Ready or Not, Here I Come!

Hide and seek: one of the most popular children’s games. What makes it so fun? For me, the most exciting part comes in the moment leading up to the final second of the countdown, right before the search begins. However, for snails, their lives become at risk as the predator begins its search. Like the seekers in hide and seek, sea stars also search for prey such as snails. Whether the snails are ready or not, hungry sea stars will find their way towards them.

How does sea star predation on snails affect other species’ abundance? Using algae to investigate these indirect interactions, we created 4 food webs:

  • Algae only
  • Algae and snail
  • Algae, snail and sea star
  • Algae and snail in water with crab chemical cues

Indirect interactions occur when a third species mediates the interaction between two species. In our case, sea stars act as the mediator because their predation influences the abundance of snails. For example, with more sea stars preying on snails, the snail population will deplete, ultimately increasing the amount of algae available due to fewer herbivores consuming it. This is an example of a trophic cascade, a phenomenon where the top predator has an indirect effect on lower trophic levels that are not immediately below it in the food chain. Contrastingly, direct interactions occur between two species without mediating species. For example, sea stars directly influence snail abundance and snails directly influence algae abundance.

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Indirect and direct interactions between algae, snail and sea star community (Photo credit: Prabh Sahota)

To further understand the direct interaction of predation between sea stars and snails, we should consider the mechanisms involved in sensing prey. How do sea stars detect the presence of the snail in the tank? One way is by detecting their prey’s odor and moving their legs in that direction. In the lab, I was lucky enough to witness sea star predation! The sea star slowly approached the snail and eventually moved itself on top of it. Why? Because unlike most invertebrates with mouths, a sea star’s mouth is on its lower surface! Unfortunately, the snail could not escape and remained cushioned underneath the sea star, analogous to its position at the bottom of the food chain.

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Sea star preying on snail (Photo credit: Prabh Sahota)

Although we focused on one food chain consisting of sea stars, snail and algae, it is fascinating to consider how changes in abundance can alter the entire interconnecting network of multiple different food chains, aka the food web. For this reason, it’s also important to keep in mind how humans, a dominant consumer may impact the food web too.

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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Algae: “Thank You, Seastar”


Trophic cascade relationships (PC:

Most people are well aware of the direct interactions such as predation, herbivory, and competition that occur within different organisms that form food chains. As those food chains mingle amongst each other, they form a larger food web. However, there are different types of indirect interactions that affect the abundance of particular species thereby altering food web structure. Indirect interactions occur when an interaction between two species is mediated by a third species. These effects can be shown via trophic cascades. One of the well-known trophic cascades involve sea otter-> sea urchin-> kelp. Sea otters have an indirect positive effect on kelp by consuming or altering the behaviour of sea urchins thereby reducing the number of urchins feeding on kelp. If the increase in kelp population is due to sea otters eating the sea urchins, it is a density-mediated indirect effect; whereas if it is strictly the presence of the sea otter that changes the urchin’s behaviour and prevents them from feeding on the kelp, it is a trait-mediated indirect effect.

We were curious to see if a similar trophic cascade existed between Littorina littorea (snail) which grazes on Ulva lactuca (algae); and the snail’s two potential predators: Evasterias troschelii (sea star) and Metacarcinus magister (crab).


Seastar preventing the snail from feeding (PC:Juhae Oh)

We set up an experiment involving 4 different environments:

  • Ulva
  • Ulva + Littorina
  • Ulva + Littorina + Evasterias
  • Ulva + Littorina + Metacarcinus cue water

removal of snails and algae after 7 day exposure (no seastar/crabwater) (PC:Juhae Oh)


Evident grazing with no predator present (PC:Juhae Oh

We compared the weight of Ulva before and after being exposed in each environment for 7 days. We predicted the snails to graze on the algae but with the presence of the sea star and the scent of crab, they would reduce their amount of feeding or even get consumed in environments with the sea star. As we had predicted, the snails were able to graze more without the presence of predators. They ate significantly less algae with the presence of the sea star, although the crab water was not as significant. Moreover, the sea stars tended to not eat the snails.

Through our results, we were able to see a positive indirect effect of algae and the sea star. Although the sea stars didn’t eat the snails, we still saw a significant decrease in the amount of grazing, meaning the snail’s behaviour was affected by the presence of the sea star. Therefore, we can categorize this effect as trait mediated.

Watch this youtube video for more information about trophic cascades:

Foodies and Food Webs

We all like food. But there are always factors that influence how much we eat on any particular day. Sometimes we eat too little or too much. Worried about all the papers you have to get done before the end of term? You might forget about eating or the alternative, stress eat and eat too much. These factors could be called trait-mediated indirect interactions because they change our behaviour and in turn our behaviour affects how much we eat. On the other hand, density-mediated indirect interactions would involve a more extreme example – a coyote devours the bunny that eats the vegetables in your garden. Here the coyote indirectly affects the abundance of vegetables in your garden by eliminating the bunny. Similar phenomena occur in food webs in the sea. In last week’s lab, we explored a food web including crabs, sea stars, snails and green algae.


Figure 1- An illustration of the food web explored in this experiment. Sea stars and crabs are both predators of snails.

Four different manipulations of the food web were set up. The first, was a tank with only algae, the second included snails and algae, and the third included snails, algae and a sea star. There were snails, algae and water containing chemical cues from a crab in the last treatment. Half of these tanks were kept at low temperature and the other half at high temperature to explore the effects of temperature on species interactions within the food web. After 7 days, the wet weights of the algae, sea star, and snails in the tank were recorded to determine the amount of feeding on each level of the food web. Then behavioural experiments to examine the crawling speed of snails in response to predation threats were conducted. Next a choice experiment was conducted- whether or not snails fed on algae in the presence of the sea star, or in crab chemical cue water without the sea star.

