If you were a predator trying to design the perfect food source, you’d probably want something that was slow moving, reasonably large, and had no annoying hard bits that you couldn’t really eat. In other words, it would probably looks something like an opisthobranch mollusk. Opisthobranch are also known as sea slugs or sea hares, and at first glance, they look like snack packs for predators.
Of course, opisthobranchs don’t take this lightly. Rather than physical defenses, they put up chemical ones. Besides any chemicals that they keep inside, many sea hares release chemicals, too. Sea hares have two chemicals they will squirt out in the presence of a predator: ink and opaline.
To test for how these two different chemicals affect predators, Nunsbaum and Derby looked at the inking behaviour of the California sea hare (Aplysia californica – a species well-known to neurobiologists for the many studies on memory that have been conducted on it; pictured above). They used blueheaded wrasse (Thalassoma bifasciatum; pictured right) as a predator.
The sea hare is a Pacific species (hence the species name “californica,” as in California) and the wrasse is an Atlantic species, so the authors obviously can’t mimic ecological interactions with 100% realism. The choice of species is almost certainly one of convenience, but you might expect that these chemicals and such are general enough that the exact species wouldn’t matter. That said, it does open up some possibilities to check whether there might be “arms races” where species in the same region have different inks or behavioural responses to animals they live with versus animals they don't.
To test the effect of the ink or opaline alone, without the actual sea hare in the tank to possibly confound things, Nunsbaum and Derby made “shrimp cubes” as a food item for the fish. They released them simultaneously with ink between the food and the fish. Ink made the fish less likely to get the food, but opaline didn’t, indicating that the ink is acting as a deterrent, partly visual.
Previous work had suggested that ink / opaline was a distraction to a predator, which would mistake it for food. They found that fish wouldn't eat alginate cubes, cubes with ink, cubes with opaline, but would eat pellets that had the amino acid components contained within ink and opaline.
What about when the ink and opaline are presented with actual food? Put any ink in it, and the fish wouldn’t eat it. Literally none. Zero. Put opaline alone in it, and the fish will eat it. All of them. 100%. This experiment (plotted right) is significant by IOT*. The more complex stats just seem like going through the motions...
When ink was around, fish took longer to get to food; this got shorter if the nose was blocked. This indicates that ink has some sort of olfactory blocking in addition to being a visual distraction.
Opaline appears to have no effect on fish at all. Curious. But this paper doesn’t speculate on what its job might be. A likely story is that opaline is a deterrent to non-fish predators, but the sea hares either can’t or don’t distinguish between predators bothering them enough to cause ink and opaline release.
* IOT = inter-ocular test. It’s something so obvious, it hits you right between the eyes.
Nusnbaum, M., & Derby, C. (2010). Ink secretion protects sea hares by acting on the olfactory and nonolfactory chemical senses of a predatory fish Animal Behaviour DOI: 10.1016/j.anbehav.2010.01.022
Wrasse picture by algaedoc on Flickr. Slug picture by user wanderinggnome on Flickr. Both are used under a Creative Commons license.