Why is the Giant Squid so giant?

[Melbe]

Is the giant squid so big because of abyssal giantism(because it lives in the deep it becomes big; as seen in sea spiders- shallow water ones are small, deep-sea can be as large as 80cm), or does it live in the deep because it is so big?
Just curious!
Melissa

 

[Steve O'Shea]

For this I have no correct answer Melissa.

To digress, I believe Architeuthis to be quite closely related to the likes of Moroteuthis (another genus with 'large' species), Onychoteuthis and Ancistroteuthis on morphological, anatomical and genetic grounds. But there would be huge debate if I were to suggest it (so pretend I never wrote this, ok).

The paralarval forms of these genera, in different families (Architeuthidae and Onychoteuthidae) are almost inseparable through to mantle lengths of ~ 10mm. At mantle lengths smaller than this the two differ slightly in shape, and in chromatophore distribution.

The adult Architeuthis doesn't really differ from its paralarval form - it is like a giant paralarva, whereas at mantle lengths in excess of 12 mm Moroteuthis develops hooks on its tentacle clubs (the expanded distal-most part of the tentacle); in Onychoteuthis and Ancistroteuthis these develop earlier. Otherwise each has 4 rows of suckers on the tentacle club in the paralarval stage (and Architeuthis retains this character state through into the adult form). I think Architeuthis has a photophore (light organ) on the end of its ink sac, although it isn't supposed to have photophores (there's a very strange structure at the end of the ink sac in New Zealand specimens at least, that looks just like a photophore; should it prove to be one then this would render the Onychoteuthidae and Architeuthidae even more similar).

So, back to your original question, in a rather convoluted manner, speciation often happens in terms of size (instead of the acquisition of a new character or character state); as such you often find 'giant species' of this, that and the other - 'giant' being a relative term only. So the speciation leap from an ancestral Architeuthis to a modern-day Architeuthis need not have been through a slow increase in size over many many millenia, it might have been a very sudden speciation event (one day the species 'woke up' and was seriously big; its' next of kin could have been a comparatively small species).

The likes of Moroteuthis live to much greater depths than Architeuthis (to 1000m in New Zealand waters, whereas Architeuthis is generally found within the water column at depths of ~ 500m), so if the two are related or have relatively Recent ancestry (that's if I ever suggested that they were related .... and of course I would never do that because people would yell at me), then Architeuthis may have suddenly grown larger and concurrently moved shallower than its ancestor.

Of course there's another reason that Architeuthis might be big, and that's because it is quite a passive beast that drifts in the water 10-or-so metres above a large shoal of large fish (hoki), dipping its tentacles into the school of fish and then quietly withdrawing them, unnoticed, expending very little energy in the process.

The blue whale, another rather friendly beast, also grew large, eating a diet of comparatively minute animals, as did the whale shark (two more examples of gross gigantisism), so you don't really have to live deep to be big (and in fact the deeper you go there's probably a tendency for the animals to get smaller). Your pycnogonid (sea spider) example is a good one, because the largest of these brutes do live down deep.

Not sure if I've answered the question at all, but until Tony builds that time machine into the new Tonmo board we'll only ever be able to guess what really happened. There is of course an alternative to what I propose above, and that is that the ancestral Architeuthis was twice the size of the modern-day Architeuthis - the species could actually be shrinking in time (like my brain). Another alternative, a consequence of a shrinking brain, is that everything written above is nonsense.

Any debate is welcome. My belief is that Architeuthis is just a giant paralarva with serious hormonal problems, subject to premature maturity.
Cheers
O

 

[nanoteuthis]

The adult Architeuthis doesn't really differ from its paralarval form - it is like a giant paralarva, whereas at mantle lengths in excess of 12 mm Moroteuthis develops hooks on its tentacle clubs (the expanded distal-most part of the tentacle); in Onychoteuthis and Ancistroteuthis these develop earlier. Otherwise each has 4 rows of suckers on the tentacle club in the paralarval stage (and Architeuthis retains this character state through into the adult form). I think Architeuthis has a photophore (light organ) on the end of its ink sac, although it isn't supposed to have photophores (there's a very strange structure at the end of the ink sac in New Zealand specimens at least, that looks just like a photophore; should it prove to be one then this would render the Onychoteuthidae and Architeuthidae even more similar).

