It's probably nothing buuuut....

Discussion in 'Physiology and Biology' started by Graeme, Sep 29, 2005.

  1. Graeme

    Graeme Vampyroteuthis Registered

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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:
     
  2. Clem

    Clem Architeuthis Supporter Registered

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    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
     
  3. Tintenfisch

    Tintenfisch Architeuthis Staff Member Moderator

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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).'
     
  4. um...

    um... Architeuthis Supporter

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    I was going to link to that TOL page, but I'll send you here instead. Phil rules.
     
  5. Phil

    Phil Colossal Squid Supporter Registered

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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!
     
  6. Graeme

    Graeme Vampyroteuthis Registered

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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
     
  7. um...

    um... Architeuthis Supporter

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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.
     
  8. Steve O'Shea

    Steve O'Shea Colossal Squid Supporter

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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.
     
  9. Vampyroteuthis

    Vampyroteuthis Cuttlefish Registered

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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..
     
  10. Phil

    Phil Colossal Squid Supporter Registered

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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.


     
  11. monty

    monty Colossal Squid Staff Member Supporter

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    sensory strategies

    This is sort of a "chicken and egg" comment with respect to evolution, but it occurs to me that the decapods are primarily visual hunters, and while octopi certainly have well-developed vision, they place a good deal more emphasis on tactile interaction with their environments, including a lot of sticking of arms into places they can't see. Perhaps the "shoot out tentacles" approach is coincident with the strong visual component to the squid and cuttle lifestyle? Of course, I imagine that open-water octopods are less tactile than the benthic ones, although the "inside-out" behavior of vampyroteuthis suggests to me that it has a tactile component to its hunting despite not being near nooks and crannies... I also recall that the optic lobes on squid brains are much much bigger than octopuses, although I don't know about non-benthic octopuses...

    just another random thought...
     
  12. Graeme

    Graeme Vampyroteuthis Registered

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    Exactly what I thought dude.

    OK, I've been racking my brain trying to think up another question, it's up to you guys which one you wanna answer, I'm just trying to come up with interesting discussion points on a regular basis (I've been on forums where a couple of people have done this, and it seemed to get quite good; started getting into really deep philosophical stuff, which isn't bad for an "anime forum":lol:).

    So next question: There are, I think, 2 species of Octopus in NZ left with a vestigial "shell" which is referred to a Pen. Now, why would the pen still be present in these 2 species when it's not in others? Could it be the same idea of the vestigial pelvis in certain Cetaceans? Could these 2 species just be elvolving a little later than most octopuses? Or could it be possible that they sevre a cartain purpose, like keeping structure, or protection (eg. Bistle-nose Catfish has barbs that stick in the predator's throat)?

    Graeme
     
  13. Steve O'Shea

    Steve O'Shea Colossal Squid Supporter

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    Hi Graeme. There are nine species here (in NZ) with the shell vestige you refer to (3 x Opisthoteuthis spp.; 1 x Cirroctopus sp.; 1 x Luteuthis sp.; 1 x Grimpoteuthis sp.; 1 x 'Cirrothauma' sp.; 1 x Enigmatiteuthis sp.; 1 x Cirroteuthis sp.; each has a name/as in it is described). These are all cirrate octopuses, the most primitive (or basal; depends if you want to attribute polarity to the group), all of which (with a global distribution) possess the shell vestige (and fins). Octopodids (conventional benthic octopuses) also have a shell vestige, but this is further reduced in this group and takes the form of 2 spindle-shapped structures in the mantle called stylets.

    Being a shell vestige - a structure slowly being phased out over the course of time - they needn't serve any purpose. In cirrates they do provide a hard structure that the muscular fins can anchor to; in incirrates (inclusive octopodids) their 'function' is a little more obscure. They are rather poorly studied structures.
    Cheers, Me
     
  14. Graeme

    Graeme Vampyroteuthis Registered

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    Ah! So it is kind of like the Cetacean pelvic bone! You know, not long after I posted, I wondered if it was 9, but I couldn't find the reference, which I found last year for uni work, so I thought I should leave it, in case I made a complete bampot of myself (looks like I kinda did anyway!).
    Thanks for that.
     

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