Sea biscuit shells are commonly found on beaches, intriguing many with their distinct shapes. These appealing objects are actually the skeletal remains of a marine invertebrate. Their unique appearance often sparks curiosity among beachcombers.
The Living Sea Biscuit
A sea biscuit is a living organism classified as an echinoid, placing it in the same group as sea urchins and sand dollars. These creatures typically live covered in short, velvety spines, which can be green, blue, violet, or purple in living specimens. They are burrowing animals, often found partially or entirely submerged in sand or mud. The sea biscuit plays a role in its marine ecosystem by feeding on organic matter and plankton found in sediment, using specialized structures around its mouth. The “shell” that beachcombers discover is the animal’s internal skeleton, called a test, which remains after the organism dies.
Understanding the Shell
Composed of calcium carbonate, the sea biscuit shell, or test, provides a durable structure, typically ranging from about 2 to 5 inches in diameter and generally thicker and more inflated than sand dollar shells. Their shape is often oval or slightly pentagonal, with a more rounded, elevated profile resembling a flattened ball. On the upper surface, a distinct five-petal pattern, known as petaloids, marks areas where specialized tube feet for gas exchange once extended. Unlike sand dollars, sea biscuit shells typically do not have lunules, which are distinct holes or slits. The overall symmetry of the sea biscuit’s test is pentagonal, a characteristic shared with other echinoderms.
Habitat and Discovery
Sea biscuits inhabit sandy or muddy seafloors in tropical and temperate marine waters worldwide. They are often found in shallow waters. These animals typically bury themselves just beneath the surface of the sand, moving through the sediment. After a sea biscuit dies, its durable test can be transported by currents and waves. This process often leads to their discovery washed ashore on beaches, particularly after storms.
Distinguishing from Sand Dollars
Sea biscuit shells are often confused with sand dollar shells due to their similar classification as echinoids, but several physical differences help distinguish them. Sea biscuit shells are generally thicker and more dome-shaped compared to the flatter, disc-like appearance of sand dollars. While both have a five-petal pattern on their surface, the petaloids on a sea biscuit shell are broader and more elevated. Sand dollars frequently have distinct keyhole-shaped lunules, or holes, in their tests, which are typically absent in sea biscuit shells. Additionally, a sand dollar’s surface is covered in small, disc-like spines, which are not present on a sea biscuit’s shell.