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Natural Science Forum / Biology / Paleontology / October 2004Large exoskeletons
Is there any reason why exoskeletal creatures have tended to remain small, relative to say, vertebrates? Is a lobster, at ~45lbs., the largest of these (including extinct lineages)? Is there anything specific reasons why they could not in theory, grow to hundreds or thousans of lbs.? (I'm assuming that the skeleton simply cannot grow to support the weight for land-based ones, but am not clear on this.) Regards, Brett Aubrey. If you simply type 'exoskeleton size limitations' into Google, you will find a few pages of articles discussing this very issue. Not to complain, but doing a little bit of background research on a broad subject like this, would allow you to ponder some of the constraints of exoskeleton size and then, if you are still not satisfied, ask more focused and more probing questions. It would also make the thread much more fun and eliminate these kind of tedious replies. > Is there any reason why exoskeletal creatures have tended to remain small, > relative to say, vertebrates? Is a lobster, at ~45lbs., the largest of > these (including extinct lineages)? Is there anything specific reasons why > they could not in theory, grow to hundreds or thousans of lbs.? (I'm > assuming that the skeleton simply cannot grow to support the weight for > land-based ones, but am not clear on this.) Regards, Brett Aubrey. Point taken (though I did get good answers from Philip Bowles). Regards, Brett. > If you simply type 'exoskeleton size limitations' into Google, you will find > a few pages of articles discussing this very issue. [quoted text clipped - 12 lines] > > assuming that the skeleton simply cannot grow to support the weight for > > land-based ones, but am not clear on this.) Regards, Brett Aubrey. >Is there any reason why exoskeletal creatures have tended to remain small, >relative to say, vertebrates? Is a lobster, at ~45lbs., the largest of >these (including extinct lineages)? Is there anything specific reasons why >they could not in theory, grow to hundreds or thousans of lbs.? (I'm >assuming that the skeleton simply cannot grow to support the weight for >land-based ones, but am not clear on this.) Regards, Brett Aubrey. It's largely to do with the way arthropods take up oxygen. Unlike vertebrates they don't actively breathe, but instead passively absorb oxygen through their skins, which then slowly percolates into the tissues through 'breathing tubes' called trachea. Diffusion is a far slower process than active breathing, and can only supply oxygen to tissues quickly enough to support small animals. Terrestrial arthropods could (and did) grow significantly larger in periods of pre-history with higher atmospheric oxygen levels than they do today, although the levels still weren't high enough to allow any to grow more than a metre or two in length (several extinct centipedes and millipedes reached these lengths, I believe). On land particularly there is an additional problem with the weight of the exoskeleton. These two factors are why the largest arthropods (the largest living is the plate crab which lives in Japanese waters and has a carapace over a metre in diameter) are marine species of cool waters - the added buoyancy reduces weight stress and, more significantly, water (particularly cold water) holds higher levels of dissolved oxygen than an equivalent volume of air. Philip Bowles Hi Philip, >>Is there any reason why exoskeletal creatures have tended to remain small, >>relative to say, vertebrates? Is a lobster, at ~45lbs., the largest of [quoted text clipped - 22 lines] > > Philip Bowles Can you supply a pointer or reference for the "plate crab" with the meter diameter carapace? AFAIK, the world's largest crab is the Japanese Spider crab (Macrocheira kaempferi), found from Japan to Alaska, with carapace up to 370mm, mass up to 20kg, and leg span up to 4 meters. The Giant Southern crab (Pseudocarcinus gigas) of the south Australian coast has a larger carapace, up to 450mm, but its legs are much shorter, so it is more compact and its mass is up to 14kg. FWIW, the largest land arthropod is probably the coconut crab (Birgus latro), with modern specimens sometimes exceeding 4 kg, although claims of much higher masses also exist. Since it is a kind of hermit crab - juveniles need a borrowed shell, adults don't - it lacks a carapace, but it can reach 400mm in body length and 1 meter in leg span. Not all arthropods rely on tracheae and spiracles. For example, the Coconut crab has an innovative breathing apparatus: the branchiostegal lung. This allows them to attain large size and engage in energetic activity in air. They drown in water due to insufficiency of their gills, despite water's potentially higher oxygen content. Best Regards, John. > >Is there any reason why exoskeletal creatures have tended to remain > >small, relative to say, vertebrates? Is a lobster, at ~45lbs., the largest > >of these (including extinct lineages)? Is there anything specific reasons > >why > >they could not in theory, grow to hundreds or thousans of lbs.? (I'm [quoted text clipped - 20 lines] > significantly, water (particularly cold water) holds higher levels > of dissolved oxygen than an equivalent volume of air. Philip Bowles Thanks for your