CyberSlug software in action. In this image, the slug faces a nutritious, helpless prey with a distinctive odor signature. The slug recognizes the odor and, having interacted with this species of prey before, associates it with a memory of high nutritional benefits and few natural defenses. It evaluates its expectations of pleasure and pain against its current hunger levels, determines that the risk is worth the reward, and decides to attack. If the actual experience of the interaction does not match the slug's expectations, its neural network will adjust its associations with the prey's odor signature so that the slug will be able to make a more informed decision in future encounters. Click image to enlarge.

What is Pleurobranchaea, and why study it?

Pleurobranchaea californica is a predatory sea slug that lives on the Pacific ocean floor. Its environment is filled with natural enemies, and most of its food sources come equipped with defensive mechanisms. In order to survive, Pleurobranchaea must evaluate when to attempt to eat something and when to run away. Sometimes the nutritional value of a particular prey is high enough to make dealing with the prey's defenses worthwhile. Other times, the slug might opt to avoid a defenseless morsel of food altogether to avoid the risk of exposing itself to predation by creatures higher up in the food chain. A slug that's starving is generally willing to expose itself to greater risks than one that has just finished a large meal, and a slug that has encountered a particular species of prey before is able to recognize that prey and possibly alter its course of action based on its memory of previous interactions.

The machinery of Pleurobranchaea's decision-making centers is fairly straightforward when compared to many other organisms. The neurons involved in the sea slug's cost-benefit analysis circuitry are relatively few in number, not protected by myelin sheaths, and tend to be fairly large and easy to access and manipulate. Furthermore, the slug's primary sensory modality is chemoreception, so many complex issues such as somatotopic mapping and multimodal sensory integration do not arise when studying the slug. Despite the relative physiological simplicity of the organism, the slug nonetheless shows intriguing behavior such as learning from experience or modulating its actions based on internal state. These slugs are also fairly easy to acquire and to keep alive. These factors make them worthwhile subjects for behavioral experimentation, physiological analysis, and computational simulation.

Pleurobranchaea californica. Viewed from the front. The antenna-like structures are the rhinophores, detectors of odorants and currents. The frond-like flap below them is the oral veil, which has a major role in odorant detection and covers its mouth. Underneath the oral veil is the through which the proboscis, containing the buccal mass, extends during feeding.

Behavioral findings

Under normal hunger conditions, when the slug is neither satiated nor ravenous, the slug will run away from painful stimuli and will attack stimuli that smell nutritious. However, a slug that's very hungry needs to focus on avoiding starving to death, so it's likely to attack stimuli even if it expects them to be painful. Likewise, a slug that's satiated from a recent large meal will actively flee from food sources. We suspect that this latter behavior is an adaptation to keep the slug from exposing itself to attack by larger predators, or sometimes even other larger sea slugs.

Observation of the slug has also shown that the Pleurobranchaea is also capable of modifying its behavior based on past experience. In particular, the slug is able to exhibit single-trial learning in its interactions with the Spanish shawl, the Flabellina iodinia. The Flabellina's characteristic odor signature contains many components that the slug normally finds appetitive, but the shawl's body is covered with natural defenses. Upon biting the Flabellina, a slug will immediately spit it out. From then on, except in the event of starvation, the Pleurobranchaea will always flee upon exposure to the Flabellina's odor. We believe, therefore, that the Pleurobranchaea is more than a simple reflex machine. The slug must have mechanisms to change its synaptic weights to be able to encode memories and to use those memories to alter its behavioral patterns.

• Open the throttle. Go to the the upper right corner, under "World Info". Click "Continuous" and move the horizontal slider bar to set your desired simulation speed. Setting it to just a bit below "Fast" works well for most people.

• Give the slug an appetite. Move down to the "Slug Info" section and click "Hunger over time". This makes the slug get hungrier as time passes.

• Prepare some dinner. To create a new prey from scratch, click on an empty square somewhere on the world grid on the left half of the program window. It should become surrounded by the blue selection box, indicating that it's the currently selected square. This square will be the location of the new morsel. Back on the right side of the window, find the "Prey Info" section and press the button labelled "Place". An icon of the new prey will appear in the selected square. The new prey will be surrounded by a blue circle, indicating that it's the currently selected prey.