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Green algae, a sea star and a snail in the experimental set-up for a choice experiment. Will the snail feed in the presence of the sea star?

There were higher amounts of algae remaining in the high temperature compared to the low temperature conditions in the sea star treatment. Also, in the high temperature treatments, sea stars gained more weight, suggesting increased consumption of snails. Snail biomass was lower in the sea star treatments and about the same in the control and crab treatments. In the choice experiment, snails fed less in the presence of a predator, but feeding did not depend on the type of predator. In conclusion, these results lend more support to density-mediated indirect interactions between the predators and the algae in high temperatures. I found these results to be surprising! I was expecting a trait-mediated interaction between the sea stars and algae because it seemed the sea stars did not eat many of the snails. Nonetheless, I definitely learned that marine food webs are more complex than they look!

To learn more about density and trait mediated interactions and marine food webs check out the following article and video!

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. 


Trophic Cascades in a Snail Shell

Who eats who is of primary importance in nature. The large-scale impacts of these interactions can be difficult to initially spot, but are in part responsible for the diversity of ecosystems we see around us. Predators consume plant-eating animals, preventing them from eating too many plants. Larger predators eat the smaller predators, preventing them from eating all the plant-eaters. These relationships can be depicted as food webs, where animals and plants are connected by feeding.

A special type of feeding effect is called a trophic cascade: it is when a species higher up in a food web effects the abundance of species lower down by eating their consumers. A well-known example is the sea otters of the Pacific coast. Otters eat sea urchins, keeping urchin numbers low, but when the otters were hunted for their firs their numbers plummeted; in the absence of predators the urchin population grew and ate much of the kelp forests in the region. In a healthy system the otters produce a trophic cascade by eating urchins, which allows kelp abundance to rise. 


Picture 1: The right shows an urchin barren, while the left shows a healthy kelp forest.

We wanted to see if we could observe a mini trophic cascade involving snails, seaweed, crabs, and sea stars. Snails graze on seaweed, and both crabs and sea stars feed on snails, so we wanted to see if adding the predators changed the amount of seaweed the snails ate. We used live sea stars to see if the presence of a predator influenced snail feeding, and water with crab scent to see if just being able to sense a predator would lower grazing.

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Table 1: These are the four food webs we set up.


Picture 2: One of our tanks with seaweed, sea stars, and snails.

We found that in the presence of sea stars the snails ate significantly less of the seaweed, while the crab water appeared to have little effect on snail grazing. Unsurprisingly, without any predator influence the snails ate the most seaweed.

There are two main types of interference we could see in our experiment. One is density: the predators eat the grazers, thereby reducing the amount of plant material eaten. The other is behavioral: the predators may not eat many of the grazers, but their presence still reduces grazing because the grazers spend time avoiding danger. Our experiment seems to show that the sea stars have a behavioral effect on the snails, because the sea stars did not eat many snails over the course of the test but still lead to lower amounts of grazing.

Our mini trophic cascade belies the complexity of natural food webs. Large food webs can include hundreds of species and thousands of interaction. Food webs are important to study because they can help researchers better understand the forces that maintain healthy ecosystems, and highlight important species to conserve.

To learn more about trophic cascades visit here.

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Climate Change: Should Hermit Crabs be Concerned?

Global warming has been a critical issue in the past and still remains a concern today. Not only does it affect us humans, but many other organisms are also affected. With human activities increasing green house gas concentrations, air and water temperatures increase, causing rise in sea level and decrease in salinity. Climate change is already evident in many oceans, but it is expected to deteriorate in our near future. Since climate change affects multiple factors such as temperature and salinity, we examined the affects of these stressors together and in isolation to visualize a potential scenario.



The species we investigated were the hermit crabs, Pagurus hirsutiusculus. We investigated their behavior by manipulating an environment they will most likely be exposed to in the future: an ocean with decreased salinity and increased temperature.


We exposed hermit crabs to 4 treatment levels for 3 days:

1)A control with optimal temperature (31ppt) and salinity (11°C)

2)Low salinity (18ppt)

3) High temperature (14°C)

4)Low salinity (18ppt) & high temperature (14°C)


We then performed 3 different experiments that we predicted would show behavior changes when exposed to the two climate change-associated abiotic stressors. We first counted the number of antennule flicks in 30 seconds, then we examined how long a hermit crab spends on feeding in 10 minutes, and lastly, we pretended to be a predator and scared it out of its shell and examined how long it took for it to return to its shell and seek refuge.  We predicted:

1)temperature would increase metabolic rates and show increased speed of behaviours

2)salinity may disrupt osmoregulation and show decreased speed of behaviours

However, our results for all experiments were insignificant.


While doing the experiments, one thing I found really cute were the extremely small shells on some of the individuals. This shell which is supposed to function as protective shelter for its soft, exposed abdomens, only covers a part of its body, leaving the uncovered areas vulnerable to predators. Thinking about the reasoning behind this left me sympathetic. Having such shell probably means that it has lost the vigorous competition to find a suitable shell or perhaps got its shell stolen in a shell fight. This made me consider taking this weak individual into an environment with no competition. Hermit crabs are fairly easy pets to care for once their tank is properly set up. As long as you can bear some sporadic chirping, they are dainty critters that can be found at beaches. Here is a link with more information about how to raise saltwater hermit crabs.



Hermit crab removed from its  unsuitably small shell (PC: Juhae Oh)

Although our experiment doesn’t represent the whole population, I am relieved that through our experiment we could predict Pagurus hirsutiusculus will tolerate the decrease in salinity and increase in temperatures we will most likely face in the future. However, we must remember that it may be a strong concern for other organisms, including ourselves. Click here to learn more about what we can do to save our environment!