The sea biscuit is an intriguing marine invertebrate whose bleached, shell-like remains are often found by beachcombers. These dome-shaped objects are the skeletons, or tests, of a type of echinoid, placing them in the same class as sea urchins and sand dollars. The common name refers to species within the genus Clypeaster that have a noticeably puffed-up appearance compared to flatter relatives. Understanding the organism’s biology helps clarify reports regarding its abundance and distribution.
What Exactly Is a Sea Biscuit?
The sea biscuit is an irregular sea urchin belonging to the class Echinoidea and the order Clypeasteroida, the same order as the sand dollar. Unlike globe-shaped “regular” sea urchins, these irregular forms have evolved a definitive front and back end, adapting them for burrowing and directed movement. The body is encased in a rigid, five-part calcium carbonate shell known as a test, which is the object typically found washed ashore.
The living animal is covered in a dense layer of short, fine spines and cilia, giving it a velvety texture and a color ranging from reddish-brown to dark brown. These structures are used for locomotion and for moving food particles toward a central mouth on the underside. Sea biscuits are detritivores, consuming organic matter mixed in with the sand and substrate.
Species like the inflated sea biscuit, Clypeaster rosaceus, are significantly thicker and more rounded than most sand dollars, sometimes reaching 8 inches in length. They possess a modified chewing apparatus called an Aristotle’s Lantern, used to grind food particles and sand. This robust structure allows some species to walk along the substrate surface rather than remaining completely buried, a behavior that differs from their flatter relatives.
Determining Rarity and Geographic Distribution
The rarity of a sea biscuit depends entirely on the location and whether one is seeking a live animal or a dead test. Living organisms are primarily found in tropical and subtropical marine waters worldwide. In the Western Atlantic, the inflated sea biscuit (Clypeaster rosaceus) is distributed from the Carolinas south through the Caribbean Sea and the Gulf of Mexico.
These echinoids prefer specific habitats, often residing in shallow waters with sandy bottoms, particularly within seagrass meadows, down to depths of about 160 feet. In these regions, such as the Gulf Coast of Florida and Caribbean islands, finding an intact, washed-up test is not uncommon. The rigid test is naturally more resilient to wave action than the thinner sand dollar test, increasing the likelihood of it remaining whole when it washes ashore.
The perception of rarity is complicated by the discovery of fossilized specimens in non-native areas. For example, reports of abundant sea biscuits on North Carolina beaches often refer to fossils dating back 34 to 70 million years. These ancient tests are churned up from offshore sedimentary deposits by storms or exposed during beach renourishment and dredging projects. Thus, while finding a modern sea biscuit outside the tropics is rare, finding a fossilized one can be locally common due to geological and human activity.
Distinguishing Sea Biscuits from Sand Dollars
Beachcombers frequently confuse sea biscuits with sand dollars, as both belong to the same order of irregular echinoids and possess the characteristic five-petal pattern. The simplest way to differentiate them is by examining the physical shape and thickness of the test. Sea biscuits are substantially thicker and more inflated, presenting a dome-like or ovate profile, which gives them their common name.
Most common sand dollars, such as those in the genera Mellita or Dendraster, are notably flatter and have a thin, coin-like appearance. Many sand dollar species, particularly the Keyhole Sand Dollar (Mellita isometra), feature distinctive perforations or slits called lunules that pass completely through the test. Sea biscuits, due to their robust shell structure, typically lack these lunules.
The five-petal pattern, which outlines the internal respiratory structures, is visible on both, but its presentation differs. On the sea biscuit, this petaloid area is often broad and raised above the rest of the upper surface. Identifying a specimen involves looking for the combination of an inflated, thick body and the absence of complete holes or slits, distinguishing it from its flatter sand dollar relative.