Winter Blues in the Intertidal

I’m so glad its Spring now, and we don’t have to worry about the snow or cold weather anymore. But for the most part, the winter is usually pretty mild for most of Vancouver. But at the Intertidal areas… it’s a whole different story. As a refresher, the intertidal is the area between the high and low tides. The climate at the intertidal experiences harsh changes in short periods of time. this means that wildlife living in these areas have to get used to this predicament!


Recently, we were interested in the life of these creatures in the intertidal. Particularly, how they fared at home during the cold winter and how changes in their choice of habitat can change their body temperatures. So, we went to Stanley Park over the winter to take temperature readings from boulders to get a sense of what the animals were experiencing.



Figure 1. Stanley Park at low tide. Photo courtesy of Chris Harley

We stumbled upon a few known predators of the lower intertidal…


At Stanley Park, by the infamous “Girl at the Westsuit” site, there were piles of Ochre sea stars (Pisaster) that were all clumped together. I believe these clumps were called aggregates. It seemed like some of the Pisaster on the edges were trying to squeeze into the middle… I wonder why… Anyways, we plucked off some of the sea stars at the middle and edges of the clusters to get a reading of their body temperature.


Figure 2 Aggregates of Pisaster. Photo courtesy of Chris Harley


We also found dogwhelks  (Nucella) around the boulders, they’re a type of sea snail! Some of the snails were near the bottom of the boulders, close to the sand, while others were closer to the tops of the boulders. Again, we took the shell and foot temperatures of these creatures from either the sand or tops of rocks.  Did you know that dogwhelks prey on other shelled-molluscs by drilling into their shells and drink up the contents like a tasty soup? They actually secrete a chemical substance so that their dinner dissolves.


Figure 3. Dogwhelks on boulders. Photo courtesy of Chris Harley

We eventually moved on to Sharon Cove in West Vancouver. At this site, we found another species of sea star, the mottled star or Evasterias. The Pisaster and Evasterias are often mistaken for one another. But a tell-tale sign of the Evasterias is that it has longer rays that taper. Here, we also took the temperature of Evaterias either in tide pools or on rocks.



Figure 4. Evasterias. Photo courtesy of Chris Harley

After going back to the lab to run some tests, we found that the organism choice of habitat (ie. on rock vs sand) had an influence on their body temperatures! Now for the next step, we just have to test if cold weather could hurt them… stay tuned!


Here are some interesting links:



The Sea Star You’ll Definitely See

While you go down to the oceans edge and admire the beautiful marine creatures that you can see, it is often not thought about why those exact species are present. For example, if you are observing the sea star populations today in Vancouver, you might notice a large number of grey or greenish sea stars in the intertidal. These sea stars are known as the mottled sea star, Evasterias troschelii and are the most abundant sea star off the Vancouver coast!

Multiple Evasterias sea stars piled together on a mussel bed. Image sourced from Hakai magazine:

The reasoning behind these sea stars being the most abundant is because their main competitor Pisaster ochraceus was heavily impacted by the sea star wasting disease outbreak in 2013. Even though the Evasterias were affected by the disease, they were not as susceptible as Pisaster. This resulted in a major intertidal population shift from the historically dominant Pisaster to Evasterias. Having a smaller Pisaster population also resulted in more food available for Evasterias because they were not eating it as well as not fighting the Evasterias for the food!

A Purple Pisaster sea star feeding on a mussel in the intertidal zone. Photo taken by Dave Cowles. Image Source: Pisaster Sea Star

Sea stars are dominant predators!

The Pisaster species has proven to be a dominant, top-predator in the rocky intertidal zone. Both it and Evasterias have shown to feast on mussels, barnacles, clams and other invertebrate species. However, in the past, Pisaster were identified as a keystone species, because of their feeding power and control on mussel populations. Without the Pisaster present in the intertidal, the mussels were shown to take over and out-compete all other prey species, such as barnacles.

