Too Scared to Eat Salad: A Story of Snails and Sea Stars

Food webs: SO MUCH MORE THAN JUST WHO EATS WHOM! Food webs are exceptionally complex, demonstrating the interconnected effects of predator populations, herbivore populations, and primary producer populations. Changes in organism numbers higher up on the food chain result in population differences at each of the levels below it; this is called a trophic cascade. Keystone species are animals for which changes in their population causes particularly large changes throughout the trophic cascade, suggesting that the ecosystem relies on them for stability.Here is a video on another trophic cascade dear to our west coast hearts, featuring a very charismatic keystone species! 

Through trophic cascades, even though predators (for example sea stars) don’t directly feed on primary producers  (for example sea lettuce), they do effect primary producer abundances. How? By effecting the herbivores that eat the primary producers! Meet this week’s favourite herbivore, the common periwinkle snail!


Littorina littorea, aka the common periwinkle! This little herbivorous snail grazes on delicious algae in the intertidal zones of eastern Canada and the United States. (

So how do predators like sea stars change the feeding patterns of herbivores like the common periwinkle, and subsequently the abundance of primary producers like sea lettuce? First, and most obviously, sea stars might eat the periwinkles! If there are lots of sea stars eating periwinkles, they might decrease the periwinkle population. With less grazing periwinkles around, you might see an increase in sea lettuce growth! This route of interaction between predator and primary producer via herbivore populations is called density mediated indirect interactions – because predators are changing the number (or density) of herbivores in an area. Another less obvious way sea stars might effect the amount of sea lettuce is that their presence might change the behaviour of the periwinkle snails. This change in behaviour could be eating more or less of the sea lettuce, either increasing or decreasing its growth in an area. Effects of predators on primary producer abundance through changes in herbivore behaviour are called trait mediated indirect interactions.


Food web simulation tank with a sea star, a periwinkle snail, and some sea lettuce.

In the wild, where and when there are sea stars, there is typically more algae growth. We wanted to find out whether this was because density mediated or trait mediated indirect effects. We set up tanks with 1) just sea lettuce, 2) with sea lettuce and periwinkle snails, and 3) with sea lettuce, periwinkle snails, and a sea star. We weighed  the sea lettuce, snails, and sea stars separately at the beginning of the week, and then at the end of the week. Sure enough, there was much less sea lettuce left after a week when snails were in the tank alone, than when snails were in the tank with sea stars! Before the study, we thought for sure that this would be because the sea stars ate the snails, causing more sea lettuce surviving after the week through density mediated interactions. However, the weight of the snails in each tank was the same at the start of the week as at the end of the week – NONE OF THEM WERE EATEN! Therefore, sea stars were just changing the behaviour of the periwinkles, causing them to eat less. Even though the sea stars weren’t preying on the snails, the snails appeared to be too scared to eat their salad. 


If You’re a Bird, I’m a Bird, but We Still Won’t Fool the Crabs

In grade 1, I had a teacher who was very into just letting us create and discover on our own. She would strum her ukulele and say things like “Okay children, I have some sticks and some yarn, make whatever you want with it for the next hour” or “Paint a picture of your house using only lunch leftovers, you magical beings of light”. That was essentially the vibe in BIOL 326 for our Shore Crab Lab, although more specifically it was “Okay stressed-out twenty-somethings, we have these buckets of Hemigrapsus oregonensis, do some science with them!”. And oh boy did we ever!

21Hemigrapsus oregonensis - yellow shore crab.jpg

The bucket of crabs in question contained Hemigrapsus oregonensis, a common shore crab here on the west coast of Canada. They come in a variety of colours, from green – purple, and are distinguished by the little hairs on their legs, called setae. Photo courtesy of

Hemigrapsus oregonensis is pretty much the poster organism for primary school field trips to the tide pools. You would flip over rocks, pick them up and they would skitter over the palm of your hand, distracting you from what ever the teacher was saying about tidal zones or not going to close to the water or whatever. These small shore crabs feed primarily diatoms and green algae, though as scavengers will eat pretty much anything. They are fed on by birds, fish, other crabs, and once by a boy named Cohen in my elementary school (who was dared to do so).

So my lab partner and I were handed this bucket of shore crabs, and told to do science. What we came up with was this: Would a crab’s reaction in the face of death-by-bird change if they were stranded out in the open, vs if there was a shelter just out of reach? We felt that left totally exposed, they would probably stay still and hope to go unnoticed, but with a shelter present, they would possibly make a dash for safety. However, all of these questions are predictions hinged on one simple assumption: that we could trick the crabs into thinking there was a bird overhead. We decided to do this by waving a notebook between them and a light, creating a sudden swooping shadow. It looked pretty good to us, but when you assume, you make an ass out of u and me.


Image credited to The Little Mermaid, gif courtesy of

80 trials later, 4 treatment groups, shelter vs no shelter, “bird” vs no “bird, my arm nearly falling off from all the notebook swooping and our data was as insignificant as it could get. Sometimes when you have collected data, and you are running your statistics you think for sure what you are seeing has to be significant, only to be crushed when you realize that it may have only been due to chance. This was not one of those times. In every one of our treatments, exactly half of the crabs stayed still, and half ran around, shelter or no shelter, “bird” or no “bird”. So what did we discover in our five hours of scientific crab discovery? That these intertidal dwellers were SO not fooled into panicking by me waving a notebook around, pretending to be a bird. And that my friends, is science.




Image and cheesy quote courtesy of The Notebook, gif credited to


Image credited to The Little Mermaid, gif courtesy of Your Tang0