Sand Dollars: More Than Just A Cool Find on the Beach

Sand Dollars (Dendraster excentricus) are one of the coolest creatures that you can find on our beautiful ocean beaches. While you might think of them as being awesome finds, which they most certainly are, there is more to them than meets the eye. While you are most likely going to find Sand Dollars washed up on the beach, they are actual living creatures who make their homes on the sandy seafloors of the ocean. Their exoskeleton is generally white in color, although as you'll see in the photos, they can sometimes be gray. Sand Dollars have a unique five-pointed shape on their backs, which resembles flower petals and stars. Not only does this make it easy to identify them, it also mirrors their internal structure. The design is known as petaloid ambulacra. However, this unique design is hidden on live Sand Dollars, which look very different to those that have washed up on the beach. Live Sand Dollars are covered by tiny gray-to-purple colored spines that are densely packed. These spines hide the flower petal/star design.

Sand Dollars are invertebrates, which means that they are cold blooded and don't have backbones. They aren't particularly large creatures, as they can grow only up to 3.5 inches across (9 centimeters). However, this size is big enough to stand out amongst other critters on the beach. Sand Dollars typically crowd together in a particular area, with up to 625 of these fine creatures living in an area one square yard (.85 square meters) in size. They typically have a lifespan of six to ten years.

Sand Dollars are members of the Phylum Echinodermata of marine animals. They are related to Sea Urchins, Sea Cucumbers, and Sea Stars.Unfortunately, large pink Sea Stars do not act neighborly toward their Sand Dollar family members. Instead, they prey upon them, as do California Sheephead and Starry Flounders. Sand Dollars are also members of the Echinoidea class, the Gnathostomata superorder, and the Clypeasteroia order.

Sand Dollars eat a variety of interesting foods, including algae, small copepods (small aquatic crustaceans), Crustacean larvae, and detritus (organic matter that is produced by the decomposition of organisms). In their sandy seafloor habitat, Sand Dollars use their spines, which are assisted by tiny hairs known as cilia, to transport food particles along their bodies to a central mouth located on their bottom side. Their mouths are home to a jaw that consists of five teethlike sections. The teethlike sections help them grind up small plants and animals to be swallowed.  While you might think that Sand Dollars, having five sections of teeth, would enable them to chew their food faster than humans, which only have two rows of teeth, Sand Dollars actually take longer. Sometimes, they chew their food for fifteen minutes before they swallow it. It can then take an additional two days for the food to be digested.

On the surface of Sand Dollars' bodies, spines and pincers (referred to as pedicellariae) are used to capture plankton. These spines bunch themselves up into small cones that are shaped like triangles. When they do this, they mark a spot where captive crab larvae or amphipods or are being held to await their journey to the mouth of the Sand Dollar that caught them. Sand Dollars get around by using their spines to move along the. On the upper half of their bodies, spines perform double duty by also serving as gills.

Sand Dollars live in the low intertidal zone, a place that is nearly always underwater, except during the lowest of spring tides. The water there provides them with more protection, giving them a better chance at survival. However, they don't go too far out to sea, as their range only extends out approximately 130 feet (40 meters) from the shoreline. They live in this zone from the cold waters of Alaska all the way to the warmer waters off the coast of Baja California. You can find washed up Sand Dollars on all of the ocean beaches in this same zone, including here in Ocean Shores.

References

1. https://www.montereybayaquarium.org/animals/animals-a-to-z/sand-dollar

2. https://www.nwf.org/Educational-Resources/Wildlife-Guide/Invertebrates

3. https://oceanservice.noaa.gov/facts/intertidal-zone.html

4. https://www.montereybayaquarium.org/animals/animals-a-to-z/copepod

5. https://en.wikipedia.org/wiki/Sand_dollar

© Ian D. Caldwell, September 2021