Purty Molluscs

Evolution of chromatophores? Well, I’ve had a bit of a think about this and I think that these were a development of the earliest coleoids. I’ll tell why I think this is, but please feel free to shoot me down in flames if you wish! Fossil coleoids, excluding belemnoids, are very rare due, of course, to their soft bodied nature. There do not appear to be many researchers working on them at present and doubtless with one or two future discoveries the whole working system will have to be reworked. Anyway…….

I think the key to this lies with Vampyroteuthis. The Vampire squid is the only surviving member of the Vampyromorpha that we know of. The Vampyromorpha were an incredibly ancient lineage, recent cladistic analyses such as the one available here indicates that the Vampyromorphs were at the root of one of the two main branches of the coleoids, the other being the belemnoid families which split from a common ancestor probably in the early Carboniferous sometime around 350mya. Most of the fossil ‘squid’ that one sees depicted from deposits such as the later Jurassic Solnhofen were members of the Vampyromorphs.

It is believed that these Vampyromorphs and the lineage that led to the modern squid, and cuttlefish, split from a branch soon after this initial divide from the ancestral coleoid, with the modern Spirula representing a comparatively unmodified ancestral form. It is believed that this split into the modern squid and cuttlefish probably happened in the early Tertiary, following the massive Cretaceous marine extinctions. The octopus lineage is thought to have split sometime in the Jurassic from the Vampyromorpha, the earliest known example being Proteroctopus ribeti (160mya)

What has this to do with chromatophores? Well, according to cladistic analyses Vampyroteuthis, the modern squid, cuttlefish and octopus all appear to have had a common ancestor that lived in the early Carboniferous and all these groups have modern examples that demonstrate chromatophores. Unless it is conceivable that chromatophores evolved independently in all these groups, which seems unlikely, it seems at least a strong possibility that the common ancestor of all these groups would have possessed chromatophores at a date of around 350mya. This ancestor had only comparatively recently split from the initial branch in the coleoids in the late Devonian. Perhaps one could speculate that possibly the belemnoids would have possessed chromatophores too, if the common ancestor of both groups had possessed them. (Practically impossible to prove though!).

Vampyroteuthis does have chromatophores though they have weak musculature. To quote from the Tree of Life pages:

“These chromatophores, however, have lost the muscles that enable rapid color change in other coleoids and are probably incapable of changing shape. A few normal chromatophores associated with photophores are still present.”

This is probably an adaption to life in the abyss; who needs a colourful display in the dark? Early Vampyromorphs were certainly not all deep water animals. The recent discovery of an Upper Cretaceous animal from Japan that has been named Provampyroteuthis giganteus is believed to have swum in the surface waters and was much bigger than the modern Vampyroteuthis. Living in an off–estuary environment it is quite possible that this animal had developed chromatophores to a higher degree than its modern descendant. Fossils of this animal have been recovered from stomach contents of Elasmosaurid plesiosaurs; these were believed to be surface or shallow water swimmers. (I can print the reference for this if anyone wants to follow it up).

I’m sure that I have managed to make something comparatively simple much too complicated. This stuff is so much easier with a diagram and timeline, you know!

:vampyro: :bonk:
 
Bald Evil said:
I was wondering if there are any known cephalopods with chromatophores that emit light frequencies outside the visual spectrum.

While I'm not able to answer that question, I do know of a few species of fish (e.g. Pachystomias microdon) that use red light from photophores under their eyes as a kind of searchlight to locate prey. Many of the animals in the deep sea, as far as I know, cannot see red light since virtually none penetrates to any significant depth. Is this the sort of thing you had in mind?

Bald Evil said:
I know this isn't really about how chromatophores evolved, but the 'why' of evolution often answers the question of 'how'.

That's sort of how I look at it. The 'how' of it is a series of steps, and each step answers a 'why'. I hope this thread touches upon various aspects of ceph body patterning and photophore use, not just the evolution of chromatophores. I don't think that question can really be answered, but hopefully it'll lead to other interesting questions and observations (like yours).
 
