Eccentric Sand Dollar
Dendraster excentricus
The sight of a sand dollar shell, whether it is in a photograph, sitting on your counter-top, or a patch, never fails to bring to mind the memories of a sprawling, west coast beach.
The Eccentric sand dollar is the only species of sand dollar in the Pacific Northwest. This does not in any way mean that they are rare, or that they are difficult to find. In fact, some beaches have populations of sand dollars so dense that children are unable to build sandcastles on them! Ironically, in British Columbia, it is the beaches of the Salish Sea that feature the most sand dollars, as opposed to the west coast beaches they are most often associated with. When visiting such beaches, it is very important to watch your step. Just because there are lots of sand dollars around, doesn’t mean they are any less special.
This artwork is inspired by my photograph of the Eccentric sand dollar below:
July 2018. The vacant test of an Eccentric sand dollar washed up on the beach near a sand dollar ‘bank’ in Oyster River, B.C.
📖 Description 📖
Sand dollars are very special animals for many more reasons than just their good looks! These animals are actually a variety of sea urchin, which may come as a surprise to many. It makes much more sense to imagine a sand dollar as a sea urchin that has been flattened like a pancake.
People are much more familiar with the empty, dried-out shell (or test) of the sand dollar than they are to the actual, living creature. In reality, living sand dollars are deep purple in colour and are covered in tiny, club-like spines. These spines help the sand dollar to dig through sand, and also assist in the capture of food. They can be difficult to see when viewing a sand dollar on the beach, but just like all echinoderms, sand dollars also have tube feet.
Eccentric sand dollars vary in size depending on the habitats they are found in. In some regions, they may reach up to 10 centimeters across [1].
March 2021: A closeup shot of an Eccentric sand dollar showing spines and tube feet. Some small portions of captured food can be seen between some of the spines. Photographed in Campbell River, B.C.
🌎 Distribution 🌎
The Eccentric sand dollar has been documented as far north as Haida Gwaii, and as far south as the tip of Baja California [2]. As one travels north along the coast of British Columbia, the number of beaches suitable as habitat for sand dollars diminishes, possibly explaining the limit of their range. To the south however, the Pacific coastline of the United States & Baja California could in some ways be interpreted as one giant beach!
Distribution of the Eccentric sand dollar, Dendraster excentricus. Suitable habitat depicted in red.
🏝 Habitat 🏝
As the Eccentric sand dollar is a big fan of beaches, it is no surprise that they are a species that is only rarely seen at depth. The vast majority of sand dollars live very close to the surface with many of them choosing to live in the intertidal zone. On occasion however, they have been spotted as far deep as 160 metres deep [3].
Depth of suitable habitat for the Eccentric sand dollar, Dendraster excentricus. Suitable habitat depicted in red. Not to scale.
Eccentric sand dollars are always found living in sand or mud, whether that be in the intertidal or subtidal zones. If you ever see a sand dollar trying to crawl its way over rocks or gravel, it might be lost! Their small knobby spines and short tube feet are developed for the express purpose of digging through sand & mud. Sand dollars spend much of their time at the surface, however at low tide, hot weather, and during storms, they will burrow into the sand for safety [4].
🌊 Diet 🌊
Eccentric sand dollars are planktivores, meaning that they feed on plankton (tiny, plant-like organisms and animals) that drift above the sands. When there is plankton in the water to eat and it is safe to feed, groups of sand dollars will ‘stand up’ on their edges, parallell with the surf or current. When plankton touches the sand dollar, it is captured with its spines and tube-feet, and is carried towards the mouth located on its underside. On occasion, Sand dollars will also search for organic debris underneath the sand.
February 2020: A collection of sand dollars in a feeding formation, perfectly aligned with the swell of the waves in Monterey, California.
💰 Life Cycle 💰
Sand dollars, like most echinoderms, are broadcast spawners, meaning instead of mating with each other physically, their eggs and sperm are instead released directly into the surrounding seawater. The sand dollars will find the perfect time to spawn, so that every sand dollar in the area is spawning within the same period of time to maximize the number of offspring produced.
Fertilized eggs will develop into something called a pluteus larvae. Interestingly, the pluteus is not quite what will develop into the sand dollar, but instead has the sand dollar growing inside of it. At a certain stage, the pluteus settles on the sea floor, ‘cracks open’, and the newborn sand dollar crawls out to begins its new life [5].
February 2022: Another angle of feeding Eccentric sand dollars in Monterey, California.
📚 References 📚
[1] Fretwell, K & Starzomski, B. (2013). Eccentric sand dollar. Central Coast Biodiversity. https://www.centralcoastbiodiversity.org/eccentric-sand-dollar-bull-dendraster-excentricus.html
[2] World Register of Marine Species. (n.d.) Dendraster excentricus (Eschscholtz, 1831). Retrieved January 27, 2022, from https://www.marinespecies.org/aphia.php?p=taxdetails&id=513215#distributions
[3] Ocean Biodiversity Information System. (n.d.) Dendraster excentricus (Eschscholtz, 1831). Retrieved January 27, 2022, from https://obis.org/taxon/513215
[4] Merill, R. J. & Hobson, E. S. (1970). Field Observations of Dendraster excentricus, a Sand Dollar of Western North America. The American Midland Naturalist. 83(2):595-624. https://www.jstor.org/stable/2423965
[5] Mcedward, L. R. (1984). Morphometric and metabolic analysis of the growth and form of an echinopluteus. Journal of Experimental Marine Biology and Ecology. 82(2-3): 259-287. https://doi.org/10.1016/0022-0981(84)90109-6
