National Geographic reports the discovery a lovely new freshwater crab, Insulamon palawanense, which sports a purple carapace and red claws. It was found on the Philippine island of Palawan, and is small (1-2 inches across).
(All photos by Hendrik Freitag.)
The crab’s brilliant hues may simply help the species recognize its brethren, said study author Hendrik Freitag, of the Senckenberg Museum of Zoology in Dresden, Germany.
“The particular violet coloration might just have evolved by chance, and must not necessarily have a very specific function or reason aside from being a general visual signal for recognition,” said Freitag, whose study was published in February in the Raffles Bulletin of Zoology.
I’m a bit dubious about bright colors as “species recognition” signals. That might be the correct explanation, but doesn’t explain why other species are more drab. Perhaps individuals of this species need brighter colors because there are fewer individuals and need more obvious signals to find each other. But there could be other explanations as well, including mutual sexual selection or aposematism (color warning of toxicity, though that seems unlikely). But Freitag saying that “it could have evolved by chance” (implying genetic drift) doesn’t comport with the selective explanation—individual recognition of conspecifics—that he offers immediately thereafter.
Large Insulamon males—such as this I. johannchristiani, another of the newfound species—sport a reddish color, possibly to signal their power, Freitag said.
Smaller, less dominant Insulamon males and females are purple, he noted.
Beautiful! Although I have to admit,sadly, that my first thought on seeing the photo was,’I’ll bet that crab is in RGB and not
CMYK’.
Could it be that the report is wrong in putting the Senckenberg Museum in Dresden? Last I know, it was in Frankfurt am Main.
Perhaps the researcher works for the Senckenberg research society, and out of Dresden?
Never mind, I checked the link. I was only familiar with my hometown branch.
An alternative explanation:
Pacific Proving Grounds
;>
regardless why his colours evolved, he’s rather gorgeous
After you touched on the asymmetry of lobster claws in the narwhal post (shredder vs. crusher claws), I wondered was whether crabs, esp Callinectes sapidus (Maryland Blue Crab) have asymmetrical claws too. Can’t tell from these pix if any of these guys do.
“That might be the correct explanation, but doesn’t explain why other species are more drab”
This assumes that ‘bright’ or ‘drab’ to our visual system corresponds to ‘bright’ or ‘drab’ to theirs. Perhaps some of those ‘drab’ colors appear very conspicuous to others of that species.
That highlights a question I had. From what I can tell with a quick search, at least some sea arthropods have tetrachromatic vision (UV and RGB). That would easily allow a drab crab to have patterns we can’t see (such as with many flowers, which look much different to their insect pollinators).
Beyond that, the “smaller and less dominant” bit makes me wonder if the purple males are passing as females to mate on the sly, which would mean the coloration is at least influenced by sexual selection.
This is something I never understand:
How can we say we know “why” something evolved the way it did? All we know for sure is that such-and-such a trait persisted and spread to successive generations, presumably because it conferred a reproductive advantage. Can we state anything more than that definitively?
Maybe these crabs’ coloration assists them in some way, but can we know whether that particular advantage is the reason possessors of that trait fared better? Isn’t it a matter simply of more or less plausible scenarios, with no answer key lurking out there somewhere for us to check, as soon as we find it? You could test certain ideas: is this trait more efficient at something or other than another trait? But wouldn’t that just tell you the idea was plausible (or more plausible than any other proposed ideas)?
By the way, this isn’t me being a jerk. It’s just me not understanding.
I think the problem here is that this is something from a Natl Geographic website and not a peer-reviewed paper. “Why” is just a shorthand we are all probably guilty of lapsing into, when we mean that “this trait appears to have conferred this or that advantage,” or, in the case of these purple crabs, “what advantage this trait may have conferred remains obscure.”
Also, does anyone wonder why blue crabs are blue? Let me venture that blue and purple are just hard to see underwater. It’s just that we aren’t used to seeing a purple crab, and so then people want to ask why, assuming there must be a special reason.
