# Science Question: Large Arthropods



## Dannyalcatraz (Mar 14, 2009)

I know that millions of years ago, Earth's atmosphere had a higher Oxygen content that allowed terrestrial arthropods to grow quite large.

Nowadays, the largest terrestrial arthropod is the Coconut Crab (Birgus latro)- at (supposedly up to) 40lbs and about 3' across, its no lightweight, to be sure.

Coconut crab - Wikipedia, the free encyclopedia

There are other arthropods out there that get quite large, notorious among them the tarantula family of spiders.  Yet none of the tarantulas known to science are even close to the Coconut Crab in size.  There are stories of spiders big enough to attack dogs and cats from certain areas of the world- creditable enough for some cryptozoologists (legit ones, not the ones who claim to have "Bigfoot" samples) to go looking for them, but as yet, none have been found.

My question is this, though- given the size disparity between the Coconut Crab and the largest known tarantulas, is there some anatomical difference between the crab and the spiders that lets the former have a larger top size?


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## Atanatotatos (Mar 14, 2009)

I think it's a pretty interesting question, especially if there's someone who can answer properly, but actually, isn't it a bit like asking why rhinos aren't as large as elephants?


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## Achan hiArusa (Mar 14, 2009)

The limiting factor in the size of arthropods is their respiratory system (and their exoskeletons).  Tarantulas use a delicate set of lungs called book lungs.  They are a thin set of 15 folded and attached sheets that deliver air to the circulatory system.  The crab's lungs are a lot more complex and somewhat analogous to our own.  Thus they are much more able to deliver oxygen than tarantulas.  I'm sure the difference in toughness of the exoskeletons play a factor as the stronger crab skeleton is able to support more mass than the softer exoskeleton of a Tarantula.


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## Umbran (Mar 14, 2009)

Dannyalcatraz said:


> My question is this, though- given the size disparity between the Coconut Crab and the largest known tarantulas, is there some anatomical difference between the crab and the spiders that lets the former have a larger top size?




Well, given the precedent of to coconut crab, the answer should be that any anatomical differences could be overcome with a bit of evolution.  The question is more why they'd do so.

Mammals have a distinct advantage over arthropods in speed and strength at larger sizes.  The coconut crab is a scavenger, eating mostly fruit.  It doesn't generally tangle with large and lively prey.  So, a coconut crab sized spider, competing with mammalian predators in that same size range - mostly felines and canines, probably loses out.

So, as you note, there's stories of big spiders - the suggestion is that there have been big spiders, and that they've gone extinct.  Usually, you go extinct due to mishap, or inability to compete.

So - spider have probably been big, and failed in the attempt.


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## Dannyalcatraz (Mar 14, 2009)

Thanks for the responses!

1) Yes, it is a bit like asking about rhinos and elephants.  But I already know why elephants are bigger- a combo of becoming too big for most predators to bring down and being able to access food resources that most other critters can't get.

2) I wondered if it was about the book lungs- I knew spiders have them, but I didn't know that crab lungs were more sophisticated.  That's probably the key.

3) Most of the big spiders are ambush predators, and their venom helps neutralize the advantages that warm blooded vertibrates have over them.  For a "super-spider," a pounce, a bite and a strategic retreat, and Fluffy is dinner.


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## Goldmoon (Mar 14, 2009)

I suspect it has a lot to do with diet. Coconut crabs feed on scraps as perviously stated and most spiders are predators. I would like to say that I live in Guam and I have seen a few rather large ones. Creepy little (relatively speaking) bastards.


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## Knightfall (Mar 14, 2009)

Wow! 0_o


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## Goldmoon (Mar 14, 2009)

This is the one that always gets me.....


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## Achan hiArusa (Mar 15, 2009)

Goldmoon said:


> This is the one that always gets me.....




I just want to know how it would taste with butter.


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## Umbran (Mar 15, 2009)

Dannyalcatraz said:


> 3) Most of the big spiders are ambush predators, and their venom helps neutralize the advantages that warm blooded vertibrates have over them.  For a "super-spider," a pounce, a bite and a strategic retreat, and Fluffy is dinner.




Dude, look at that coconut crab, and watch some videos of them in action.  It isn't pouncing on anything, even a coconut.  Even with the better lungs, it is carrying a lot of weight in skeleton for its size, limiting the speed and mobility.


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## Dannyalcatraz (Mar 15, 2009)

> I just want to know how it would taste with butter.




From what I understand, the CC is considered edible, and thus, is probably butteriffic.



> Dude, look at that coconut crab, and watch some videos of them in action. It isn't pouncing on anything, even a coconut.




