This Week In The Lab

Urchins on hunger strike

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.

Tethered urchin

Tethered urchin

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.

Urchins in experimental "stalls"
This Week In The Lab

Urchins on leashes

Unfortunately, the urchin corrals (see previous post) proved unsuccessful.  After numerous adjustments to the design, escapees continued to find their way to freedom.  By the end, the only prototypes that were successful at containing urchins were also trapping algae and confining other species that will need to move freely in and out of experimental plots in the field.  So, I’ve moved on to plan B: urchins on leashes.

Tethered urchin

Tethered urchin

This is a rather sad alternative, as it requires piercing the side of the test (exoskeleton) of urchins and threading it with monofilament line. While it seems likely that this is uncomfortable for them, a close look at their anatomy gives me reassurance – the area that I’m puncturing is basically a hollow cavity without organs. Urchins are also very different from us in their make up, and a limited exposure of the interior part of their body to the surrounding environment does not have the same implications for them as it does for us.  Indeed, the individuals I have tethered are recovering well and appear to be going about life as usual.

The method I’m using is borrowed from previous work by Nick Shears, Russell Babcock, and Anne Salomon.  Shears and Babcock describe the method in a paper they published in Oecologia in 2002 (link). Anne Salomon was also gracious enough to offer some additional tips via email based on her experiences tethering purple urchins, Strongylocentrotus purpuratus.

While urchin tethering has been used successfully in several previous studies, it has primarily been used as a means for quantifying predation on urchins themselves.  This is a common technique in marine ecology; Remember the goat they put into the T-Rex enclosure in Jurassic Park? It’s kind of like that but with lots of replicates and on a much smaller scale. By tethering urchins and placing them in the field, previous studies have evaluated relative rates of predation on urchins across different sites. (It’s only relative because the process of tethering may itself alter predation rates).

In my case, however, the question is not about predation on urchins, but about the impact of urchin feeding on the rest of the biological community. In other words, I’m not interested in how many goats the T-Rex will eat – but in how the goats impact the grass and flowers and shrubs when they are present.  Goats have a big impact on the biological community (just type ‘rent a goat’ into your google browser if you want proof); How about urchins?

In order to use tethered urchins as an experimental “treatment” in the field, I need to first see whether being tethered affects their feeding behavior.  So this is what I’m doing now at MaST aquarium. I’ve got 24 “stalls” housing individual urchins.  Half of them are tethered and half are not.  They have each been given a pre-weighed quantity of food.  I am running multiple trials of this lab experiment, but in each trial, I re-weigh the remaining algae from each urchin stall to see how much they’ve eaten after a few days.  I should have a comparison of feeding rates between tethered and un-tethered urchins compiled and available within a few weeks. Assuming all goes well, it will then be off to field for these little guys, where they’ll have the opportunity to eat their way through macroalgae that has grown wild. Just like rent-a-goats.

Urchins in stalls with algae

Tethered and untethered urchins in their stalls with pre-weighed pieces of macroalgae.

This Week In The Lab

Urchin corrals

I have installed the first of what will likely be several different contraptions to contain urchins. It’s an urchin corral! You can see it here in the kiddie pool tanks at MaST aquarium:

Photograph of urchin corral out of the waterClose up of urchin corral

It’s kind of like a little urchin boxing ring!

If you’re wondering why on Earth I’m doing this, you might check out this previous post.  These urchins will eventually be used in a field experiment to test how their feeding affects the subtidal communities on artificial rocky substrates.  In order for the field experiment to work, I need to be able to keep the urchins from wondering off experimental plots.Photo of urchin pressing against edge of corral

Enter little urchin boxing rings… this is likely the first of several iterations of the urchin corral/boxing ring idea.  Over the next week I’ll be trying to figure out whether urchins are escaping and how exactly their doing it.  If I can develop a design that contains urchins effectively, the next Photo of urchin corral in kiddie poolstep will be to put something like this into the field and evaluate whether it seriously alters flow, traps algae, excludes other important critters, or changes some other aspect of the physical environment on seawalls.

In its current form, the boxing ring/urchin corral is constructed from bolts protected by pieces of artificial sponge (thanks to Rus Higley at MaST aquarium for this fantastic idea) and monofilament line (this idea compliments of my advisor, Ken Sebens). I’ll let you know what happens!

This Week In The Lab

Urchins tanks are ready

Last week, I told you about experiments I would be starting at MaST aquarium in Des Moines, WA.  I’m happy to report that the urchin tanks (aka kiddie pools) are up and running! Urchins in kiddie pool A small number of specimens have been moved into the tanks.  I’ll let them acclimate for a week or so to ensure that everything is functioning properly.  Then, it will be time for experiments.

I’ll be trying a variety of different approaches – urchin corrals, sponge barriers, and tethering.  The objective is to find a way to contain urchins without significantly altering physical conditions (such as water flow).  With each method, and particularly with tethering, I’ll be testing whether the technique alters how urchins feed.  This is important because the urchins themselves will later be the experimental treatment in the field when I test whether urchin feeding alters the biological community on subtidal riprap.

Urchins in kiddie pools at MaST aquariumIf you have ideas about how you would contain urchins in a fixed area, I’d love to hear about it!  Submit an idea or drawing on the Contact Me page.  Or, send me your idea at:

Eliza Heery
University of Washington, Department of Biology
Box 351800
Seattle, WA 98195-1800

Photos of benthic habitat when urchins are present versus absent
This Week In The Lab

Herding urchins

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!