Here you go!

Kanie, Y., Hasegawa, Yoshikazu, Okazaki, Y. and Tatematsu, Yoshiko; 1998:

Vampyromorphs: past and present; - Cretaceous vampyromorph (Coloidea: Cephalopoda) as the diet of a plesiosaur; Bulleton of Gunma Museum of Natural History (Number 2) pp: 11-23

If anyone tracks this down , please let me know! Providing it's not all in Japanese, of course. :biggrin2:
 
I have just discovered a wonderful article:

Messenger JB (2001) Cephalopod chromatophores: neurobiology and natural history. Bio Rev 76: 473-528

I have yet to actually read any of it, but it appears to be fairly comprehensive (for its length).

5 pages of references. Hooray! More bloodshot eyes!
 
I just realized that yesterday was my one-month tonmoversary. I'm going to use this as an excuse to have a little extra :beer:, and I invite everyone to do the same (provided you have achieved the relevant legal drinking age).

You know you want to.


(and remember: JD and Coke might be a good mix, but drinking and driving aren't!)
 
Steve O'Shea said:
um... said:
I just realized that yesterday was my one-month tonmoversary.

Happy Tonmoversary um...... It's been great to have you online
Cheers
O

Thank-you, sir. It's certainly great to be here, interacting with like-minded individuals for a change.

tonmo.com is my happy place.
 
Messenger (2001) seems inclined to think that "the chromatophores may have evolved primarily for concealment" from "sharp-toothed predators", which became necessary as the external shell was lost. How effective was the shell at keeping predators from eating the occupants? In Ammonites ( :thumbsup: :thumbsup: ), Monks and Palmer write:

Ultimately, cephalopods have had to trade-off increased mobility and swimming efficiency against the defensive properties of the shell. The shell can be thought of as being under a constant 'crushing attack' from the surrounding water. The thinness of the shell and the lack of structural support between each septum, mean the shell has little extra strength to resist the additional forces from a predator's bite.

The shell is obviously going to provide some measure of protection, but more sturdiness will eventually come at the expense of buoyancy. Looking at an ammonite shell, I get the impression that buoyancy is a pretty big deal.

Andrew Gray, in Shell loss in mollusc evolution, suggests that:

The internalisation of the shell provides coleoids with several advantages, most of which are related to the development of more efficient swimming. (The coleoids may originally have evolved in response to increasing competition from predatory fish, with which they are convergent in many ways.) Liberation from an external shell helps the coleoids to float horizontally in the water, and has allowed the development of fins for better locomotion, and a highly contractible mantle cavity that can squirt out water violently, moving the animal by jet propulsion.

Basically, I'm getting back to my Jean-inspired epiphany of several posts ago: Might the early development of body patterning have been driven by its use in communication at least as much as by its use in concealment?

If the shell is overrated as a defensive adaptation, and coleoids end up being more mobile (and presumably harder to catch) without large external shells, how badly would crypsis be needed? I'm also making the big assumption here that a chromatophore system in the very early stages of development would not be anywhere near as effective as the finished product, and might even be occasionally harmful.

Anyone care to set me straight?

:bonk:
 
Hmm…

OK, here’s a bit of a half-baked argument against what I just said.

Consider the case of a young early coleoid, possibly living a life something like a young cuttlefish’s (i.e., benthic). This guy is bite-sized and can’t do as much running or fighting as his larger conspecifics. He is in much greater need of camouflage, since hiding is going to be his shtick. Perhaps early chromatophore systems produced less structured patterns that were more along the lines of Disruptive, and were in fact sufficient to provide some degree of protection for this little guy when hiding over a mottled rocky bottom or partially burying himself into sand. Breaking up his outline might be a good start, even if he can't blend into the background as well as a modern octopus could.

That still leaves me with a million questions…

:confused: , and still :bonk:
 

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