response. We're having a theoretical discussion over at sci.bio.evolution dealing with what alien life might look like. From what I've gathered here and at various web sites it might have been evolutionarily *possible* to have an actively breathing (i.e. lungs or similar) exoskeletoned creature (i.e. evolving on a different planet under fairly similar circumstances) of sufficient size to support a brain large enough for what we view as intelligence. Comments (other than to question the validity of the thread)? TIA. Regards, Brett. >Thanks for your response. We're having a theoretical discussion over >at sci.bio.evolution dealing with what alien life might look like. From [quoted text clipped - 4 lines] >brain large enough for what we view as intelligence. Comments >(other than to question the validity of the thread)? TIA. Regards, Brett. From what I've heard arthropods on Earth, at least on land, have never been able to grow large enough for the weight issue to be a problem - it undoubtedly would prevent them reaching thousands of pounds, but I don't know exactly what the upper limit would be. In water there would be less of a problem - there were large Cambrian inverts (Anomalocaris or something similar?) one or two metres long and later 'sea scorpions' somewhat larger (I think those may have been the largest arthropods that ever lived - I don't remember what they were called, but I do remember that Arthur C. Clarke's "Songs of Distant Earth" featured semi-intelligent versions), though I have no idea what they weighed. Aside from practical difficulties in discovering fire and the like it seems conceivable that an intelligent life form could be aquatic, so it is conceivable that something not far off human size could develop. As for normal breathing, I'm not sure whether that would be possible with the rigid constraints of an exoskeleton, as it wouldn't allow room for the lungs to inflate. I'm sure I've heard it explained why arthropods aren't physiologically able to breathe normally, but I'm not sure if that's the explanation or not. Exoskeletons would have to be energetically expensive for a sizeable animal to carry around, even in water, and this would necessarily reduce the availability of oxygen to power higher motor functions. On balance I doubt that giant intelligent insects are feasible outside horror films, but that's based on the way arthropods have evolved on Earth - however, on any planet there would probably have to be a trade-off between flexibility, allowing such things as lung expansion, and size, which requires rigid supporting structures, whether internal or external. Of course there may be ways around the problem that haven't evolved on Earth, maybe some way of actively breathing without significant changes in the volume of internal organs or a mechanism for speeding up diffusion (organs close to the skin surface,, for instance, although the trade-off here might be that the bloodstream would be less efficient at transporting oxygen to the organs from elsewhere in the body than if the vital organs were more centrally-placed). Philip Bowles >As for normal breathing, I'm not sure whether that would be possible with the >rigid constraints of an exoskeleton, as it wouldn't allow room for the lungs to [quoted text clipped - 3 lines] >carry around, even in water, and this would necessarily reduce the availability >of oxygen to power higher motor functions. Just as a quick followup here; exoskeletons aren't as rigid as one might think. Take a look at an active honeybee, or hornet next time you get the chance. Their abdomens can be seen rapidly compressing and expanding. As for lungs, I would like to point out that most extant spidersl have lungs (they're called book lungs) to coincide with their trachae, so they can evolve in critters with external skeletons. These lungs might also explain why spiders grow larger than insects. It is important to note that, while spiders have lungs, the mode of oxygen uptake remains the same (spiracles on the body). The spiracle method is another reason why arthropods don't get as big as vertebrates. The bigger one gets, the larger (or more numerous), the spiracles would need to be. So, a human sized arthropod, would either have a few gaping holes in its body, or would look like Swiss cheese. Neither of these options are good for water storage. A large, terrestrial arthropod, would have to evolve a method of pumping oxygen into its body, if it were to grow larger. Anyway, just some quick change on the matter. Jura >>Thanks for your response. We're having a theoretical discussion over >>at sci.bio.evolution dealing with what alien life might look like. From [quoted text clipped - 63 lines] > > Philip Bowles Your ribs bend when you breathe. As noted by others better methods of taking in oxyen have been developed by several kinds of large arthropods so that doesn't seem to be the barrior. >> >Is there any reason why exoskeletal creatures have tended to remain >> >small, relative to say, vertebrates? Is a lobster, at ~45lbs., the [quoted text clipped - 35 lines] > (other than to question the validity of the thread)? TIA. Regards, > Brett. A lobster at forty pounds is as large as a many first grade students. Some few adult humans are that small according to the book of records with normal intelligence so the answer would