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:
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!
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).