Tire covered in green urchins
Happenings, Sites and Critters

Urchin take-over?

Plastic penguin statue (Seacrest Park, Jan 2015)

Green urchins, Strongylocentrotus droebachiensis, on a plastic penguin statue (Seacrest Park, Jan 2015)

There’s been much talk in the local dive community recently of an urchin take over in Puget Sound’s urban waterways. Divers at some of Seattle’s busiest dive sites have noticed a sudden influx of green urchins, Strongylocentrotus droebachiensis. They travel in hungry mobs, marching with the help of lots of little tube feet, presumably in search of greener pastures. At dive sites like Seacrest Park, in West Seattle, you can see them covering submerged objects by the hundreds, clearing the substrate of algae and smaller invertebrates as they go.

Traffic cone (Seacrest Park, Jan 2015)

Urchins on a traffic cone (Seacrest Park, Jan 2015)

Long-time divers note this is the first they’ve seen green urchins out in such force. What’s more, the apparent invasion seems timed with the recent die off of large sea star predators due to sea star wasting syndrome. In particular, the sunflower star, Pycnopodia helianthoides, was hit hard by the disease in the fall of 2013, and is known to be an important predator of green urchins. Any diver who’s seen green urchins clambering over one other in response to an approaching Pycnopodia will agree that urchins take the threat quite seriously. So perhaps the two events are connected….

Has the absence of Pycnopodia caused the urchin population to explode?  To address this, here are a few points I think are important to consider:

– The majority of green urchins that I’m seeing at sites in Seattle are 25-30mm in diameter or larger. Here’s the size frequency distribution I found back in November (below) when documenting the urchin barren that recently formed at Elliott Bay Marina Breakwater. The urchins I’ve seen at Seacrest Park are of a comparable size range.

Size frequency distribution of Strongylocentrotus droebachiensis at Elliott Bay Marina Breakwater in November 2014.

Size frequency distribution of Strongylocentrotus droebachiensis at Elliott Bay Marina Breakwater in November 2014.

– Based on published growth parameter estimates for S. droebachiensis (below; see Chapter 18 by Scheibling and Hatcher in  Edible Sea Urchins), we know that individuals of this size range (25mm in test diameter or greater) are at least 2-3 years of age. Green urchin recruitment occurs in the winter and spring, so we expect baby urchins that have settled since Pycnopodia populations disappeared in fall 2013 are no bigger than 15mm across at this point.

Growth curve for Strongylocentrotus droechiensis based on  published estimates of von Bertalanffy parameters.

Growth curve for Strongylocentrotus droechiensis based on published estimates of von Bertalanffy parameters.

Therefore, we cannot conclude that the high urchin densities we’re seeing at some sites are the result of an increase in population size. While juvenile urchins could very well be experiencing lower predation rates in the wake of sea star wasting syndrome, those that have actually arrived since the disease hit are not the ones we’re seeing out in force at dive sites like Seacrest Park.

So what is going on?

It’s important to note that urchins of all kinds are commonly found in very patchy distribution patterns. In some cases, this is the clear result of patchiness in predation, as Jane Watson and Jim Estes have documented beautifully in their work on red urchins around Vancouver Island (Watson and Estes 2011). In other cases, the reasons for their patchy distribution pattern are unclear.

It’s possible that the mass mortality of Pycnopodia has shifted the behavior of green urchins, allowing them to aggregate and travel more freely from one location to the next. The formation of urchin mobs may just as easily be the result of a localized depletion of algae and other food resources, however. Since we don’t even know where these urchin aggregations were living before they showed up at common dive sites, it’s quite hard to tease apart what caused their mobilization in the first place.

Lastly, I’ll add that patchiness in the distribution patterns of species like green urchins is a phenomenon that occurs over both space and time. Who knows what green urchin populations looked like 100 years ago, or even 500 years ago. If in fact the increase in urchin densities proves to be widespread and persistent, we can certainly expect to see some changes in the community of species that dominate Seattle’s subtidal ecosystems (urchins are quite effective at clearing space on hard structures and facilitate high turnover rates of sessile invertebrates and macroalgae). Whether such changes are “desirable” or “undesirable”, “natural” or “unnatural”, “destructive” or “restorative”, depends very much on your perspective and your objectives and goals for how urban marine ecosystems, which are in a sense our own creation, should function.

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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!

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