The past year I have immersed myself into the world of intertidal ecology and have found myself obsessing over sea stars. Most people actually share my fondness of these charming animals, but maybe not to my extent…. so I naturally found myself doing my final project on them.
One of the patterns I have observed around Vancouver, is it’s pretty challenging to find sea stars on the rocky shore during the summer. Sea stars are actually the most abundant during winter. I expect few people to know this because they likely spend their winter nights inside their warm houses, but not me! I bundle up and spend my winter nights counting these critters, and let me tell you, there are tons of them! There are two main sea star species you are likely to find around Vancouver, the mottled sea stat, Evasterias troschelii, and the ochre star, Pisaster ochraceus, . I have additionally observed that Evasterias seems to completely disappear from the intertidal where as Pisaster can still be found occasionally. So why do sea stars retreat from the intertidal in the summer and why does Evasterias do it more than Pisaster? Obviously, an experiment was needed to settle this.
Maybe sea stars are not able to handle the extreme conditions experienced in the intertidal during the summer, and they retreat to the subtidal to escape. Vancouver is by the Fraser River and each summer the river’s output increases and makes the sea water less salty (lowers the salinity). Temperature also increases as the sun cooks the intertidal. Sea stars do poorly in low salinity, check out this study, so maybe this summer condition was driving the sea star retreat. Temperature may also be a player here, so I decided to test this too. Specifically, I wanted to see how lowering salinity and increasing temperature, affected the activity of sea stars, and whether any negative effects were more harmful to Evasterias.
To test this, I placed sea stars of both species into tanks of low salinity and high salinity, across a range of temperatures from 12 degrees Celsius to 18 degrees Celsius. To see how activity changed for each species, I flipped sea stars and timed how long it took for them to right themselves. This felt a bit cruel at times, especially for the ones that never made it back rightways up.
I found that low salinity, but not raised temperature, decreased the activity of both species, and that Pisaster was just as affected as Evasterias. So it looks like salinity could be driving the retreat of sea stars during the summer, but maybe other factors are causing Evasterias to disappear entirely. Maybe biological factors may be driving the disproportional retreat between my two species. My next future experiment…. effect of gull predation!