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!
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!
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.
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 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!
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!
BIOL 326: Experimental biology of invertebrates 