I'm currently in sunny southern California for some experiments at UCSD. This is the place where one can find the marine snail Aplysia in its natural habitat. As I've been working with Aplysia for about ten years now, I felt it was about time to see Aplysia in the wild and observe what these animals do when they're not in a tank waiting to be experimented upon. Just this morning, before heading out to UCSD, I went snorkeling in La Jolla Cove in the hope of seeing some specimens. All these years working on Aplysia and I had never seen one in the wild. What do they do? Why can they learn? Do they only learn about how to handle their food? I always imagined these snails as sort of the cows of the sea, living happily on their food and, being hermaphrodites, not really having any trouble finding mates, either. Any other forms of learning are probably purely coincidental of neurons in general being plastic. Unfortunately, I didn't find any Aplysia at all today. However, when I checked the table of contents of thelatest issue of the Journal of Neuroscience later in the evening, I found this great paper that was so long overdue: "Lobster Attack Induces Sensitization in the Sea Hare, Aplysia californica".

Sensitization leads to a form of non-associative memory that leads to the animals to respond more strongly to a weak stimulus after having experienced a very strong, often noxious stimulus, compared to animals that did not experience the strong stimulus. Sensitization is also one of the most intensively studied and best understood forms of learning. Finally, after roughly 40 years of research on sensitization in the marine snail Aplysia using electric shock in the laboratory, the new paper by Watkins et al. provides evidence that Aplysia sensitizes to natural, ecologically relevant stimuli: lobster attacks.

Sensitization of sensori-motor synapses in Aplysia is one of the first and most basic models for the cellular and molecular basis of memory. The Nobel Prize for physiology or medicine in 2000 was awarded, in part, for work using this preparation. For more than four decades, laboratory work has unraveled the neurobiological processes underlying this simple form of memory. These studies were done either in intact animals using electric shock as sensitizing stimulus, in reduced preparations with either shock or the biogenic amine serotonin leading to sensitization (serotonin is thought to be released in response to electric shock), in isolated ganglia or even in cell culture with individual sensory and motor neurons. For all this time, the researchers studying these processes have had to speculate what the ecological relevance of the processes that they were studying was, if they had any. The paper by Watkins et al. provides first evidence suggesting that sensitization may be an adaptive response to attacks by lobsters.

The authors put individual Aplysia in the same tanks as starved spiny lobsters and compared the head withdrawal and the siphon and mantle withdrawal reflexes of animals who were attacked with the reflexes of those who weren't. They found that 30 and 60 minutes after the attack, the duration of the withdrawals was increased in both reflexes in the animals that were attacked compared to the non-attacked animals, constituting compelling evidence for sensitization. Granted, Watkins et al. used tank experiments with starved lobsters attacking the slugs. However, both species are sympatric and the authors cite observations that these attacks do occur in the wild. Also granted, the fitness effect of sensitization on future predator attacks is rather speculative, but at least there is a testable prediction now: sensitized animals should be a tougher prey than naive animals.

Be that as it may, at the very least, as of today, neurobiologists finally have a plausible story to tell when they are asked, as I have often been asked and been asking myself, why Aplysia needs to learn anything.

Small aside: It is amusing to note that the authors report the carapace length of the lobsters they used to have been between 80-90cm. Animals with an overall body length of that dimension would be regarded trophy size. Lobsters with a carapace length of 80-90cm would probably have a body size of around 180cm, a rather average height for a human male and a lobster size worthy of its own publication. Now I'd love to have the tail of a 6 foot lobster for dinner!

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Watkins, A., Goldstein, D., Lee, L., Pepino, C., Tillett, S., Ross, F., Wilder, E., Zachary, V., & Wright, W. (2010). Lobster Attack Induces Sensitization in the Sea Hare, Aplysia californica Journal of Neuroscience, 30 (33), 11028-11031 DOI: 10.1523/JNEUROSCI.1317-10.2010