have to be yes. If you have a species like ants that have a feeding phase taken care of by adults that does a metamorphases to adult size that doesn't molt you should be able to get even larger beings. >>Is there any reason why exoskeletal creatures have tended to remain small, >>relative to say, vertebrates? Is a lobster, at ~45lbs., the largest of [quoted text clipped - 34 lines] > > Philip Bowles Spiders have book lungs. Some land crabs are fairly large. Some insects have been shown to pump air in and out. I think a more serious issue is that our types or arthropods have to shed their exoskeleton inflate themselves then spend a long time in a semi helpless state untill the exoskelton hardens. To explain I had some students put a crayfish in a fish tank with some gold fish. I left it alone. It chased gold fish until it had to shed and then one ate it because it was unable to hide. If the exoskeleton were living skin that could grow without having to shed I see no reason they could not get human sized or large but not as big as the dinos due to the reasons you gave about the exoskelton being to heavy. On the other hand they do work better at small size than an indo skeleton and the time for the new exoskeleton to harden is reduced. >Is there any reason why exoskeletal creatures have tended to remain small, >relative to say, vertebrates? Is a lobster, at ~45lbs., the largest of >these (including extinct lineages)? Devonian Eurypterids were a couple of meters long. Weight unknown, but surely closer to Arnold Swarzenegger than to a 20kg lobster. And, of course, marine mollusks reach very large sizes. I think some Paleozoic nautilids were in the 5-6 meter range and the giant squid -- if it exists -- is ... well .. gigantic. But it doesn't have an exoskeleton > Is there anything specific reasons why >they could not in theory, grow to hundreds or thousans of lbs.? (I'm >assuming that the skeleton simply cannot grow to support the weight for >land-based ones, but am not clear on this.) Regards, Brett Aubrey. Although turtles are endoskeletal from a biological point of view, they are exoskeletal from a predator's viewpoint. They can reach quite impressive sizes. A couple of meters in length and 800-900kg. I think that some of their Cretaceous forebearers may have been even larger. === One additional point is that it would probably take some sort of crack and fill mechanism for an animal with an exoskeleton to grow without moulting. I can't think of any living or fossil creature that does that. So a large animal with an exoskeleton is going to have to molt and presumably go through a soft shell stage fairly often. That's probably not a survival trait. >>Is there any reason why exoskeletal creatures have tended to remain small, >>relative to say, vertebrates? Is a lobster, at ~45lbs., the largest of >>these (including extinct lineages)? > >Devonian Eurypterids were a couple of meters long. Weight unknown, >but surely closer to Arnold Swarzenegger than to a 20kg lobster. Ah yes, those were the large 'sea scorpion' things. Thanks. And, >of course, marine mollusks reach very large sizes. I think some >Paleozoic nautilids were in the 5-6 meter range and the giant squid -- >if it exists -- is ... well .. gigantic. But it doesn't have an >exoskeleton It, and indeed the recently-described (this year) colossal squid (which is bigger) certainly do exist, but while they don't have exoskeletons they do have internal skeletons of sorts in the form of cuttlebone, the internalised mollusc shell. >Although turtles are endoskeletal from a biological point of view, >they are exoskeletal from a predator's viewpoint. They can reach >quite impressive sizes. A couple of meters in length and 800-900kg. >I think that some of their Cretaceous forebearers may have been even >larger. I'm not sure on that one - I have a feeling the extant leatherback (up to 1,000lbs/1 tonne) is close to the greatest size turtles have ever attained, if not quite there, and that's a lineage that predates the Cretaceous. Philip Bowles >I'm not sure on that one - I have a feeling the extant leatherback (up to >1,000lbs/1 tonne) is close to the greatest size turtles have ever attained, if >not quite there, and that's a lineage that predates the Cretaceous. > >Philip Bowles Actually, leatherbacks (_Dermochelys coriacea_) are rather small on the gaint turtle scale. The pleurodire _Stupendemys_ (Pliocene) was in the 2-3 tonnes range. Then there's _Colossochelys atlas_ (Pleistocene) which came in around 2-4 tonnes, and the most famous ancient turtle of all: _Archelon_ (Cretaceous) which was believed to have weighed 4-5 tonnes. There's also the meiolaniids (awesome ankylosaur like turtles), which grew to lengths of 8 & 10 ft, but I don't know of any weight estimates for them. I'm thinking they probably didn't get much larger than 1,000 lbs. Jura >>I'm not sure on that one - I have a feeling the extant leatherback (up to >>1,000lbs/1 tonne) is close to the greatest size turtles have ever attained, [quoted text clipped - 10 lines] >2-4 tonnes, and the most famous ancient turtle of all: _Archelon_ >(Cretaceous) which was believed to have weighed 4-5 tonnes. Someone also e-mailed me with an example of a particularly large turtle as well, so I stand corrected. Philip Bowles |
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