Posts Tagged Avoidance Learning
Exactly how happy are clams? Not very, according to Mark Miller of the Huffington Post (http://www.huffingtonpost.com/mark-c-miller/happy-as-a-clam_b_901054.html), who cites Dr. Patra Gupta of the Kerala Institute of Undersea Study. Dr. Gupta states that “the clams’ liquid secretions are identical in DNA structure to human tears. Clams also have less mobility than almost any other living creature, one of the sure signs of depression. They don’t fight back, don’t react to pain, take no interest in their appearance, don’t play or communicate. I’ve seen suicidal individuals with more zest for life, coma patients with a greater level of activity. These clams have less than zero interest in living; we might as well eat them.”
Gupta’s team attempted to generate some degree of happiness or life in the clams, introducing them to the far peppier shrimp, scallops, crab, lobster, even angel fish. But nothing. “Those clams couldn’t have cared less; they scarcely peered out of their shells. It was quite rude, actually. We’re getting in touch with a fish therapist to see if counselling might help, but quite honestly, I’m not holding out a lot of hope for it. I think we’re just going to have to face the fact that clams as a species are severely depressed.”
Maybe they were depressed because they were examined at low tide because the complete expression is, “happy as a clam at high water” (http://www.phrases.org.uk/meanings/as-happy-as-a-clam.html). High tide is when clams are much less likely to be predated, a good reason for happiness. Apparently the phrase originated from the US in the early nineteenth century.
The idea of happiness implies some kind of functioning nervous system, which makes me wonder, not about the clam’s state of mind but, carrying on from the previous post, can clams feel pain? Lobsters and crabs do have what can be termed a brain and, despite what our lobster eating friends would have us believe, do appear to feel pain and don’t appreciate being boiled. In a 2008 study a noxious stimulus was applied to the antenna of prawns. The prawns immediately began grooming the treated antenna and rubbed it against the side of the tank. This activity did not occur if the prawns were treated with benzocaine, a local anaesthetic (Barr et al 2008). In a much earlier study lobsters tossed into boiling water took up to seven minutes to die, all the time writhing, thrashing and convulsing (Baker 1975) (http://www.shellfishnetwork.org.uk/facts/fact4.htm).
While this appears extremely disturbing it is important to determine if these movements are the result of an organism in pain or merely a reflex to a noxious stimulus without a conscious perception of pain. While this may seem counter intuitive the brain of so called lower life forms is not as all important as it is for higher life forms. Many tasks are delegated to the spinal cord or other clusters of neurones. This can even be seen in higher vertebrates. Many years ago, while necropsying a freshly dead horse, I cut through a nerve in the groin region and promptly received an impressive kick for my trouble. The horse was well and truly dead, could feel no pain and yet reacted to the stimulus. A similar event occurred during a lizard necropsy. The lizard had a severed spine and, at the conclusion of the procedure, I was left with the lizard’s tail, back legs and pelvis. When I pinched a toe the leg and tail both wriggled, even though they were no longer attached to the top half of the body. (As a digression freshly dead reptiles often still have beating hearts when they are opened up. They are, however, definitely dead. If the heart is removed from the body it will continue to beat, lying on the table by itself, for up to an hour. It certainly creeps the students out and can make it somewhat tricky to confirm death in a reptile.)
It can be difficult to differentiate reflex from pain response in a live animal of limited reactions. I feel that, if the animal responds to pain killers by not reacting to the noxious stimulus, like the prawns and the fish in the previous post, then it is probably experiencing pain. The implication here is that the animal has pain receptors that can be chemically blocked. Pain and opioid receptors have been identified in snails, nematodes, crustaceans and insects (http://en.wikipedia.org/wiki/Pain_in_invertebrates).
But the clincher, for me at least, is the phenomenon of avoidance learning. To demonstrate this effect a light was shone on a crayfish. Ten seconds later the crayfish received an electrical shock. The crayfish learned that the light was associated with the shock and rapidly moved away when the light came on, thus avoiding the shock. A similar phenomenon was reported for Drosophila flies. In this case the electrical shock was paired with an odour. The flies quickly learned to fly away from the odour whenever they detected it (Elwood 2011). This cannot just be reflex. These animals are displaying a learned response and acting peremptorily to avoid a painful stimulus.
Which brings us back to clams. As far as I can tell clams do not have a brain as such but do have clusters of nerve cells called ganglia, which allow them to respond to certain stimuli. Pain receptors have been identified in their snail relatives and presumably exist in clams. Given that the avoidance of pain is universally beneficial to all forms of animal life and most clams are capable of movement, at least some of the time, I am prepared to give them the benefit of the doubt and say that they can feel pain. I hope that, at least, makes them happy.
Dr. F. Bunny
Baker, J.R. 1975. Experiments on the humane killing of lobsters (Homarus vulgaris) and crabs (Cancer pagarus). Part 1. The killing of lobsters by gradual heating. Scientific papers of the Humane Education Centre 1: 1-10.
Barr, S., P.R. Laming, J.T.A. Dick and R.W. Elwood. 2008. Nociception or pain in a decapod crustacean? Animal Behaviour 75: 745-751.
Elwood, R.W. 2011. Pain and suffering in invertebrates? Institute for Laboratory Animal Research 52: 175-184.