thanks everyone.
posted by jeb at 12:23 PM on April 25, 2012 [1 favorite]

Thumb sized mammal.
posted by cmoj at 12:41 PM on April 25, 2012

There are some fairly small vertebrates (aka "squishy outside), but my guess as to why you can have ant and flea-sized (mite sized!) creatures with exoskeletons is that their organ systems are vastly simpler and can thus be scaled down much further than vertebrates can. Perhaps someone who is an actual scientist can speak to whether that theory holds any water.
posted by SomeTrickPony at 12:54 PM on April 25, 2012

If you want to frighten yourself to death, have a look at the giant isopod. It's not the only huge creature with an exoskeleton. There are fewer of them, but they do exist, and they are terrifying. Try Googling "giant arthropod." You'll get pictures of things I can't bring myself to look upon, let alone research enough to describe. There was one that was apparently a ten-foot-long millipede from Indonesia. Also something about an ancient sea scorpion that was more than two meters long. Which is six and a half feet. Which is only a little smaller than a horse.
posted by brina at 5:08 PM on April 25, 2012 [1 favorite]

Best answer: Subject of a famous essay, Haldane, On Being the Right Size (1926).
posted by oddovid at 5:22 PM on April 25, 2012 [2 favorites]

Here's your smallest vertebrate (it's a frog, and it's freakin' adorable).

Pure speculation on why they don't get smaller: They need to be anatomically more complex to support their defenses, namely active defense (fighting or running), plus defenses like poisons or at least bad taste, which require special glands.
posted by anaelith at 6:45 PM on April 25, 2012

Also we don't fish horses.
posted by lalala1234 at 9:57 PM on April 25, 2012

Also we don't fish horses.

But they do shoot them.

Can you imagine the leader you would have to use when fishing for horses? The teeth on those things!!
posted by wenestvedt at 9:24 AM on April 26, 2012

Response by poster: It turns out this is a lot more complicated than just the square-cube gas diffusion issue:
Giant insects, with wingspans as large as 70 cm, ruled the Carboniferous and Permian skies. Gigantism has been linked to hyperoxic conditions because oxygen concentration is a key physiological control on body size, particularly in groups like flying insects that have high metabolic oxygen demands. Here we show, using a dataset of more than 10,500 fossil insect wing lengths, that size tracked atmospheric oxygen concentrations only for the first 150 Myr of insect evolution. The data are best explained by a model relating maximum size to atmospheric environmental oxygen concentration (pO2) until the end of the Jurassic, and then at constant sizes, independent of oxygen fluctuations, during the Cretaceous and, at a smaller size, the Cenozoic. Maximum insect size decreased even as atmospheric pO2 rose in the Early Cretaceous following the evolution and radiation of early birds, particularly as birds acquired adaptations that allowed more agile flight. A further decrease in maximum size during the Cenozoic may relate to the evolution of bats, the Cretaceous mass extinction, or further specialization of flying birds. The decoupling of insect size and atmospheric pO2 coincident with the radiation of birds suggests that biotic interactions, such as predation and competition, superseded oxygen as the most important constraint on maximum body size of the largest insects.
Environmental and biotic controls on the evolutionary history of insect body size

"Biotic interactions" means "interactions between living things" such as predator/prey relationships, plant/herbivore relationships, competition, and symbiosis (*). The interaction between the bugs and the oxygen level of the atmosphere is a called an "abiotic interaction". So…these biotic interactions sound like a pretty classic "insanely difficult to model" system with chaotic dynamics, but either way, it's not just the oxygen diffusion thing that limits the size of squishy-inside creatures. Or, perhaps it is, if the reason the biotic interactions reduce the population of ginormo insects to zero is because of something like reduced fitness when competing for the same resource, or reduced fitness when serving as prey to birds/bats because of the oxygen-diffusion limits (flying things, like cars, I'm guessing, are highly oxidation-of-a-fuel-source limited things)

Now, this only covers flying bugs and birds. I wonder if anyone has done the same thing for land beasts like those nasty crabs on Indian Ocean islands. Nothing hunts those, though, which at least takes one biotic interaction variable out of the mix.
posted by jeb at 9:04 AM on June 8, 2012

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