Science Question: Large Arthropods

[Dannyalcatraz]

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.

 

[Achan hiArusa]

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.

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.

 

[Dannyalcatraz]

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.

 

[Achan hiArusa]

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.

 

[Aeolius]

I am reminded of these guys :

 

[LightPhoenix]

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

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.

 

[Achan hiArusa]

I am reminded of these guys :
picture snipped

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