It's probably nothing buuuut....

Graeme

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So there I was speaking to another guy in my class, after a meeting with our Hons Proj co-ordinator, about the fact it's weird how octopuses are octopods, as opposed to decapods like other ceph's; in that they lack the two tentacles. So he put across an interesting point: Which do you reckon happened first? Did octopods lose their tentacles, or did squid and cuttles develop a new pair of limbs? Got me thinking. I dunno iffen this's been discussed before, or if there's even a more mundane explanation like "octopods are completely seperate from the other two, and elolved pretty much in parallel, or that all three are different and squid and cuttles became similar through convergent evolution. I can't comment on nautiloids or argonauts as I know little of the former and nothing of the latter!

So anyhoo, any takers??

Graeme- trying to sound clever, but probably just sounding like a rambling fool! :hmm:
 

Clem

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Graeme said:
So there I was speaking to another guy in my class, after a meeting with our Hons Proj co-ordinator, about the fact it's weird how octopuses are octopods, as opposed to decapods like other ceph's; in that they lack the two tentacles. So he put across an interesting point: Which do you reckon happened first? Did octopods lose their tentacles, or did squid and cuttles develop a new pair of limbs?
Hello Graeme,

Excellent questions. I think this subject has been bandied about in the Fossils & History section of TONMO; have you tried searching in that forum, yet? I'm not qualified to make any arguments one way or another. However, as a starting point you might consider the strange and primitive vampyromorphs, cephalopods that straddle the line between octopod and decapod. Vampyroteuthis infernalis has eight arms and a pair of retractile filaments which tuck away into pouches on the animal's body. They're very fragile structures, and are thought to perform a sensory function.

That might be something to think about as you all ponder the evolution of tentacles as feeding arms: did they start out as sensory organs and then develop an offensive capability? If you're a cephalopod using two tentacles to detect potential prey items nearby, you'd be a more efficient predator if those limbs could also capture and draw the prey up to you, rather than having to put on a burst of speed (and energy) to rush at and grasp what the weak tentacles detected.

Cheers,
Clem
 

Tintenfisch

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Here's the start of what Tree of Life says about the retractile filaments in Vampyroteuthis (which, so very oddly, are in between what would be Arms I and II on a squid, rather than between Arms III and IV, where every Recent squid has them):

'The primitive (plesiomorphic) arrangement of arms in coleoid cephalopods is thought to be 10 equal arms. This arrangement is known in some fossil coleoids (belemnoids) and the presence of ten unequal arms in modern decapods is easily derived from such a condition. Therefore, octopods, with eight arms, have apparently lost one pair. If the vampire filaments represent modified arms II and if this is the pair that is lacking in octopods, then strong support would exist for a vampire-octopod affinity. Embryological evidence suggests that the missing arm pair in octopods is either arms II or III (Boletzky, 1978-79).'
 

Phil

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Thanks Um, I'd forgotten about that thread!

If I may be so bold as to link to my own diagram, please have a look at the Family Tree of Cephalopods I knocked up a short while ago. A picture paints a thousand words they say, and it's much easier than writing it all out again. Please ignore the silly Intelligent Design versions later in the thread, especially if easily offended!

As Kat has mentioned above, the belemnoids had ten arms of equal length and are the most 'primitive' coleoids we have soft-bodied fossils for. All later non-shelled cephs seemed to have taken this basic plan and modified it in their own manner.

An interesting question is, where did the belemnoids derive the pattern from, as Nautilus has 80-90 tentacles? Nautilus is but the tip of a once massive and very diverse group, the nautiloids, and it's quite likely they had varying soft-bodied plans, some with ten arms, some with more. Perhaps the modern squid and octopus originally stemmed from a very distant ten-armed nautiloid.

We need more soft-bodied fossils!
 

Graeme

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That's a pretty groovy pic, I've only managed to grab a quick look as I'm on a uni comp and have to go over a lecture, like, NOW! When I get spare time I'll pur over it properly, but I grabbed the gist of it!

Next question- Squid and Cuttles are very active pelagic hunters (for the most part) and also have tentacles; Octopods are for the most part benthic predators that favour a more ambush-like approach (cammo, and descending on prey like a net). Do you reckon this is coincidence, or do you reckon that living in the water column requires some sort of harpoon-like hunting apparatus, which octo's don't need, since they have a modified membrane between each arm, and just need to smother the prey. Or put it this way, do you reckon that squid have more use for a pair of tentacles as the prey they catch are more or less fast, active, pelagic animals, like fish where closing the distance will be a problem, whereas octopods do not require tentacles since they ambush the prey on the sea floor using cammouflage and envelop it at close quarters? I hope you get the idea of what I mean, as I find it easier to say it rather than type it. I dunno if it's been discussed before, or is the general theory, but I came to the conclusion via observation. So any takers on this 'un??

Graeme- being a right pain in the butt now
 

um...

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Pain in the butt? Are you mad? We need more threads like this.

There was a paper in Nature a couple of years back that discussed the expression of Hox genes in embryonic Euprymna scolopes. (Blogged about here, where I first learned of it.) I'm wondering if anybody might be aware of anything similar being done for other cephs, especially octopods.
 

Steve O'Shea

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.... indeed an interesting thread.

Most octopuses (greatest diversity of species) are found in the Octopodidae, and are primarily (probably exclusively) benthic forms (although capable of jetting off from the sea floor into the benthopelagic realm). However, most framilies of octopuses are pelagic (Alloposidae, Ocythoidae, Argonautidae, Tremoctopodidae, Amphitretidae, Vitreledonellidae), and lack tentacles. So, the presence of tentacles (not always propelled out to capture prey, even in squid) is probably not something that is absolutely necessary for a pelagic existence (to capture prey at some distance). Those squid with the longest tentacles of all, Chiroteuthidae, Mastigoteuthidae, Grimalditeuthidae (some of these families may have been synonymised - it depends on what treatment you follow), don't propel them out at all, but rather let them dangle below. Architeuthidae has inordinately long tentacles also ... and I'm still inclined to think of this thing as a tentacle dangler, at least in submature-to-adult form (non-paralarval form), despite this recent imagery (as a 'dangler/lunger'; this is how we proposed it fed quite some time ago, and I don't see the new imagery refuting this). Then, a number of squid families lose the tentacles as adults, chiefly Octopoteuthidae and Lepidoteuthidae, and these have those wicked hooks on the arms, used to restrain prey, so even here, tentacles need not be required to restrain fast-moving prey from a distance.

I'll think about it some more.
 

Vampyroteuthis

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My thoughts are: the prescense/abscense of tentacles depend on where they hunt. Octopi normally hunt on the ocean floor, among rocks and such, so wouldnt long tentacles just get in the way? Squids on the other hand, hunt more in the open, where a pair of elasticy fish catchers would be quite handy.
Rambling on with just a hunch..
 

Phil

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As an aside, the earliest octopod fossil known from a date of 296 million years ago, actually has a total of ten arms, though two of these formed a modified pair. Even at this early date the animal was dwelling in a warm and shallow-water habitat with well developed fins.

I am unsure which pair was modified and whether it was the same pair that was lost in subsequent octopuses. I expect it was probably arm pair II, the same as the modified pair in the vampyromorphs, their close relatives. There was clearly some evolutionary selective pressure to lose the pair, as by the time of our next fossil octopus, at 164 million, we have our familiar eight-arm plan.


 



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