Scardy Snails

Can you scare snails with a sea sear? What if you just have some water that smells like a crab, would that be enough to scare a snail so much that it won’t eat? That’s what we set out to find out this week.

We had some tanks with seaweed in them, and either added snails, snails plus a sea star, or snails plus water that smelled like a crab to make the snail THINK that there was a scary crab nearby. We also had some tanks with just seaweed, to act as a control (a ‘nothing’ treatment for us to compare our results to). Half of these tanks were kept cold in a seawater table, and half were kept at room temperature.


Our snail (Littorina littorea) eating some seaweek (Ulva). Photo: Emily Lim

HOWEVER. A bunch of pesky shore crabs managed to crawl into the cold tanks and shredded most of the seaweed, so our results are pretty muddled from those treatments. The crabs also escaped the lab completely and were terrorizing the offices down the hall, but that’s neither here nor there.

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A pesky shore crab. photo from Island Nature, edited by Emily Lim

We found that the snails ate a lot of seaweed when they were by themselves, but not very much when there was a sea star with them. Interestingly, smelly crab water didn’t deter them from eating. This is what we call a trophic cascade, where the predator has an indirect effect on the stuff the herbivore is eating (the vegetation) by eating or scaring the herbivore. In our case, this means the sea star keeps the snails from eating seaweed, resulting in more seaweed left over.

Next, we wanted to find out how afraid of our predators the snails are. Our instructions included the phrase: “gently use [the sea star] like a miniature cattle prod” (science is weird, folks), which basically translates to us poking snails with sea stars and seeing how fast they crawl away. We also recorded how fast snails crawled sitting in crab water or plain seawater, and after being poked with a pencil. We found that the snails didn’t really care about the pencil or sea star, because they crawled just as fast (slow) as they did when they were in plain seawater. Interestingly, they crawled even SLOWER in the crab water and we don’t really know why.

In the end what this tells us is that snails are scared of sea stars when they have to live together for a week, so much so that they don’t eat as much seaweed as they do when they’re alone. This is called a trophic cascade, where the presence of a predator indirectly increases the amount of vegetation by scaring or eating the herbivore. And overall, snails don’t really care about smelly crab water.

If you’re interested in learning more about the famous trophic cascade that happens just off our shore, involving kelp, urchins, and sea otters, click here

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