By Eliza Heery Posted on

Herding urchins

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This week in the lab!.. Setting up kiddie pools on the dock at MaST aquarium.  Why you say?  I’m preparing for an experiment with urchins!

Urchins have a very patchy distribution in the Seattle area.  Using field surveys and underwater photography, I have found differences in the marine community between sites where urchins are present and absent (see image above).  Rocky sites with urchins tend to be characterized by encrusting algae, bare rock surfaces, and occasional large blades of kelp.  Rocky sites without urchins are dominated by a diverse range of red macroalgal species, which may support a different suite of mobile invertebrates.

Photos of benthic habitat when urchins are present versus absent
Sample images of the marine community at (a) a site in Elliott Bay where urchins were present and (b) an adjacent site where urchins were absent. Underwater photography allows us to collect data on the flora and fauna at different sites much more efficiently than was possible in the past. Photos are taken on SCUBA dives using a randomized survey design. Back in the lab, we test for differences in the biological community at two sites by quantifying the percent cover for each algal and invertebrate species in the frame and then using multivariate statistics.

Although I was able to detect differences in the biological community at sites with and without urchins, I don’t know whether these differences are actually caused by urchins until I test this hypothesis experimentally in the field. Starting in late August, I will transplant urchins from the kiddie pools at MaST to a site in Elliott Bay where urchins are currently absent. The urchins will be kept there for several months while I monitor the algal and invertebrate life around them and watch for changes in community structure.

Urchins are faster than you might expect and keeping track of them once you’ve transplanted them to a new site turns out to be quite challenging. In a pilot study, I was only able to find 4 out of 10 urchins after they had been transplanted to a new site. In the full experiment, it’s important that I be able to keep urchins on experimental plots. Otherwise, I won’t know whether the plots were actually subjected to urchin feeding. So, I need to develop a way to contain them within a fixed area. That’s exactly what I’ll be experimenting with in the kiddie pools at MaST.

Thanks to the facilities provided by MaST, I can develop and test alternative urchin containment techniques in a controlled setting prior to implementing them in the field. My objective is to find the least invasive urchin containment technique that meets the following criteria:
(1) Contains urchins without significantly altering the physical conditions (such as water flow) of experiment plots
(2) Does not affect urchin feeding
(3) Allows for free movement of other invertebrate species (such as chitons, snails, crabs, etc.) in and out of experimental plots

Tanks should be up by next week.  I’ll keep you posted!