The results are in. After several months of tethering urchins and measuring their feeding rates, it seems that we can now conclusively say that tethered urchins go on hunger strike. Given that the whole operation was rather comical (try putting urchins on leashes, building little urchin boxing rings, and feeling normal!), I am tempted to present this conclusion jokingly and without the context you probably need to understand why it matters. The bottom line is that I am still without a means of quantifying the effect of urchins on urban marine ecosystems in the field. So that’s unfortunate.
A huge thanks to the folks at MaST aquarium for letting me set up kiddie pools in on their dock and use their flow through seawater system. Throughout August and September, I used these tanks to test whether tethering impacted the way that urchins feed. Urchins exhibit an extremely patchy distribution in urban marine ecosystems. When they are present, the algal community appears to be considerably different, with less foliose red algae and a different suite of sessile invertebrates. My overarching question is whether urchins alter the community structure on rocky habitats when they are present in urban marine ecosystems, or whether these differences are the result of some other process. In order to do this, I ideally would conduct a transplant experiment, moving urchins to sites where they currently are absent and measuring any changes in community structure that result. But pilot studies demonstrated that transplanted urchins are not easy to keep track of – they move away from transplant sites quickly, often disappearing into deep crevices between the rocks. If they don’t stay on experimental plots where they’re transplanted, I can’t effectively quantify their effect. Tethering was the last of several attempts to contain the urchins within experimental plots and would only have been effective if they continued to feed once tethered. Since they did not continue to feed, we can rule it out as an approach for measuring the impact of urchin feeding in the field.
What was striking about the results of the experiment was that the differences in feeding rates between tethered and non-tethered urchins were consistently so significant. I conducted the experiment with three different types of algae: Ulva sp. (fleshy green algae), Chondracanthus exaperatus (a red alga known as Turkish towel), and Laminaria saccharina (“sugar kelp”). Urchins that had not been tethered consistently ate 2-3 grams of algae per day, while tethered urchins ate less than a gram or nothing at all. Statistically, this led to highly significant differences in feeding rates. Before-and-after weights of algae in stalls with tethered urchins did not, on the other hand, differ significantly from empty stalls where urchins were left out as an experimental control.
For now, I’m taking some time to regroup and reconsider how we can test the effect of urchins on algal communities. It is an issue that we’d love to understand better, particularly since urchins play such an important role in temperate marine ecosystems in less urbanized environments. The lesson of the day is that designing effective field experiments can be more challenging than one might expect. We’ll keep working at it though, and will let you know develops.