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.