By Nicole Webster
You can see the lovely holes in urchin tests used creatively. Credit: Norm Wiens
by Amanda Kahn
If you liked the SEM images from Getting to the Heart of Urchin Spine Attachment, then check out this recent post over at the Echinoblog. These echinoderms are not from around Bamfield, but they are done on regular urchins that we do have around BMSC, including Strongylocentrotus franciscanus and S. purpuratus (versus the irregular, or heart, urchins that I wrote about). Those two urchins can become so large, they’re like spikey grapefruits roaming around on the seafloor, mowing down algae in their path (in the case of urchin barrens) or hiding in cracks waiting for algae to drift by (in the case of kelp forests).
by Amanda Kahn
Sand dollars live in aggregations. In the time-series photos below, from research done in part by Dr. Fu-Shiang Chia, a researcher from the University of Alberta, sand dollars that were strewn haphazardly within a cleared area moved together to form dense aggregations. These aggregations can be as dense as 600 individuals per square meter. At that density, having a million sand dollars means having about 1.7 square kilometers of sand. To have a million people, well, that’s about the population size of Calgary.
Sand dollars have two modes of feeding: suspension feeding and deposit feeding. Suspension feeders remove particles from the water, including small drifting pieces of kelp but also smaller particles. Deposit feeding happens when the sand dollars snuffle along the seafloor, removing organic matter from around sand grains. A neat study in 2007 observed something neat about sand dollar feeding: the proportion of time spent feeding either as deposit- or suspension feeders depended on the density of the sand dollars present! A study published in 2007 found that as densities increased, the proportion of sand dollars that depended on deposit feeding (instead of suspension feeding) decreased. That means greater densities had to depend on what was in the water column versus what they could scrabble together from organic material on the seafloor.
This brings in questions about intraspecific competition, something I’ve been thinking a lot about lately. When the sand dollars change their mode of feeding based on density, that makes it seem like there is not enough food from deposit feeding alone that can sustain them. Instead, at higher densities, more sand dollars depend on suspension feeding, meaning food that has moved through the water column and flows and refreshes with the currents. Looking at the figure above, it looks like even at low densities, some sand dollars preferentially rely on suspension feeding anyway. I wonder if there is a point at which even suspension feeding cannot sustain the numbers of sand dollars (or other filter feeders) in a given body of water. Maybe currents change and suddenly there is less food than before, or filter feeders just get too efficient at pulling food from the water and deplete it continually.
All of this thinking is making me hungry. I think I’ll go forage now–some sand dollar cookies sound like good brain food for the train of thought I’ve been following! (Check out the recipe for the cookies below by clicking on the picture of the cookies.)
Hmm, looking back, this post wandered a bit…Getting back to the title of this post, if I had a million dollars…
…they’d probably all be suspension feeding. And cookies are delicious.
Birkeland, C., and F.S. Chia (1971). Recruitment risk, growth, age and predation in two populations of sand dollars, Dendraster excentricus (Eschscholtz). Journal of Experimental Marine Biology and Ecology, 6(3):265-278.
Fodrie, J.F., S.Z. Herzka, A.J. Lucas, and V. Francisco (2007). Intraspecific density regulates positioning and feeding mode selection of the sand dollar Dendraster excentricus. Journal of Experimental Marine Biology and Ecology, 340(2):169-183.
by Amanda Kahn
Hi folks! Just a short post today. I wanted to feature this amazing video I saw, produced by Rendezvous Divers, a diving resort operated in Barkley Sound. Since I recently wrote about the amazing predatory abilities of sea stars, I thought I’d follow up with this video of yet another animal’s dramatic response once it senses a sea star nearby.
Whoa!! That escape response must have cost the sea anemone so much energy! But I guess it’s better than being eaten. Have a great weekend, and keep an eye out for five-armed predators!
by Amanda Kahn
Sea stars are intense invertebrate predators. Like, eat-everything-in-sight voracious. It might be hard to imagine at first. Some sea stars are soft and look cuddly, like the Pteraster pictured below, which always makes me think of the stars from Super Mario Brothers games.
The Pteraster pictured above harbors a more sinister side. Namely, the oral side, where there are five mouth plates with mouth spines on each (that’s right, pentaradial symmetry means they don’t just have TWO jaws like we’re used to, but 5 chompers!).
