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?
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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?
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