This is of concern for scientists because of how major the impact of the disease outbreak was on the top-predator, Pisaster, population. In order to properly understand if our future rocky intertidal zones will have huge loses in the number of species because of mussels, it is important to recognize if another species exists that can fill Pisaster’s feeding role. This is where the closely related Evasterias species comes into play.

Photo of me reaching into buckets of Evasterias sea stars in my garage to check the temperature. Image provided by myself.

As an independent project in Biology 326, the question regarding if Evasterias can fill the role of Pisaster as a keystone predator feeding on mussels, was initially proposed to be looked at through conducting feeding trails in the Harley lab at UBC. This was, however, made impossible because of the University closing down due to the COVID-19 epidemic. The sea stars were taken home and kept in my garage, along with a bucket of mussels to feed them.

The study species were kept in their collected buckets seen here, the two buckets on the right containing Evasterias and the one on the left containing mussels. A black aquatic pump cord can be seen going into each of the buckets. Photo provided by myself.

The garage did not turn out to be an ideal location to keep the sea stars alive and after a week, and a non-significant amount of feeding, the sea stars passed away. If I were to attempt this feeding experiment from home again, I would try and find a place to keep them other than my garage because the ideal temperature of 12-15 degrees Celsius was hard to maintain. The garage door constantly opened during the day and it was relatively warm outside.

In order to try and answer this question and continue my independent project, I then looked into every laboratory experiment conducting on Evasterias and Pisaster feeding on the intertidal mussel species. The Evasterias and Pisaster were not found to have different feeding rates and were described to feed in a similar way. They both have been seen to feed throughout the summer months and barely feed in the winter as well as both have similar feeding behaviour. This was found to suggest that the Evasterias, the currently most abundant sea star, may be able to take over the feeding role of the decreased Pisaster population, preventing mussels from taking over the intertidal!

A photo of an Evasterias sea star taken by Claire Mackenzie. Image source:
Photo of myself in Bamfield, B.C. holding a Pisaster which is feeding on a mussel. Photo taken by Claire Armstrong

For further insight into the eating behaviour of sea stars, check out this video of a sea star eating a mussel. It feeds by inverting its stomach which is visualized by a cleverly hidden camera!

Sea star crawling along the ocean floor and feeding by using their inverted stomach technique on mussels! Video taken from: Shape of Life Vimeo Channel

Baby, it’s cold outside

The intertidal zone is a unique ecosystem that serves as a home for a diversity of marine critters. It is unique because temperature and other environmental conditions can be very different between the lowest shore level and the highest shore level. 

Why does this happen? We can thank the tides! Yes, something as far away as the moon has an extremely large impact on the life of sea stars, snails, crabs, mussels and many other small creatures that you can find along the beach. The highest points on the shore are exposed to the air for a much longer duration than lower shore levels during low tides. And since high shore levels are exposed to the air for a long time, they experience very hot temperatures.

This is important for the ecology of intertidal organisms because they are ectotherms. This means that they cannot maintain their internal body temperature using their metabolism or by sweating, unlike humans. Which means that intertidal species that are less resistant to extreme temperatures are restricted to low shore levels, where they can take refuge in the sea water (The temperature of sea water stays the same throughout the year because the ocean is so huge). To learn more about the intertidal zone, watch this video 

Hot temperatures are not the only threat to intertidal organisms. Extreme cold weather during the winter can also be dangerous. This is why Christopher Harley and his team set out to determine if behavioural decisions made by intertidal predators influenced their body temperatures during the winter.

Results showed that predators can stay warm by choosing different positions within the intertidal zone. For instance, the ochre sea star is warmer in the centre of aggregations than on the edge of aggregations. You can think of an aggregation as a bunch of sea stars huddling together. Kind of like emperor penguins, who huddle together during the severe Antarctic winters to stay warm and take turns shielding their neighbours from the wind (however, there was no evidence that sea stars were taking turns. The ones on the edge just got the short end of the stick). Results also showed that mottled sea stars were much warmer within tide pools than on the top of rocks. And dogwhelks stayed warmer if they were touching the damp sand. 

