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.