Any debate is welcome. My belief is that Architeuthis is just a giant paralarva with serious hormonal problems, subject to premature maturity.
Cheers
O

Fascinating. So the implication of this is that A. dux would simply be a huge but physiologically "arrested" form of Moroteuthis, Mesonychoteuthis, and even Taningia (referring to A. dux's vestigial or prototypical photophore).

IMHO your "giant paralarva" theory is not farfetched at all. In fact it seems analogous to the development of the amphibian Axolotl, which if I am not mistaken is simply a "prematurely mature" Salamander, a giant paralarva with external gills.

Definitely would like to hear more about this!

Tani

 

[Steve O'Shea]

Here's a definition of neoteny (word of the day) for you Tani, which is probably what we are talking about in giant squid:

"A slowing of the rate of growth of the somatic (nonreproductive) parts of the body of an animal, so that larval characteristics may persist when it reaches sexual maturity. The axolotl is a neotenous salamander that rarely assumes the typical adult form under natural conditions, although metamorphosis can be triggered by injection of thyroid hormone. Neoteny is also known in certain tunicates (primitive marine chordates)."

And another:
"1. Retention of juvenile characteristics in the adults of a species, as among certain amphibians. 2. The attainment of sexual maturity by an organism still in its larval stage."

...and yet another:
"The persistence in the reproductively mature adult of characters usually associated with the immature organism."

But here's another word, paedomorphosis, and here's the only online definition I can find for it:
"Paedomorphosis - condition in which a larva becomes sexually mature without attaining the adult body form"

Now many years ago I remember someone explaining to me that there was a significant difference between neoteny and paedomorphosis. Perhaps some bright spark who reads this has something to contribute to the discussion - I look forward to it.
Cheers
O

 

[nanoteuthis]

Many thanks! Just looked 'em up and found this in a search:

http://science.kennesaw.edu/~rmatson/neoteny.html

But I still don't get the difference between the two terms. Enlighten me?

:?

 

[Steve O'Shea]

I'm no wiser myself Tani, though I do wonder why they would have two 'big' words that both describe the same thing. We need to find ourselves an evolutionary biologist to answer this one.

With regard to Taningia, I'm afraid it is only distantly related to Architeuthis (despite having photophores). You'll find 3 illustrations of Octopoteuthis hooks in figure 6 of the 'guide to frequently used.....'; Octopoteuthis and Taningia are classified in the family Octopoteuthidae - one of the characteristics of which is possessing these hooks on the arms (and in having 8 arms in the adult form). Architeuthis lacks these hooks entirely. Many onychoteuthid squid possess photophores.

Plans for the future are to have online descriptions and illustrations for all of these different kinds of squid (but we'll probably do the octopus first, as I'm far more familiar with them).
Cheers
O

 

[Phil]

With reference to Octopoteuthis, is it true that the arms can actually detach if the animal is threatened? I've read that the arms wriggle and flash as a decoy. this seems a somewhat extreme action to take! I suppose the tissues must have a weak point at the base as with Ameloctopus. If so, what exactly triggers the detaching, is it an external pull, from, say, a predator, or a reflex muscle spasm within the base of the arm pinching off the rest of the arm?

How long does it take for the arms to regenerate, or are they lost for good?

Loads of questions there. Hope you don't mind!

 

[Steve O'Shea]

Interesting question Phil!

The genus Octopoteuthis is in dire need of systematic revision; a few species are reasonably well described but the majority are not. Therefore attaching a name to any one species is difficult (there will always be some uncertainty there). Now, having said this, and the reason for saying it, I recognise three species of Octopoteuthis in New Zealand waters.

