Why do crabs lose their legs when they get to cold. Is it voluntary, or a failure?
My understanding (based on material read many years ago) is that it’s thought to be adaptive evolutionarily: When temperature drops, the limbs falling off will improve circulation in the body’s core. Limbs can get regrown, but if the body gets too cold to live, that’s it.
And I’m not sure the crab has a complex enough nervous system to call anything it does “voluntary”.
For the purposes of the question, let’s say it is ‘voluntary’. This, as opposed to a ‘structural failure’ or a ‘design flaw’. That is, if it’s an evolutionary adaptation, then it’s supposed to happen.
I assumed evolutionary adaptation, for the reason you said. But while watching Deadliest Catch a fisher showed what he called a ‘puck’ – a crab that had lost all of its appendages. I couldn’t see how that would be a survival advantage, as it would not be able to evade or fight predators nor eat. ISTM it would be eaten or starve before regenerating limbs.
Perhaps what Johnny is asking is “Is losing limbs in the cold something valuable enough to crabs that there’s a particular mechanism for making it happen (presumably genetically selected for in the past), or is it just an accident of crab physiology?”
Or, shorter but easier to misunderstand “Is losing limbs in the cold an advantage to crabs?”
[On preview, this does seem to be what he meant. ]
Probably a less certain fate than definitely freezing to death.
I suspect the clawlimbs would be the last to go. And perhaps the wacky crabber assisted in the removal of all the limbs before presenting it to the cameraman.
Speaking of adaptive behavior, I really enjoyed watching Sig the other week when he managed to quit smoking for a whole 3 1/2 hours. And that was by using nicotine patches…
Part of the problem is that you’re talking about crabs being brought out of the water into air temps down to (at one point they mentioned) -40F. Obviously this is not at all a natural temperature that the crabs would typically find themselves at. I suspect there doesn’t have to be an evolutionary mechanism at work, as the crabs would almost never have a reason to be out of the water during such temperature extremes.
Are you sure it wasn’t -40ºC? 
I always assumed it was a design failure, brought about by extreme, unusual conditions.
They are coming up through a significant pressure differential, with lots of dissolved gasses in their systems. They then flash freeze. (-40F is colder than any freezer I’ve been in, or heard of for commercial operation). The squishy membrane like stuff that makes up the leg joint freezes hard, and being flexible is probably exceptionally brittle when solid, resists as long as it can, then fails spectacularly due to forces of gasses coming from inside the crab, and the weight of the leg. Any motion by the crab would accelerate the failure mode.
That’s my theory anyway. Crabs are tolerant, for a bit, of conditions outside that normally found a couple hundred feet down in the Bering Sea, but begin dying once out of that element. -40F, in air, at 1ATM pressure, is just a bit much. How would you do out there naked? (Please post pics if you try this) Though, your limbs will probably stay attached…
I believe the crabs and the pots are so warm (relative to the air temp and surface temp) that steam can be seen coming from the crabs/pots when they are brought to the surface in those episodes where the claimed air temp was -40F.
So, you are talking about creatures specialized for the environs, and it is has not been key to their survival to be able to survive a shift in temps like that, so losing limbs is a consequence, but in the crabs’ day-to-day existence, this consequence doesn’t present itself often enough (if at all) to seriously reduce the population.
BTW: “Design failure” is language just begging for a drubbing.
On a tangeant: The crabs are dumped into a holding tank of water, presumadely so that they stay alive long enough to be considered “fresh” when they get offloaded back in port.
What’s the temp and pressures in the tank?
I assume the tank’s water pressure is not that of the floor of the Bering Sea. Does this not imply that the crabs are a tad more pressure insensitive than a nekkid human?
Crabs can certainly survive at a wide range of pressures… however, rapid transition from one pressure level to another can be pretty traumatic no matter what species you are. Human divers can get the bends due to much smaller transitions than the crabs are experiencing.
The temp in the tank is probably what they’re used to - really cold, but above freezing.
The smilie means you know the relationship between -40C and -40F, doesn’t it?
You have to use smileys because deadpan doesn’t work very well on a message board.
Temperature is probably somewhat similar to that of the ocean bottom water. Until it freezes, the water can’t get much colder than the low 30s F until it freezes (I know the freezing point is somewhat below 32F for seawater, but it’s a matter of a few degrees in this case).
My point is that they can handle the pressure difference for a while, and stay alive, when the temperatures are above freezing. When they crash as far below that, to -40F (or C), the crab freezes at the outside edges, and the pressure differential isn’t so tolerable.
They can only live in the tank for about 10 days, not sure why that is.
Tank water is simply pumped in from the available sea water.
One dead crab releases toxins that can kill up to 10 other crabs, and the chain continues… so holding them in tanks is risky business.
i’m no crab sympathizer, but it’s not like they lost their legs naturally… they are housed in pots with other crabs grabbing onto walls made of metal wire openings… they are hauled out of the water into freezing temperatures, shaken around, manually pushed/prodded/slid/lifted/dropped… in sub-zero temperatures, leg are going to be lost…
And then found, boiled and served with butter. MMmmmmnumnumnumnumnum!
I just don’t understand you PETA people.