The Great Mussel Feast starring Hungry Hungry Hemigrapsus

Rising ocean temperatures and decreasing salinity are predicted to affect (and are affecting) the world’s oceans… but what does this mean for our favourite intertidal invertebrates such as shore crabs?

For my independent project, I chose to study the shore crab Hemigrapsus oregonensis and the effects of temperature and salinity on their feeding behaviour. H. oregonensis inhabit muddy banks in rocky intertidal zones from Alaska to California (Sliger 1982). I wanted to measure how many minutes out of a five minute period they spend feeding on their favourite prey, crushed mussels.

I predicted that in the high temperature conditions the crabs would spend more time feeding because the rate of metabolic activity is higher at high temperatures and the crabs face greater metabolic demands. At low salinity conditions, crabs consume more oxygen suggesting higher metabolic activity. This led me to predict that the crabs would also spend more time feeding at low salinity. When the two conditions, high temperature and low salinity, were paired together I expected the crabs to spend the most amount of time feeding when compared to the high temperature or low salinity conditions alone.

To test my predictions, I set up four tanks with different combinations of temperature and salinity as shown below:

Screen Shot 2017-04-12 at 11.13.09 PM

In the center of each tank I placed some crushed mussels. Then I placed one crab in the tank at a time and recorded how much time they spent munching away on the mussels. The crabs had been in the lab for 2 weeks and were starving at this point, so they were quick to attack the mussels and start eating!

Screen Shot 2017-04-12 at 11.28.19 PM

H. oregonensis devouring mussel tissue. Photo credit: Arpun Johal

I found that salinity and the temperature-salinity interaction did not have significant effects on the amount of time that the crabs spent feeding. However, temperature did have an effect. The crabs spent less time feeding in the warm temperature compared to the high temperature. This was surprising because I had predicted the opposite effect!

After looking at some previous studies on temperature and Hemigrapsus, I decided that my results may have something to do with the crabs’ thermal tolerance. The warm temperature (22°C) was higher than what the crabs are normally used to and this temperature may have been too extreme for the normal function of the crabs. Hence the observed decrease in feeding under high temperatures instead of the expected increase. Increases in temperature only increase the rate of metabolic reactions to a certain extent. The graph below demonstrates this:

The following video also explains the connection between optimal temperature and metabolic activity.

Although my experiment did not find high temperature and low salinity to have a significant effect on the feeding behaviour of Hemigrapsus, the effects of increasing temperature and decreasing salinity on marine intertidal species and communities still remains an important question.

The following video explains the effects of climate change on the world’s oceans.


Sliger, M. C. (1982). Distribution and microhabitat selection of Hemigrapsus oregonensis (Dana) and Pachygrapsus crassipes Randall in Elkhorn Slough, Monterey County, California (Doctoral dissertation, California State University).


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.

Screen Shot 2017-03-20 at 12.27.44 AM

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!

Rainier than Raincouver?

What’s rainier than Raincouver? “Is that even possible?” some fellow Vancouverites, especially those who despise rain, may ponder. After a weekend trip to Bamfield Marine Sciences Centre and a tour of the temperate rainforest at Bamfield, I learned that yes, Vancouver is not the rainiest city in B.C. To my surprise, Bamfield receives much more precipitation than Vancouver (199 days compared to 168 days out of the year). This precipitation can come in the form of fog or rain and it influences what species of trees can grow in the forest. The species found in the temperate rainforest at Bamfield Marine Sciences Centre differ from the species found in the temperate rainforests in Vancouver. Precipitation plays a larger role in the biodiversity of a forest than I had originally speculated!

Since the forest is exposed to high levels of precipitation, the soil is washed off with the runoff, and the nutrients needed for growth are found near the surface of the soil. As a result, the trees extend their roots outwards from the base of the tree and the roots do not dig deep into the soil. The ground is also very spongy and bouncy due to the lower amounts of soil.

Disturbances such as logging have divided the forest into different levels of growth. In areas that are more recently disturbed, the understory is richer in plant species such as sword fern and deer fern. This is because when the trees are younger, the overhead canopy is less dense and more light can pass through to the forest floor and is available for photosynthesis and growth.

This forest displayed many significant biotic relationships between different species. One prime example would be a Western Hemlock tree looming out from the side of another tree! Also fallen trees called nurse logs provide nutrient-rich habitats for other forest species such as the Western Hemlock to grow on as they decay. It’s common for wind to uproot the trees in this forest due to the shallow roots and hence when knocked over, the fallen tree can become an ideal habitat for seedlings. Bracket fungus was also seen growing on the Western Hemlock trees. The white part of the bracket fungus faces downwards since it produces spores, and hence this position maximizes dispersal of the spores. On one such fallen log, the bracket fungus was seen curving slightly to the right in an attempt to reorient the white part towards the ground and ensure efficient dispersal.

Bracket fungus growing on a Western Hemlock tree (Photo by

Bracket fungus growing on a Western Hemlock tree. The white parts facing the forest floor produce spores (Photo by

A nurse log facilitating the growth of various other forest species (Photo by

A nurse log facilitating the growth of various forest species (Photo by

Lower soil levels and shallow roots make the temperate rainforest in Bamfield more susceptible to disturbances such as the wind. These disturbances increase the chances of trees falling and becoming nurse logs. To learn more about nurse logs and the species diversity they support, check out the following blog and short video below!