  Tips: You can also control the action in the World View with your keyboard. You can move the selection box with your arrow keys, and you can place new prey by pressing "P". When it comes time to make many copies of your prey, you can press "C" to place a copy of the currently selected prey into the selected square (provided it's empty).

• Name your food. Now that you've created the prey, you need to add some basic information about it. In the "Prey Info" section there's a text entry box labelled "Name", with the contents "Nameless Prey". Erase it and type in a name for your prey, for instance, "Squid" or "Shrimp". The icon of the prey won't change, but the name under it will.

• Servings per container. At the bottom of the "Prey Info" section are two slider bars. One, with a green smiley face near it, is labeled "Nutritional Value". The further to the right this bar is set, the greater your selected prey's nutritional benefits will be to the slug. Set this slider bar to a desired value for your prey. The other slider bar, with a red frowning face, represents the prey's defensive capabilities, which you should set to your desired level as well.

  Tips: Remember, if the prey is extremely nutritious, it will be so filling that the slug will be full after a mere one or two bites, and will never be able to finish a meal. Also, though natural defenses are a deterrent to the slug, remember that an extremely hungry slug will eat almost anything, no matter how badly it expects to be stung.

• Smells like slug spirit. The "World Info" section contains a list of odors that the prey can exude and the slug can recognize. This list starts out empty. Create a new odor by typing the odor's name into the text entry box and pressing "New". Your new odor will appear in the list below the text entry box, and will be selected as the current odor. The odors in this section are intended to model amino acids (for example, betaine or taurine) that the slug's chemoreceptors can detect, but for purposes of demonstration they can represent any higher-level odor descriptors of a prey. Squid, for example, might smell slightly bitter and very salty. To give the slug receptors for these properties, type "Bitter" into the text entry field and press "New", then type "Salty" and press "New" again. Both of your new smells will be shown in the list, and "Salty" will be selected as the current smell.

• Sign on the dotted line. Give your prey an odor signature by using the slider bar in the "Prey Info" section labelled "Intensity of current odor". Select an odor from the list in the "World Info" view to be your current odor, and move the "Intensity" slider bar to represent how strongly your prey smells of that odor. Do this for each odor your prey exudes.

• Send in the clones. Make sure that your prey is currently selected (it should have a blue circle around it), and begin copying it. You can do this by selecting an empty square somewhere on the grid and then clicking "Copy" in the "Prey Info" section, or just by pressing "C". An exact copy of your prey will appear in the selected square, and this copy will now be your selected prey.

  Tips: You can make as many copies as you like, but keep them spaced far enough apart to give the slug room to move around. Remember that these copies might be named the same, but they're not linked to one another in any way, so modifying the odor signature or nutritional properties of one copy will have no effect on any of the other copies.

• Lights, Camera, Action! Start the action by going back up to the "World Info" section and clicking "Step".

The Interface: What Am I Looking At?

When you start CyberSlug, you'll see the slug's simulated world, called the World View, on the left half of the window, and the info panel, or Info View, on the right. The slug will be facing East, or to the right. The simulation begins with no prey and no recognizable odors.

World View The World View on the left shows your slug, its prey, and your current selections. The direction in which the slug is facing can be determined by noticing the orientation of the slug's oral veil. Graphically, the oral veil can be seen as a flat protrusion from the slug's body, and it keeps the slug icon from looking perfectly circular. The purple icons around the screen are the slug's prey. Even though the prey graphically all look the same (the icon is actually from a photograph of the Flabellina iodinia, or Spanish shawl), they are not all necessarily the same species. The name of each prey appears below its icon. The prey wiggles as time passes in the simulation. Your currently selected square is shown with a light blue border, or selection box. You can move the selection box with the arrow keys or by clicking on the grid with your mouse. When you create or select prey or move the slug around, this box will let you control the location of your action. If you have a currently selected prey, it will be surrounded by a light blue circle, or selection circle. There are no prey on the screen when you first start the program, but when you create them, you can select them by clicking on them, or by pressing the Space bar when your selection box is over your desired prey. When you modify the name, odors, nutritional value, or other properties of a prey, the selection circle lets you control which prey to modify.