This study was very important because it demonstrates how intertidal organisms may be able to withstand cold temperature events caused by climate change. While most people know that climate change causes global warming, it is less well known that it also increases the frequency and intensity of cold weather events! Here is a video that explains why:

Although they are not esteemed as the most intelligent animals, intertidal organisms are able to make behavioural decisions to keep them warm throughout the winter. 

To learn more about intertidal organisms, watch this video.

(not) Niche to meet you! Animals in the intertidal intrude on their neighbors space

Think about your favorite type of weather. Snowing, sunny, or a thunderstorm perhaps? Thankfully, your favorite weather falls in your fundamental niche. An organism’s fundamental niche is the potential habitat based on the set of environmental conditions that allow it to live. For you and I, this includes conditions such as an outdoor temperature of between 4℃ and 35℃ (for tolerable long-term living). Those who like the freezing cold are out of luck! Our bodies need warmth eventually. We also need at least 11% oxygen in the air, meaning we cannot live for long periods of time above altitudes of around 4700 meters (to read more about the limits of human survival, see: Just like human’s, barnacles also have a set of environmental conditions they can handle! According to a 1969 study, young barnacles start to die at around 42℃, pretty similar to people! As we learned in class earlier this year, barnacles do not perform well and slow their feeding at salinities below 30 ppt. In terms of actual habitat, this means that barnacles can up to a certain height on the intertidal (between 3-4 m above chart datum around Vancouver) and down to a certain height as well. The lower potential limit of barnacles is more difficult to know for the reasons below! So keep reading!

Though each organism has a potential habitat that it could live in, rarely do organisms actually occupy all of this space. In reality, there are other interactions between species that restrict (or expand!) the liveable habitat of a population. For instance, competition between mussels and barnacles in the lower intertidal leaves both species living in a smaller habitat than they theoretically could live in. Usually we see a distinct line between the mussels and barnacles that is the result of this competition! Though mussels push back the niche of barnacles, they expand the niche of other intertidal invertebrates by providing moist habitat in places that these small invertebrates like amphipods and isopods. 

Clear intertidal donation exists in this picture, taken in Bamfield, BC. I have added red lines to show where barnacles live in this picture. Between the red lines is the realized niche of the barnacles!

For part of my independent project, I looked into how predation affected the niche of barnacles in Vancouver. As part of my study, I measured the lowest height above chart datum (the tidal baseline) that barnacles appeared in two places. One of these places had no predators, the other had copious amounts of Evasterias troschelii and Pisaster ochraceus sea stars, which are known predators of barnacles. What I found is not surprising. In the site with no predators, barnacles existed much lower in the intertidal than in the area with sea stars. This likely means that the fundamental niche of barnacles extends lower in the intertidal, but sea stars eat any barnacles that settle in the area. 

During pandemics, the best scientific equipment is often a brother! Here, my brother holds his arm up so I can use him to measure how high above the waters edge barnacles can live. Plus, it was great to have a friend with me when measuring tidal heights of the barnacles.

Predation is an important mediator in the intertidal. Predators like sea stars help open up space by eating barnacles and mussels allowing other creatures that would otherwise be outcompeted by mussels or barnacles find a place to live. Read about Bob Paine’s work on these creatures here:

An interesting article that contrasts mussel-barnacle predation – sometimes they get along!

A really fantastic look into how complicated intertidal systems can be – Pisaster aren’t the only keystone sea stars!

Climate Change is Making You Sick & Shellfish Are Involved

As climate change causes ocean temperatures to rise, seafood lovers are at risk for experiencing upset stomach and food-poisoning symptoms. Shockingly, clams, oysters, and mussels are mainly to blame.

Abundant in both marine and freshwater areas, clams, oysters, and mussels are part of a class of two-shelled creatures called bivalves. With a lack of a head or mouthparts to take in food, these creatures draw water into their bodies to capture tiny food particles from surrounding bodies of water.

The role of filter-feeding bivalves is varied. These organisms are known to take in anywhere from approximately 2 – 4000 mL of water per hour, cleaning the water columns they live in while they do so. When contaminants make their way into bodies of water, through wastewater or runoffs that lead into aquatic waterways, they also make their way into the bodies of filter feeding bivalves.

