Sand dollars are marine invertebrates, recognized by their flattened, disc-like shape. They are a type of sea urchin, classifying them within the class Echinoidea and order Clypeasteroida. Sand dollars typically inhabit sandy seafloors in temperate and tropical regions, often found just beneath the sediment surface. Their distinctive appearance, especially after they have washed ashore and bleached white, earned their common name.
Typical and Maximum Sizes
Most sand dollars generally measure between 2 to 4 inches (5 to 10 cm) in diameter. While many species fall within this average range, some can grow larger. The Pacific sand dollar, Dendraster excentricus, for instance, averages about 3 inches (7.6 cm) but can reach 4.7 inches (12 cm) in width.
The largest recorded sand dollar measured 5.8 to 6.3 inches (14.8 to 16 cm) across. These larger specimens highlight the variation in size that can occur within the broader sand dollar family. When found alive, these animals are covered in velvety spines and fine hairs, appearing in shades of green, blue, violet, or purple, rather than the white “shell.”
What Influences Their Size
A sand dollar’s size is influenced by species-specific characteristics and environmental factors. Different species of sand dollars have inherent genetic predispositions for varying maximum sizes. For example, some species are naturally smaller, while others, like the Pacific sand dollar, grow larger.
Age also plays a role, as older sand dollars typically grow larger, approaching their species’ maximum size. Scientists can estimate a sand dollar’s age by examining growth rings on its exoskeleton, similar to how tree rings indicate age. Environmental conditions, such as the availability of food like plankton and detritus, significantly impact their growth rate and dimensions. Water temperature and the quality of their habitat, including nutrient levels, can also affect their growth.
Life Cycle and Growth
Sand dollars begin their lives as microscopic larvae, developing from eggs fertilized externally in the water. These free-swimming larvae float in the ocean, gradually developing arms over several weeks. After this stage, they settle onto the seafloor, metamorphosing into a juvenile form resembling a miniature adult.
Growth is continuous throughout a sand dollar’s lifespan, slowing as they age. They develop their hard, disc-shaped skeleton, known as a test, as they mature. Sand dollars typically live for 6 to 10 years, allowing them to reach their full size. During this time, they use thousands of tiny spines and cilia to move, burrow into sand, and capture food particles.