Purty Molluscs

um...

Architeuthis
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Melissa said:
um...

You aren't talking to yourself, we're just still thinking it through and reading to catch up enough to offer a reply. I should have one around November 14 at the earliest!

Melissa
I didn't mean to imply that I felt alone here, just that I tend to argue with myself too much. I was suggesting to myself that maybe I should shut up a little, and stop replying to my own posts. Other people tend to have more interesting things to say.

:bonk:
 

Bald Evil

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Here's an intriguing thought: we know that many cephalopods are pretty bright (for molluscs), and seem to have very highly developed sensory systems (most notably the eyes). What if the advanced chromatophores of many species are not the result of ongoing evolution, but a remnant from a time when cephalopods were much more intelligent? Imagine a species of now-extinct cephalopods with the capacity for advanced communication, moreso than dolphins or other cetaceans, using light instead of sound. We know that dosidicus (for one) hunts in packs; could this be an instinctual remnant of more advanced cooperative thinking? This is purely speculative, but it is certainly worth investigating.
 

Phil

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Here’s a thought. As chromatophores are used for visual display, then it follows that they must have evolved in tandem with eyes capable of observing that display. The nautiloids almost certainly had very poor eyesight; think of the modern Nautilus with its pinhole camera type eye. I think it is unlikely that these animals would have evolved these organs. Besides, chromatophores are not present in the modern Nautilus, which uses its striped shell as camouflage.

There is no evidence of eyes in the ammonites; not that they were not present, but we have no soft bodied ammonite fossils to be used as evidence. The ammonites are a bit of a red herring here anyway, as they split from the common ancestor with the coleoids somewhere in the mid to late Devonian, roughly about 370million years ago and are only very distantly related to modern squid. This common ancestor, the Bactritida itself had probably split from the nautiloids somewhere around 400 million, only a few million years before.

The belemnoids and the vampyromorphs, (from which the squid and octopus recently evolved) both of which were probably highly visual animals, themselves divulged from the bactritids in the early Carboniferous, only a few million years later. This could imply that the evolution of complex eyes systems and consequently chromatophores must have happened in a short space of time, probably in the mid Devonian-E. Carboniferous. This is when the earliest common ancestor of all the coleoids lived. Unless all the modern coleoid groups evolved chromatophores independently, they must have been established at this early date.

The upshot of all this is that it seems to me key lies with the obscure bactritids, which was a linking group between the nautiloids, coleoids and the ammonoids. They existed in the short window of time in the Devonian between the externally shelled nautiloids, with no chromatophores and poor eyesight, and the appearance of the internally shelled coleoids with chromatophores and good eyesight.

Perhaps I’m making too many assumptions here, and apologies this was a bit repetitive of my post above. Please forgive, I’m still trying to think this through…..
 

Clem

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Um,

Great thread you've started. My head hurts, but in a good way.

The presence of chromatophores on the viscera of some cephalopods has got me to wondering where on the early cephalopods chromatophores first began to appear: on the dermis, or in the guts? Did they take hold first for their value in obscuring the appearance of internal organs (and the contents of the gastric tract) visible through the transparent/translucent bodies of larval cephalopods, or did they "migrate" internally to mitigate the loss of an opaque calcareous shell's light-blocking qualities?

:?:

Clem
 

um...

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Bald Evil said:
What if the advanced chromatophores of many species are not the result of ongoing evolution, but a remnant from a time when cephalopods were much more intelligent? Imagine a species of now-extinct cephalopods with the capacity for advanced communication...
What if they're still out there? Why not?
 

um...

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People around here are pretty good at making feel dumb (see any post in this thread). :)

Clem said:
The presence of chromatophores on the viscera of some cephalopods has got me to wondering where on the early cephalopods chromatophores first began to appear: on the dermis, or in the guts? Did they take hold first for their value in obscuring the appearance of internal organs (and the contents of the gastric tract) visible through the transparent/translucent bodies of larval cephalopods, or did they "migrate" internally to mitigate the loss of an opaque calcareous shell's light-blocking qualities?
I never even thought about the need for chromatophores in larval cephalopods. It's funny how I keep forgetting that 'natural selection' applies to all stages of the life cycle. (I think this might be due to the fact that most of my initial learning about evolution was picked up while studying genetic algorithms. I’ve been reading about ontogeny recently, though, so that’s a pretty weak excuse.)

As for whether chromatophores first appeared on the dermis or in the guts, the only evidence I can think of is the fact that older (black-brown) chromatophores lie closer to the epidermis than younger (orange-yellow) chromatophores. Yeah, I know that’s pretty weak.

You make an excellent point regarding the need to hide the guts. These little guys might be quite likely to eat some bioluminescent plankton, which is no good for hiding in the dark if their innards are mostly transparent. At the same time, being transparent is pretty handy during the day (especially for pelagics). So, what would be ideal is to be pretty much transparent during the day but more opaque at night. Simple pigment cells are only going to help with the latter, but even the crappy hormone-regulated type of chromatophore might provide a workable all-around solution. (How did that kind evolve, by the way? :twisted: )

Any thoughts on what the precursor to the chromatophore organ might have been? Anyone?