They are (provisional identifications, until comparative material can be secured): Octopoteuthis megaptera (many specimens, the majority mature ), Octopoteuthis sicula (one specimen, immature), and Octopoteuthis sp. "giant" (one specimen, mature; possibly one other, immature - need to do detailed examination to confirm).

Every specimen that I attribute to O. megaptera from NZ waters has lost its arm tips. This is extremely frustrating as I cannot determine whether they had, and what the shape was of any photophores on the arm tips (a rather important systematic character); all specimens have been caught in nets of one variety or another, and therefore you can expect their capture to have been a traumatic affair, which might explain the loss of the arm tips).

The single specimen of O. sicula and the ?2 of O. sp. "giant" have all of their arm tips intact. Very unusual this - both were caught in nets (they differ in other characters - so I am not simply attributing them to a different species because their arms are intact).

So, the loss (jettison) of the arm tips during traumatic periods would not appear to be a characteristic of all species of Octopoteuthis, although for at least O.megaptera it would appear the normal behavior. How they do it and whether the arms can regenerate afterwards (I would imagine they would) I do not know.
Cheers
O

 

[Phil]

Thankyou for the information, Steve. Interesting stuff.

Popular books on animal life, such as the one in front of me now, seem to catagorise creatures into distinct pigeon-holes and give the impression that the systematics are cut and dried. They rarely point out that the relationships between groups and individual species is constantly undergoing revision. It's easy to forget this and take what one reads as gospel!

For the record, the species that I have the reference to for the detaching arms that continue to flash is for Octopoteuthis nielseni, accredited to the Eastern Pacific. I wonder what sort of evolutionary advantage could be gained by the loss of the two feeding tentacles in the juvenile. After all, one might would think that the tentacles would aid in food collection and be an advantageous feature to retain. Not really knowing much about the feeding habits of Taningia I wouldn't really like to hazard a guess!

 

[Steve O'Shea]

Tentacles are interesting things. For some squid they most definitely are used in the act of prey capture, especially when the structures are inordinately long (as in Mastigoteuthis, Chiroteuthis and probably Architeuthis), but that is not saying that these structures are propelled out at great speed to catch and restrain prey some distance from the animal. For many kinds of squid I believe the structures dangle naturally.

In squid like Nototodarus (ommastrephid squids - or common arrow squids), oceanic beasts with shorter tentacles, they are not shot out all-that-often either; on many (probably the majority of) occasions the prey is actually caught and restrained in and by the arms. The tentacles are actually held out in front of the squid, held together and 'functioning' or 'serving to function' as a pointer or guide for the squid to track its prey. The squid scrunches its eyes forward, lining the prey up along the tentacles (and distal-most tentacle clubs), then launches forwards and both catches and restrains the moving prey with its arms (rather than having the tentacles held within the arm mass, tracking the food with its eyes, having no guide whatsoever, propelling the tentacles out to restrain it, then withdrawing the struggling prey to the arms for restraint).

Octopoteuthis obviously cannot line up any prey along the tentacles, as it lacks them, so it has to do something different in order to localise, catch and restrain prey. The photophores probably play an important role here, but whether they function to attract prey, disorient prey, confuse predators, all or none of the above I really don't know (probably a combination of them). Its comparatively short and muscular arms are endowed with two rows of seriously wicked hooks, and its fins are enormous (relative to its overall size) and very muscular; probably in combination these hooks, arms, and fins render this animal an adept killing machine.

....actually, having hooks on your arms (like those of Octopoteuthis) and long tentacles probably isn't too good a thing. The tentacles themselves would constantly get snared and hooked on the arms. This raises an interesting point O (excuse me whilst I think out loud) .... hmmmm .... so how do squids of the Family Enoploteuthidae get on, as they have both hooks on the arms and long tentacles? I'll go have a look and see if there's some protective sheath somewhere.

...off I go