So Sci was scrounging around the internet for blog topics recently. Every time she does this, she is of course completely overwhelmed by the piles of cool and bloggable science out there (seriously, I’ve got like 20 topics for the next week, of course not all will make it). But she’s also surprised to see some familiar faces. A lot of times it’s a familiar face related to her field and the stuff she’s been recently interested in (for example, Yavin Shaham just wrote an interesting editorial on binge eating and food addiction in Nature Neuroscience which Sci might have to blog), but everyone once in a while it’s something like this:
OMG! Sci knows that guy!!
So of course Sci had to go and ask if I could blog it. And of course the gracious Zen said yes!
Puri and Faulkes. “Do Decapod Crustaceans Have Nociceptors for Extreme pH?” PLoS ONE, 2010.
And first of all, I would like to introduce you to today’s three species:
Louisiana red swamp crayfish (Procambarus clarkii)
(Handsome little devil, no?)
White shrimp (Litopenaeus setiferus)
(OM NOM NOM)
And the grass shrimp (Palaemonetes sp.)
(By far the coolest because they are see through).
And now, let’s talk about nocioception.
Nociception is, technically speaking, the detection of stimuli that are noxious. Colloquially speaking, we like to call it pain, but that’s really not being specific enough. What it actually is is the ability to detect a stimulus which might bring harm to the creature involved. This can be anything from something mechanical (excess bending or tearing), to something temperature related (like excess heat or cold), to something chemical (like high acidity).
Of course I’m sure everyone can think of all sorts of good reasons for studying pain. But why study pain in invertebrates? Because studying nociception in invertebrates often translates really well to the human condition. Finding receptors responsible for nociception can sometimes lead to discoveries in humans for similar receptors, and so can change the way that we study pain in humans.
And why decapods? Well, obviously, because they’re cute!
(Can you say no to that face?!)
First off, you all should know that decapod means “ten footed”, meaning that these little dudes have 5 pairs of legs. Sci is drawn to these little guys, in part because she studied one of their relatives, the amphipods (not closely related and a lot more legs) in her undergrad days. And also because they are delicious.
But why decapods in particular, you might ask? Because nociceptors, the receptors that process these noxious stimuli, have been found in mammals, worms, and insects, but not a lot has been done in decapods. One study found what could be nociceptors in prawns, but the results were kind of ambiguous. Not only that, nociceptors often are similar in broad groups of species but different in specific species (like the naked mole rat). And so to determine whether decapods in general have nociceptors, Zen and his grad student decided to look at the three species shown above. They first looked at the behavior of the animals in response to known noxious stimuli, really basic and really acidic substances, and then they went in with electrophysiology to see what the neurons were doing.
So how, you might ask,does one study the behavior of decapod crustaceans and their response to noxious stimuli?
Check the antenna.
If a crustacean is exposed to noxious stimuli (like acids or bases) on their antenna and it is capable of detecting them, presumably it’d want them OFF, right? So a good measure of whether or not the animal behaves as though it can sense noxious stimuli is whether or not is grooms its antenna when noxious stimuli are dropped on it. You basically grab your crustacean, take it out of the water (I guess you’d probably dry it off a little), and use a cotton tipped swab to swab some acid, base, or control on one of the antenna, with the other antenna serving as a control. Then, watch for grooming. You can see the results here:
You can see that the three species, when exposed to acid (on the left) and base (on the right), they didn’t show any real difference in the amount of antenna grooming. So it looks like, behaviorally, the crustaceans can’t sense the acid or base. They even tried a local anesthetic, benzocaine, which apparently produced grooming in another paper, and didn’t get a response.
They then checked out the neural activity. And this part is the part Sci thinks is really cool. You take a crawdad. You put it on ice to anesthetize it (lobsters on ice, anyone?), and cut off one of their antenna. You put that antenna in a dish, and you can then DISSECT out the nerve and record from it!!! Sci wishes she could do this…in a brain electrophysiology is considerably more difficult and annoying. But anyway, you dissect out the nerve, and bathe it in solutions, either innocuous or noxious, and see what happens to your recording.
And they found what they expected to find here, a good basal level of cell firing. But like the behavioral responses, it appeared to be unaffected by acids, bases, or benzocaine. Bummer.
This paper actually goes AGAINST a previous paper which indicated that decapod crustaceans have nociception. So who’s right? Sci doesn’t know, but give the variety of species tested here and the variety of tests, she’s putting her money on Zen.
Now, you’re going to say, does this mean decapods can’t feel PAIN?! No. Not at all. It just means that they don’t response to noxious stimuli in the same way that other species do. They could still have nociceptors that work in a different way. After all, how often is a crustacean going to run into a solution that is strongly acidic in its life? In many species changes in pH hit the same nociceptors that things like heat and mechanical stress do, but it’s possible that decapods could be an exception. They could have nociceptors that respond to bending or pressure or heat or cold, and not to acids and bases.
And here’s where Sci is going to go off on a tangent. Yes. This paper was a big pile o’ negative data. You know what? That’s GREAT. We NEED this data. Sci personally feels that negative data is not published half as often as it should be. This is for two reasons: (1) it’s really hard to publish negative data because it’s often seen as boring, and (2) it’s often not what you expected. As far as point (1), that’s an issue with journals and reviewers. Negative data often doesn’t have the “impact” of a huge wild positive result to a hypothesis. And for (2), well, scientists are human, and we get very attached to our hypotheses. When something just…constantly fails to work, people are often more likely to blame the person working on it than admit their hypothesis is wrong.
BUT, that doesn’t mean this data isn’t important. On the contrary, it is JUST as important as the possible positive result would have been. And we produce a LOT of it. Negative findings are still data, and still say something important about the world. There is a possibility that decapods do not sense “pain” or noxious stimuli the way other species do. This could have a lot of implications for nociception and how we study it, for decapods as a group of species, and in other species of animals. And who knows, this paper full of negative data could one day turn an entire field on its head. You never know.
So Sci just wants to take a moment to stand up for negative data. Don’t be afraid to publish it. It deserves to see the light of day. No matter what, you’ve found something important about the world (or a tiny part of the world, studying an effect that is true under certain specific conditions. It’s still the world, dangit). Be proud of your negative data! And Sci is proud of Zen and Sakshi, for a good job, with some pretty data, and for showing something interesting, even if it’s something the world didn’t expect.
Sakshi Puri, Zen Faulkes (2010). Do Decapod Crustaceans Have Nociceptors for Extreme pH? PLoS ONE, 5 (4) : 10.1371/journal.pone.0010244