And as far as the big ones being red, are they necessarily still reproductively capable? If they aren’t, there’s no advantage to them remaining hard to see, and so the old ones are more likely to be eaten, increasing the odds that the young ones get a chance to reproduce.
You’re not wrong. Evolution, like history, is just just one damn thing after another. The goal of studying it is to discern patterns that can be repeated. Convergent evolution, as an example, shows that such patterns definitely exist.
Once you hypothesize a particular pattern (a reason for a given trait’s existence), the trick is to devise tests of said hypothesis that are specific enough to increase your confidence that it’s real – not just plausible, but probable, or even virtually certain.
What you’re seeing about the crabs coloration in the paper cited here is nothing more than guesswork, and not very good at that (in my opinion). It’s not presented as anything more, though, so there’s nothing wrong with such speculation. It’s not difficult, however, to imagine experiments that would narrow the range of possibilities down. The obvious first one (to my eye) would be to create a controlled environment, paint some crabs, and see how that affects mating success. With some adjustments, the same general experiment could test both species visibility and mate choice as factors for the color.
One possibility that far too few biologists seem to consider is that it could easily be both (assuming it’s not something else entirely).
Sure, a lot of it is speculation – that quote from the article, for example. However, sometimes you can do experiments or look at other lines of evidence to try and confirm or exclude a particular hypothesis. Sadly, the regular media is not good at differentiating the speculation from the more certain statements.
Pretty! He should be used as inspiration for Project Runway designs
I assume he meant that the particular colour (purple as opposed to blue or green) was by chance or drift. Then it was enhanced and brightened with species recognition as the selective pressure. So the two statements are not contradictory, it is just not well written.
Doesn’t the color differences based on size in males directly point to sexual selection?
Yes and no. Yes, but only for the large males’ colouration, the smaller ones are the bright purple ones while the large males are a more traditional crab hue, so it doesn’t really explain that. Females and non-breeding males are usually drab so they can hide from predators, rather than bright purple with orange tipped claws. Interesting.
? So this is my line of reasoning… For some reason (who knows?) an early female of gets it in her head that the kind-of-a-bit-more purple and red big male is more attractive than the others and mates. In selecting for the redder clawed guy genes for more intense purple for all of the offspring are passed on. The females in the next generation have a preference for more of the big-red stuff… over time they all get purpler as a result of selection for brighter red claws on the big males.
(I’m assuming heritability of the preference for bright red claws and the rest of the package comes along as an incidental side affect.)
must not necessarily have a very specific function or reason aside from being a general visual signal for recognition,
When I read this I thought of sexual selection. I have difficulty in seeing what other mechanism might apply.
I do not see why signal selection should not apply here. However it is still a part of Natural Selection.
Or it could be an effect of male hormones (do crabs have testosterone? #notmyfield) on the pigmentation pathway, and be completely incidental. Genetics is tricky like that.
Couldn’t they have come up with a more imaginative epithet than palawanense? Something equivalent to ‘fiery pinchers’, or the like?
It was named after the name of the island where the new crab was found. The island is called Palawan. Beautiful island, you might want to visit someday.
Yes, I know all that. But as there are other species of the same genus from this island the name doesn’t convey much. That was my point.
Don’t get crabby!
Yes, I know all that, too. As there are other crabs which maybe “fiery pinchers”, such name doesn’t convey much. That was my point. LOL. Relax.
I don’t see how the colours could be species recognition signals. In that case surely the male and females would be brightly coloured and the young ones drab for safety, instead of which the breeding males are less violently coloured than the others. Well that’s how it looks from the photos, things may seem different on-site at the beach. I suppose it’s possible the purple is less obvious than the red in real life but if that’s true then surely more crabs in the world would be purple? Maybe there’s a species of purple seaweed that washes up on the beach making purple a good colour for camouflage. I’ll volunteer to go and have a look if you like. 😉
These are freshwater crabs that don’t live near the sea.
That’s an extroadinary colour. How many species can even “see” that?
The full article is here –
http://rmbr.nus.edu.sg/rbz/biblio/60/60rbz089-100.pdf
but no time to read it now!