Phil said:
The upshot of all this is that it seems to me key lies with the obscure bactritids, which was a linking group between the nautiloids, coleoids and the ammonoids. They existed in the short window of time in the Devonian between the externally shelled nautiloids, with no chromatophores and poor eyesight, and the appearance of the internally shelled coleoids with chromatophores and good eyesight.

Perhaps I’m making too many assumptions here, and apologies this was a bit repetitive of my post above.
Far from being repetitive, this complements your post above by attacking the question of timing from the early end. We mustn’t forget about the bactritids. You make a pretty strong case, based on the available evidence (I’ll have some questions for you about the available evidence, later). It’s also good to be reminded where ammonites don’t fit in.

I’m inclined to believe that chromatophores would have evolved after some kind of reflecting cells. Anybody feel like tackling the question of leucophore/iridophore evolution?

:read:

[this post has been edited]
 

um...

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For Clem:

Clem, when you mentioned light-blocking, what light did you have in mind? Were you thinking along the lines of bioluminescence caused by prey, light produced by photophores (presumably used for counterillumination), or something else?


For all:

From Messenger (2001):

We should also make clear that cephalopod chromatophores are always assumed to lack a direct response to light (the so-called 'primary' response, common in many invertebrates, Weber, 1983). However, Packard & Brancato (1993) claim that, in Octopus vulgaris and O. macropus, light may act directly on the skin and this obviously merits further investigation.
I certainly agree. It would be quite nice to know whether or not such an effect existed, and what mechanisms might be involved in such a response. I'm attempting to track down the article referred to:

Packard, A. & Brancato, D. (1993). Some responses of Octopus chromatophores to light. Journal of Physiology, London 459, 429P.

I don't suppose anyone knows any more about this, eh?


I know a lot of people might have already seen this clip of Octopus vulgaris from CephBase, but I found it so amazing that I was compelled to post a link to it (I recommend the mpeg over the avi).

Gotta do some work now, will babble more later...
 

Clem

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um... said:
For Clem:

Clem, when you mentioned light-blocking, what light did you have in mind? Were you thinking along the lines of bioluminescence caused by prey, light produced by photophores (presumably used for countershading), or something else?
Um,

I was thinking mostly about luminescent prey items: animals that generate their own light (primary) and animals that consume bioluminescent organisms which might remain active light-producers for a short time after consumption (secondary). Bio-luminescence that persisted after maceration, radular grinding and transit through the esophagus into the GI tract would seem to require either an active (chromatophores on the gut) or passive (dark pigment on the mantle wall) counter-illumination strategy on the part of the cephalopod in order to maintain low visibility. Larval and juvenile cephs with transparent/translucent bodies that ingest small luminescent organisms would be at high risk, unless they restricted their pursuit of food to the daylight hours, and kept close to the surface. I might be totally wrong about this, but I'd expect that cephs with internal, light-blocking chromatophores would have a decisive evolutionary advantage, by being able to feed at all hours and with diet restricted only by availability of manageably sized prey.

On a tangent (sorry!), I noted a reference in Messenger ("Cephalopod Chromatophores: Neurobiology and Natural History," p. 5) to cephs whose external chromatophores retracted completely upon "exposure of cephalopod skin to ammonia fumes." Do the ammoniacal squids have to do something differently, then?

Um, thanks for posting the PDF link for the Messenger text. Great stuff, and I've lots to learn.

:bugout:

Clem
 

um...

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Clem said:
I was thinking mostly about luminescent prey items
I'm glad my assumption was correct. I was going to bring up the photophore issue, but then decided that it might be better to leave those worms in the can for now.

Clem said:
On a tangent (sorry!), I noted a reference in Messenger ("Cephalopod Chromatophores: Neurobiology and Natural History," p. 5) to cephs whose external chromatophores retracted completely upon "exposure of cephalopod skin to ammonia fumes." Do the ammoniacal squids have to do something differently, then?
I'd like someone to explain to me exactly how ammonia acts to produce that effect. I am almost completely ignorant concerning matters of physiology.

Ammonia is generally quite toxic, is it not? It reacts with water in a rather exothermic reaction which produces highly alkaline ammonium hydroxide (burns, burns, burns). I was under the impression that ammoniacal squid actually use an ammonium chloride solution for buoyancy, and that the ammonium (NH4+) cation is much less toxic than ammonia (NH3). However, there is a very good chance that I don't know what the **** I'm talking about. I would appreciate some enlightenment on this issue.
 

Clem

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um... said:
I was under the impression that ammoniacal squid actually use an ammonium chloride solution for buoyancy, and that the ammonium (NH4+) cation is much less toxic than ammonia (NH3). However, there is a very good chance that I don't know what the **** I'm talking about. I would appreciate some enlightenment on this issue.
Um,

Trust me, I don't know what the **** I'm talking about. Can you say "rank amateur?" It's spelled C-L-E-M. (I need to finish reading the Messenger text.) This is a good place to ask clumsy questions, though.

As for the photophore can of worms...I'm not touching that with a 10-foot flying gaff.

:roll:

Clem
 

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