The captivating discs found on shorelines hold a deeper secret. These intricate objects are not just remnants of the ocean, but the skeletal remains of fascinating marine creatures. Understanding these unique animals reveals a hidden world beneath the waves.
What Exactly Are They?
Sand dollars are living animals belonging to a group of marine invertebrates called echinoderms. They are classified within the class Echinoidea, making them a type of flattened sea urchin. Other common names for these creatures include sea cookies or pansy shells. The familiar flat, disc-shaped object found on beaches is actually their rigid skeletal remains, known as a “test.” This test is composed of calcium carbonate plates, which are what give it a white appearance once it has died and dried out.
Unique Physical Features
A living sand dollar appears different from the bleached tests often collected. When alive, their bodies are covered in a dense layer of tiny, velvety spines, which can range in various colors depending on the species. These spines give them a fuzzy or hairy texture.
The distinctive petal-like pattern on their upper surface, known as petaloids, consists of five paired rows of pores. These pores are part of their ambulacral system, through which small tube feet extend for gas exchange. The mouth of a sand dollar is located on its underside, at the center of its body, while the anus is positioned towards the rear edge rather than on top, a notable difference from most sea urchins.
Life in the Ocean
Sand dollars inhabit sandy or muddy seafloors, primarily in shallow coastal waters. They often live in large groups, which aids in reproduction and finding food.
These animals use their numerous small spines and fine, hair-like cilia to slowly move across the seabed and to burrow into the sand. They can completely bury themselves to hide from predators or anchor against currents.
Their diet primarily consists of detritus, microscopic algae, and plankton particles, which they filter from the water or collect from the sand using their spines and cilia. Sand dollars contribute to the marine ecosystem by breaking down organic matter, helping to keep the ocean floor clean and recycling nutrients.
Finding and Understanding Them
Distinguishing a live sand dollar from a dead test is simple. Live sand dollars are dark in color and covered in tiny, moving spines. If held gently, the movement of their spines can be felt.
In contrast, dead sand dollar tests are typically bleached white, lack spines, and are rigid. When exploring beaches, it is important to collect only dead sand dollar tests. Many regions have regulations against taking live marine animals.
Live sand dollars should be gently returned to the water, as they cannot survive for long out of their natural environment. Even dead tests can be fragile, requiring careful handling.