In the past few months there has been some attention paid to pain in crustaceans.  Even here in the Grand Rapids region, the Press ran an opinion piece discussing the possibility of pain in lobsters. The presence of pain in an animal, or even another human, cannot be known for sure.  It is a subjective perception, yet  we can often infer pain via physical and behavioral reactions.  The current view is that all vertebrates can feel painful stimuli, and that certain invertebrates, like cephalopods, might as well.

But what we seem to understand of decapod crustacean pain is that crabs at least will avoid an electric shock and that they can learn to avoid places where they were shocked before.  But what we don’t know concerning crustacean pain is vast. For example, do decapods process an electrical shock in a similar way that vertebrates process noxious stimuli?  Moreover, do crustaceans even have nociceptors, which are specialized pain receptors for noxious stimuli, and if they do then what other types of noxious stimuli do they avoid.

In this most recent article, Magee and Elwood used electric shock as their choice of a noxious stimulus.  Electric shock is in fact used in many studies of pain, but it does have a big problem as my students from Human Physiology will recognize and that is it’s not very specific in terms of what it will activate.  As my students learn, many cells are excitable by electricity, because well that’s what the nervous systems uses to function.  Receptor and action potentials are nothing more than changes in membrane potential cause by the movement of ions, i.e. electrical charge.  Thus, electrical shock will activate any cell that is electrically excitable that includes sensory neurons, motor neurons, and muscles.  It is difficult to know just what a researcher is doing to the animal’s nervous system when using electric shock because the cells potentially affected are numerous and need not have anything to do with pain.

Even if one thinks that electric shock is painful to a crab or lobster, that does not automatically imply that decapod crustaceans experience pain in any sense of the word similar to that of vertebrates.  For example, it is not uncommon for the crayfish in our lab to get injured during fights, even to the point of tearing off claws or legs.  Yet their behavior seems relatively unchanged when such injuries occur, they will continue to fight, eat or even mate in spite of the amputation.  Losing a limb is likely going to stop such behaviors in vertebrates for an extended period of time.  This is a reminder that what might seem painful to us is not necessarily painful to a different animal.