The sand dollar or Dendraster excentricus is one animal that continues to surprise me every time I see it. Long before I started learning about the ocean and marine life, I remember seeing faded-white sand dollar shells (former sand dollar endoskeletons) and not thinking much of them. Now I find sand dollars fascinating because they are just so strange. Drum roll please…here are seven swinging sand dollar facts:
1. Sand dollars live in…you guessed it! The sand. Typically, the species Dendraster excentricus is found close to shore in the low intertidal zone to as deep as 30 feet from Alaska to Baja California. (The low intertidal zone is the area close to shore that is usually covered with water except at very low tides.)
2. For sand dollars, living right next to each other in very large groups or beds is the way to go. Sometimes there are neighborhoods of sand dollars that are several square feet (suburban dwelling) and others stretch for miles across the sandy ocean floor (urban dwelling). (Here’s a good photo of sand dollars hanging out together.)
3.There are many species of sand dollars living around the world with a variety of common names including sea cookie and sand cake. Sand dollars also come in a variety of colors such as green, blue or black; the local California species, Dendraster excentricus, is purple.
4. Sand dollars are members of the phylum Echinodermata, which means they are echinoderms and closely related to sea stars, sea urchins and sea cucumbers.
5. It’s hard to believe, but this flat and round-shaped creature is designed for burrowing in the sand. Their bodies are covered with tiny spines (similar to a sea urchin’s spines but super small) that they use to dig. Once they’ve burrowed into a good position (see above photo) sand dollars keep their butt end above the sand’s surface to capture food.
6. The feeding strategy of sand dollars is fascinating. Their bodies are covered with tiny appendages to capture food particles small and large. Tiny cilia (extra small hairs) on the sand dollar’s spines sweep up small bits of food and tiny tube feet adeptly collect larger food pieces. Once food is caught the tiny appendages on the sand dollar work together to sweep food towards the mouth, which is located at the center of the five-petal flower pattern on bottom. The mouth has a five-toothed set-up called Aristotle’s lantern for chomping food. (Five pattern symmetry is a characteristic of echinoderms, sea stars have five arms, etc.)
7. Sand dollars reproduce by spawning; male sand dollars release sperm and female sand dollars release eggs into the water during spring. Reproduction is assisted by sand dollars living so close together. In fact, this post is very timely since it’s May 1st, coinciding with the beginning of sand dollar spawning season that continues through July. Sand dollars begin their lives as larvae and go through several larval stages before developing skeletons and settling on the ocean floor as the first step to adulthood. An adult sand dollar is about three inches in diameter and lives approximately eight years.
(Note: The above sand dollar facts are for the species Dendraster excentricus, other sand dollar species may have slightly different adaptations depending on where they live in the world.)