But sea stars are slow-moving, aren’t they? They couldn’t possibly be effective predators against other invertebrates that can move…right? Well first of all, they’re not that slow. Those little tube feet can stick and unstick quite quickly, easily overtaking other benthic invertebrates. Crabs are quick, and usually can get away…but a race between a snail and a sea star isn’t really a race at all.
But as seen with the sea cucumber above, animals that normally move slowly have special escape strategies that, while energetically quite expensive, can work to free them from a sea star’s inexorable reach. Snails do a similar trick, torquing their shells around and causing them to tumble away downhill and, hopefully, down away from the sea stars. Scallops clap their two valves (shells) together, allowing them to swim away from the area with the offending predator. So there are strategies for avoiding the occasional sea star encounter, at least based on typical densities of sea stars.
However, ChrisM, founder and author of the Echinoblog, pointed out a phenomenon that has been noted a few times in the waters of British Columbia, in which super dense swarms of the tube-footed predators carpet the seafloor.
Check out his post for the full story, amazing pictures from Neil McDaniel, and some hypotheses for why these swarms might occur. I’ve reposted the info here because I’m curious–have any divers noticed this happening in the Barkley Sound area? Other accounts have been off the coast of Vancouver, within the Strait of Georgia, but I can’t imagine it would be too different in Barkley Sound, and we definitely have lots of divers that go out of Barkley Sound who could have noticed something like this. Leave a message in the comments below if you have, do check out the full story of these swarms on the Echinoblog, and try to forget that image of FIVE sets of mouth spines before you go to sleep tonight!
by Amanda Kahn
My first SCUBA dive in Bamfield was at a site called “Aguilar”, although by some it’s also called the “Love Shack.” I don’t know the history of the building that sits perched on the rocky intertidal zone in the cove (maybe a local can chime in on that in the comments below), but the picturesque cove is an interesting, curiosity-inspiring place above- and below-water.
A dive beneath the Love Shack features reveals a shallow field of cobbles housing sea squirts, sponges, snails, sea stars, and TONS of sea urchins. Aguilar sits at the northeastern tip of Bamfield’s West Side, which means that from the marine station, it is accessible only by boat. It is a good site for a training/practice dive, which is what we used it for, because it is protected from swells (it was a stormy day the day we dove), shallow, and there are open expanses on the seafloor where one can practice skills without quite as much worry about bumping into a fragile animal (though buoyancy control should never be undervalued).
This site is an urchin barren, meaning if you go, you’ll find a habitat that’s overrun with urchins. Urchins are often opportunistic feeders who sit in burrows and feed on kelp that happens to drift by. However, urchins in urchin barrens come out of their burrows and scour the benthos (seafloor), eating through live kelps. What results is a habitat that contains mostly urchins, and very few kelps. Pictured in the video below is an urchin barren filmed near BMSC.
Farther south, studies have shown that the loss of one of urchins’ key predators, sea otters, is implicated in facilitating the swap from a kelp forest habitat to one of an urchin barren. Whatever causes urchin barrens up here, once a habitat becomes an urchin barren it is fairly stable–it would be difficult to recover the habitat back to a kelp forest. Likewise, if a kelp forest is robust and healthy, it is difficult to convert it into an urchin barren. In the video below, notice that there are tons of urchins visible, but no kelp in sight.
This interesting phenomenon is called “alternate steady states” by ecologists.and is interesting because it shows that no one habitat is the best for a specific location. Can you think of other habitats, whether in land or in the water, where there can be alternate steady states? The urchin barren/kelp forest is the classic example, and I realize that I haven’t really thought about other possibilities. If you can think of some, please leave a comment below!
On the west coast you may have noticed sea urchins, specifically purple ones (Strongylocentrotus purpuratus) are hidden in crevices in the rocks, some that seem not only the perfect shape, but so deep the urchins can’t escape! This is not a trick of your mind, but ‘intentional’. The purple sea urchin is one of several species of urchin that actively bore into rocks, both by abrading with their spines and grinding with their teeth.
Some species, or younger urchins leave their protective home at night to graze, returning home during the day. In some cases the urchin bores too deep, or grows to quickly and becomes wedged in its hole. I’ve seen that at Keeha beach, but no photo available. In that case it depends on the food to drift into the hole.
The purple sea urchin is quite famous for it’s boring, and has even been noted boring directly into steel pilings in California!
I found another boring urchin in Cuba, I believe it is Echinometer lucunter – the red rock urchin or rock boring urchin. Don’t take this identification too seriously, simply matching photos and biogeography is not the most accurate way to make a positive ID.