Predatory Sea Slug Learns What Not To Eat Through Trial And Error
April Flowers for redOrbit.com — Your Universe Online
A predatory sea slug that normally isn´t very picky about what it consumes has more complex cognitive abilities than scientists previously thought, according to a new study from the University of Illinois. These cognitive abilities allow Pleurobranchaea californica to learn the warning cues of dangerous prey and avoid them in the future.
Found off the west coast of the US, Pleurobranchaea is a deep-water species of sea slug with a relatively simple neural circuitry and related behaviors. UI professor of molecular and integrative physiology Rhanor Gillete says that Pleurobranchaea is a generalist feeder, meaning it “seems to try anything once.”
Flabellina iodine, called the Spanish shawl because of the orange outgrowths called cerata that covers its purple back, lives off the west coast of the US as well. The Spanish shawl, unlike Pleurobranchaea, is a picky eater. It eats only one type of food, an animal called Eudendrium ramosum. The Spanish shawl consumes all of the Eudendrium´s body except for its embryonic, developing stinging cells — which are transported to the shawl´s cerata where they mature. The Spanish shawl co-opts its victim´s bodily defenses to protect itself.
The Pleurobranchea-Flabellina study, published in The Study of Experimental Biology, began with a happy accident that involved showing a lab visitor how predatory Pleurobranchaea is.
“I had a Pleurobranchaea in a small aquarium that we were about to do a physiological experiment with, and my supplier from Monterey had just sent me these beautiful Spanish shawls,” Gillette said. “So I said to the visitor, ℠Would you like to see Pleurobranchaea eat another animal?´”
Gillette dropped the Spanish shawl into the tank, expecting Pleurobranchea to attack. In that, he was right — Pleurobranchaea smelled and bit the bright orange and purple visitor. Pleurobranchaea was stung by the Flabellina´s cerata, causing Pleurobranchaea to reject the Spanish shawl as food. The Spanish shawl did its typical “flamenco dance of escape,” and Pleurobranchaea also managed to escape with an avoidance turn.
Gillette was intrigued and put the Spanish shawl back into Pleurobranchaea´s tank. Pleurobranchaea immediately started an avoidance turn rather than attacking again.
“I had never seen that before! We began testing them and found that they were learning the odor of the Spanish shawl very specifically and selectively,” Gillette said.
Gillette´s team replicated the experiment by placing a Pleurobranchaea in a training arena approximately four to five inches from a Spanish shawl. They recorded the behavior of the Pleurobranchaea, repeating the experiment 24 and 72 hours later.
When Pleurobranchaea´s feeding threshold was too high (they were already full) they would not participate. When the feeding threshold was too long (they were extremely hungry), the Pleurobranchaea completely consumed the Spanish shawl. The Pleurobranchaea who were hungry, but not starving, exhibited the learned avoidance-turn behavior, even when placed with the Spanish shawl 72 hours later.
These experiments revealed that Pleurobranchaea was selective in its food choices, but only on a case-by-case basis. The slugs already trained to avoid the Spanish shawl were not that picky about a closely related species called Hermissenda crassicornis. This ability comes in handy in Pleurobranchaea´s natural environment.
“If you´re a generalist like Pleurobranchaea, it´s highly strategic and advantageous to learn what´s good and what´s not good so you can decide whether or not to take the risk or of attacking certain types of prey,” he said.
The “simple” Pleurobranchaea is much more complex than originally thought, the researchers say.
“We already knew the neuronal circuitry that mediates this kind of decision,” Gillette said. “Finding this highly selective type of learning enlarges our perspective of function, in terms of the animal´s ability to make cost-benefit decisions that place it on a rather higher plane of cognitive ability than previously thought for many sea slugs.”