Why Eight Arms?


Mar 27, 2003
Here's a question for anybody that would like to hypothesize or otherwise give insight...

...Why do octopus have 8 arms? Why not 6, 7, 10, or any other number?

Mel :biggrin2:

Well, here's my two cents... I think it has something to do with a mixture of symmetry, and neuro-muscular control. Nature tends to favor even numbers in symmetry, though I'm not sure why. *sigh* I never make sense late at night, sorry...

I think that its a selective pressure based on the design of the cephalopod "brain". Chances are they have a lot fewer limbs than their ancestors and probably have a great deal more control over said limbs. More limbs would probably be harder to control, or at least tax the resources of the nervous system.

An interesting thing to consider is the diversity of the cephalopoda in general, as some "squids" have only eight limbs, and how the limbs vary in shape and size across the Class. Maybe selective pressures wil move them toward fewer limbs, but more complex behaviours and limb usage? Beats me...

Anyway, I hope I wasn't over-technical or under-technical for you. Truth is, I think Steve O'Shea and Tintenfisch would provide a much better answer than I. Take care, and have a great night....

Good idea. I was considering doing the same thing myself a few weeks back (in December, actually). There are some other topics that I wouldn't mind seeing revived, as well.

I presently have nothing useful to contribute, but I like where you were headed, John. I had some thoughts of my own, related to what you wrote earlier, but they're all lost in a bit of insomniacal fog right now.
Afraid I'm brain dead on this one myself, but it is a jolly good question. I'd also like to know who cooked the first curry! Was it an invention, an accident, and did they realise just how popular and varied the dishes would become.

.... on that note, off to munch on a curry.
[Edit: I spoke with my animal physiology instructor today... he told me that to him it would seem that, from an evolutionary standpoint, there is a certain amount of "evolutionary streamlining", or that it may be less taxing on the system to have more than you need. However, he also made an important point about selective pressures.]
Firstly, I'm with FS and um..., there are some great threads on this board, so if/when inspired, please do resurrect away!

My :twocents: (which is outrageously overpriced 'cause I've always done poorly in science) on their eight arms is that I think it's very much along the lines of symmetry, but I don't think there was any environmental need that dictated it... I imagine that it's just the way things turned out by nature, and lucky for cephs it worked out for their survival...

Regarding symmetry, cells divide, and as they divide, they replicate, and just before they split, they're one symmetrical entity. That would seem to be a microcosm of the end result of the entire being. That is, we talked somewhere else (Intelligent Design perhaps?) about how if you split us down the middle we're the same on both sides, save a few internal organs. The same seems to apply to cephalopods.

I mean, I have no idea -- what makes things grow, then stop growing? Cells divide at a furious pace in the early stages of life, and then things take more form and eventually stablize... right?

I would venture to say that humans are "lucky" that we're built as we are, and we've learned to work well with what we've got. But how much do we attribute to "luck" and how much to evolution? I tend to think of evolution as something that happens with fringe features, but not the core. I mean, our earliest known ancestors may look a lot different than we do today, but they were at the most fundamental level the same. What gives us two arms, two legs, balance, a skull with brains and a heart and all that stuff? Those elements haven't changed at their core.

Uh, so can anyone cite an example of what beast, what being, has evolved most dramatically that we know of? That'd be interesting to know...

Wow what a great thread!

Just to throw my hat in the ring, IMO- there is more symmetry involved, than the actual split down the mid. Taking in consideration, the location of the eight arms around the beak. It appears to me as if octopods, seem to "throw" their arms, haphazardly and then their tentacles start to explore. Perhaps 8 is a good number to sit in a circle and through out fishing poles for the best odds and minimal effort, who knows, I don't!

However, I have ALWAYS felt the best answer to why, is WHY NOT. There is no reason they need any different number if it works for them! 8)
Well, from a slightly different perspective and looking at the fossil record and evolutionary history, this question is quite interesting. Maybe the question should be ‘Why do coleoids have ten arms’, rather than ‘Why does the octopus have eight arms’? Let me explain…..

The basic stock from which all the coleoids are believed to have originated are believed to have ten arms, all of equal length. This is demonstrated in very well preserved belemnites that have ten arms of equal length which was probably also the case with their own ancestors. Those said ancestors quickly split into two other distinct lineages, the Decembranchiata which includes the squid, Spirula, and cuttlefish. This group slowly modified arm pair IV into seizing tentacles. The other group, the Vampyromorpha, evolved in a different direction, also retaining the ten arms but modifying a different pair, pair II, into filaments which can be seen in the modern Vampyroteuthis, the only known modern survivor of the group. The Octopus evolved from the Vampyromorphs and lost the second arm pair entirely. The earliest known Octopus from the late Jurassic, Proteroctopus shows these eight short arms. I attach a diagram, adapted from the family tree to hopefully demonstrate what I am driving at.


To me the real question is how did this ten-armed plan from which all modern coleoids have arisen evolve from the nautilus? The modern nautilus is so different from the other extant cephalopods it is very hard to picture them evolving from that lineage. After all, the nautilus has 90-100 arms, depending on which source one looks at, and an external shell, obviously. It also has two pairs of gills, (the other coleoids have one pair), much cruder eyes, a brain of a different structure to name but a few. These are all fundamental differences let alone the many behavioural differences. Perhaps we are looking at a scenario that the ancient Orders of nautiloids, of which the modern Order Nautilida is the only survivor, were a disparate group with many varied forms of soft-bodied morphologies. The modern coleoids may have developed from a long extinct branch of nautiloid with more modern and familiar features, possibly varying greatly in form from today’s Nautilus.

Going back to octopuses though, I think I agree with John about selective pressures favouring fewer limbs and more complex behaviour. Think how crude the hunting strategy is for the archaic Nautilus with over ten times the number of arms, compared to the highly supple and more complex octopus. You could be right about the circle idea, Neptune, works for me!

(Please remember that I am no biologist, and these are just my opinions….)
Phil said:
Perhaps we are looking at a scenario that the ancient Orders of nautiloids, of which the modern Order Nautilida is the only survivor, were a disparate group with many varied forms of soft-bodied morphologies. The modern coleoids may have developed from a long extinct branch of nautiloid with more modern and familiar features, possibly varying greatly in form from today’s Nautilus.

That's been my assumption. Having almost complete ignorance of any fossil evidence, I figured that the two groups represented diverging modifications of some sort of precursor. Turning :nautilus: into :octopus: is far too weird.
Wow, Phil that was really something, and it goes with my feeling shared by many others that Octopods are the most complex cephalopods, first the losing of the shell and now the loss of one pair of arms. Now I don't mean to jump to any conclusions, but there might be a slight chance that if the animal could evolve into terrestriality, it could lose more arms and adopt a humanoid form, but I digress....
Cheers, O.V, neptune, um... and FJ.

I think that between us we have got to find a site of exceptional fossil preservation somewhere round about the late Devonian/early Carboniferous in date. This seems to have been a crucial time in cephalopod evolution with the emergence of the ammonoids and primitive coleoids in a comparatively short space of time from the nautiloids. The discovery of fleshy preservation amongst the weird linking-group bactritida would answer so many questions, though doubtless would raise many more....

The discovery of a lagerstatte containing soft-bodied cephalopods would be goldmine. One day!

Well, the selective pressures/complexity argument might have a bit of a problem... Check out this thread about cognitive mapping. Our member Brown has some interesting things to say, and I hope to read more of his posts.

But I still am pretty hung up on the idea of selective pressures/complexity, so I'm not giving up yet.

Sushi and Oyako-don

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