I know- the CC is more like a Brontosaurus...er...Apatosaur, slow and massive, depending upon its size (and because its an arthropod, its shell) for defense.

The larger Tarantulas are more like T-Rex- opportunistic predators, but not as big as some of the herbivores around them.  I was just saying that while mammals have certain advantages, the Tarantulas do bring down birds and mammals nearly their own size.  Then, extrapolating to one that had the anatomical advantage that a CC has (better lungs) or something similar, a spider that size should be targeting mammals like small to medium sized dogs or cats.


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## Achan hiArusa (Mar 16, 2009)

Umbran said:


> Well, given the precedent of to coconut crab, the answer should be that any anatomical differences could be overcome with a bit of evolution.  The question is more why they'd do so.
> 
> Mammals have a distinct advantage over arthropods in speed and strength at larger sizes.  The coconut crab is a scavenger, eating mostly fruit.  It doesn't generally tangle with large and lively prey.  So, a coconut crab sized spider, competing with mammalian predators in that same size range - mostly felines and canines, probably loses out.
> 
> ...




There are some limits to change in morphology via evolution.  Apparently, its easier for gill chambers to become lungs than it is for book lungs to become more efficient lungs.  It can be a limiting factor.

There is a theory floating out there (or maybe a hypothesis) that the higher oxygen content during the Carboniferous and Permain (345 to 225 Myr ago) may have allowed for larger arthropods.  _Meganeura monyi_ was a dragonfly that grew to a wingspan of 75 cm.  Spider ancestors evolved around 400 Myr ago and the first true spiders evolved 300 Myr ago, but did not begin to diversify until 250 Myr ago.  So they may not have had a chance to take advantage of this situation.  There was a Eurypterid (sea scorpion) _Megarachne servinei_ that had a leg span of 50 cm and a body length of 34 cm that was mistaken identified as a spider that lived 280 Myr Ago on the Discover Channel.


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## Dannyalcatraz (Mar 16, 2009)

Achan hiArusa said:


> There is a theory floating out there (or maybe a hypothesis) that the higher oxygen content during the Carboniferous and Permain (345 to 225 Myr ago) may have allowed for larger arthropods.<snip>




That's what I was referring to in my initial post.  With more oxy, you don't need the best lungs to get efficient enough respiration to support mega-arthropods on land.


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## Achan hiArusa (Mar 24, 2009)

Yeah I missed that.  I think it could just be that spiders evolved too late in the game to take advantage of the heightened oxygen levels.


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## Aeolius (Mar 25, 2009)

I am reminded of these guys :


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## LightPhoenix (Mar 25, 2009)

In full disclosure, my training is in biochemistry, and not zoology.



Dannyalcatraz said:


> I know that millions of years ago, Earth's atmosphere had a higher Oxygen content that allowed terrestrial arthropods to grow quite large.




Well, first off, this is a theory.  Personally, I think that because arthropods were among the first creatures to go from sea to land, they exhibited a form of island gigantism, if you will.  The theory is that a lack of predators removes any evolutionary pressure to be smaller; larger is usually better.  With a lack of real predators for a long time, arthropods grew larger.  Evolutionary pressure pushed them back smaller once predators developed.

Additionally, oxygen usually isn't a terribly limiting factor in larger creatures.  Otherwise, you would have seen a global trend towards stuff getting smaller.  Humans are the prefect example to the contrary, giraffes another.

The biggest issue with a larger size is two-fold.  The first is nutrient uptake - the larger you are the more you generally need to eat.  If the nutrients aren't there (competition, die-off, whatever) then a smaller size is more advantageous.  The second is nutrient distribution - you need to develop a circulatory system that allows you to get nutrients (including oxygen) to the cells that need it, and spend energy doing it.

So while a higher oxygen content couldn't have hurt, I don't think it was responsible for increased size.  Correlation is not causation, and all that.

I would hazard a guess that humans have had a significant impact on crustacean populations, at the least.



> My question is this, though- given the size disparity between the Coconut Crab and the largest known tarantulas, is there some anatomical difference between the crab and the spiders that lets the former have a larger top size?




Realistically?  Probably not.  At this point it probably has boiled down to genetics.  In fact, I'm not sure there's any data linking the progression of size of spiders in the current day - for all I know they could be getting bigger.  Achan hiArusa brings up an interesting point with the lungs, so that might be something there... but I'm not convinced.


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## Achan hiArusa (Mar 26, 2009)

Aeolius said:


> I am reminded of these guys :
> picture snipped




Hey, its a Cybermat from Dr. Who.  Where are the Cybermen?


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