During their course of feeding, bivalves may pick up bacteria and other toxic contaminants from the water around them. These toxic particles build up in the creatures’ bodies and if then eaten by humans, can cause dangerous and sometimes deadly poisoning.

How toxic bacteria reach humans from shellfish (Modified from:

Warming ocean temperatures provide ideal conditions for bacteria to grow and accumulate in bodies of water. This increased bacterial growth poses a higher risk to both shellfish and the humans that eat them. Not only do these warm ocean temperatures allow bacteria to grow, but studies have shown that bivalve filtration rates (the rate at which they take in water) increase with rising temperatures.

Timelapse of oysters filtering Chesapeake Bay water (From: The Chesapeake Bay Foundation)

Across Canada, contaminated areas are closed to shellfish harvesting throughout different seasons when the risk of shellfish transmitted diseases is high — putting restrictions in place in the shellfish industry to help decrease the number of people getting sick from eating infected bivalves. Tests for bacteria and checks of water temperatures have been set up where shellfish reside.

Timelapse of oysters filtering a tank filled with water and algae from the Severn River in Annapolis, MD.
(From: The Oyster Recovery Partnership 2016)

Because of the filter-feeding capabilities of these bivalves, if they are harvested and then placed in freshwater tanks before distribution for human consumption, bacteria can get cleaned out of the shellfish via the same methods through which they entered.

The rise of toxic bacteria in warmer bodies of water can also help shellfish harvesters know where to make their catches from and modify habitats to ensure better water quality for species. If the top layers of water are too warm, animals can be lowered into deeper, colder waters to stop bacterial growth.

Significant research still needs to be done to determine the best methods to both use bivalves in wastewater management programs and to fully prevent humans from getting these diseases, but knowing the threats to bivalve environments (such as increasing temperatures) is the first step to understanding these amazing filter feeding organisms.

How climate change threatens the oceans (From: Monterey Bay Aquarium Research Institute)

Check out this video to find out more about the way aquatic bivalves filter feed:

And to gain a greater appreciation for the water cleaning capacity of bivalve filter feeding, check out this video:

20 Years later, why finding Nemo might not be so easy…


(Modified from:;

As a kid born in the mid 90s, finding Nemo had me fall in love with the animals under the sea. From the tragedy with the barracuda, to the bromance with the turtles, it was amazing to see all the animals interact with one another. But above all else, the interaction that I found the most interesting was this one:


No, not Nemo and Marlin, but the clownfish and their home. Their house is a sea anemone, a living, breathing animal. In fact, in the real-world clownfish and some sea anemones have what’s called a ‘symbiotic’ relationship. That’s a fancy way of saying a relationship where both animals benefit. The clownfish gets to shelter within the sea anemone tentacles, and the sea anemone gets to feed on scraps of food that the clownfish leaves behind. So, if one species suffers, the other will as well… and sea anemones are suffering.

Climate change doesn’t just involve warming on the surface, but warming in the oceans as well, and sea anemones aren’t handling that very well. You see, they have another symbiotic relationship with algae. These little photosynthetic plants live INSIDE the sea anemone and provide energy to the anemone in exchange for shelter (kind of like the clownfish!):

Picture3Photo courtesy of Anthony (See all that green? That’s algae!)

But these algae are not happy with the changing temperature, and so they’re leaving the anemone to go find somewhere cooler.

Picture4(Modified from:—Large)

This is called anemone ‘bleaching’, and once the algae leave the anemone are left hungry, and a lot less colourful.


All in all, climate change is causing a cascade of effects from algae, to anemones, to clownfish. If the oceans continue to warm, the clownfish will lose their home, and suffer because of it. Eventually, even an adventure across the ocean might not be enough to find Nemo…


For more fun facts about sea anemones, check out this 3-minute video from the National Geographic:

For more information on coral and anemone bleaching, check out this page by the Living Oceans Foundation: