Going Topless with Urchins

There’s nothing so satisfying as pulling back and seeing your brand new experiment out there in the water.

It’s been a crazy week or three getting this up and running, but now my first big postdoctoral experiment is soaking in the water, with urchins grazing away.

I’m testing some ideas regarding how diversity mediates the impact of disturbance by urchin grazing, and vice-versa – how disturbance by grazing can alter diversity. In essence, I’m testing a model of a community feedback process based on a framework whipped together by Randall Hughes, myself, and a few other fabulous co-authors.

But even though your ideas may be high-up and lofty, they always meet some interesting realities on the ground. Reality point 1 – my god, we built a lot of large cages.

This is about 1/4 of the cages before deployment. The rest were in the water. Thank fod for cheerful undergrad labor (fueled by brownies made from scratch – the key is to underbake them, and use a combination of eggs and egg yolks for extra gooey-ness) They look like such simple cheap affairs – some garden fencing, some PVC, some netting around the bottom…and then there’s about 1 ton worth of chain and half a ton of rebar stuffed into them. Subtidal work: unless it’s heavy, the waves will sweep it away.

Reality 2 – sometimes, you’ve gotta do it topless. Yes, the cages have no tops. This would seem the height of insanity if you want to keep something INSIDE. However, urchins appear to not like bendy flexy things. Sure, they’ll crawl up to the tops, but then they get to that wave strip at the margin, and freak out and freeze up. I’ve watched it. It’s kinda odd. And those cages that did have a top on them? That top, even if it’s mesh, creates a LOT of lift. So, a small wave washes by, and suddenly the cage top becomes an airplane wing. Unless you’ve added a huge amount of weight to your cage (see above), you may well never be able to find your cage again.

Reality 3 – nature is variable. Well, duh. See the two cages with two very different species compositions, som providing more or less biomass. I mean, the whole premise of this experiment was to use a natural gradient in species diversity as a treatment. But sometimes adding or subtracting one species can make a huge difference. Sampling (Reality 4 – ID-ing to the species level in the field on SCUBA gets pretty tedious after one hour, let alone 4 or 5) was pretty interesting, showing that large differences were indeed generated by both position on the reef, local topography, etc, as well as whether, say, tiny sea cucumbers had colonized a plot, whether the plot was full of lush Pterygophora, or the presence of the squat thick gorgonian Muricea.

Reality 5 – hungry urchins are hungry. And devious. Upon addition of urching to plots, they zoomed over to any brown algae (particularly the aforementioned Pterygophora or any juvenile giant kelp) and began munching in earnest. Some ran for the sides of the cages (and a few managed to squeeze out – Reality 6, the best laid plans of underwater mice and men… I’ll be doing some replacements this week with larger urchins). But the instand voracious consumption was really quite impressive.

I’m pretty stoked, and deeply curious as to how this will turn out. I’m sure there will be cursing, frustration, and bizarre results in the future, but for now, SCIENCE! Love it!

3 thoughts on “Going Topless with Urchins

  1. Pingback: The Story behind the Paper: Climate Change and Kelp Forest Food Webs « i'm a chordata! urochordata!

  2. Love your blog!
    And especially think this topless fencing idea to retain urchins in an area is fantastic and inspiring. Do you think other echinoderms like sea cucumbers might react similarly on flexible substrates? I am trying to find a way to similarly retain sea cucumbers without having to have a structure with a top and wondering if a similar principle could also apply? (Although probably a very different design and different mesh size etc.)

    • Huh. I’m not sure. Maybe? They really seem to deal poorly with the flexibility and narrow edge to move over. I watched a few try, and it was kind of adorable, if a bit pitiful. I would think larger cucs would work the same way, But they also have more flexibility, so it might be less of an issue. In fairness, this was a trick shown to me by Dan Reed from his days of urchin caging.

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