Sea biscuits are intriguing marine organisms that often capture the curiosity of beachcombers. Many people wonder about their prevalence, frequently asking if these unique finds are truly uncommon. Understanding what sea biscuits are and their natural behaviors helps clarify their actual abundance in marine environments. This exploration will delve into their biological characteristics, typical habitats, and the reasons why their intact remains are not frequently encountered on shorelines.
What Are Sea Biscuits?
Sea biscuits belong to the phylum Echinodermata, making them relatives of sea urchins, starfish, and sea cucumbers. They are specifically classified within the class Echinoidea, sharing this group with sea urchins and sand dollars. These marine animals are characterized by their distinct, somewhat inflated, and disc-shaped bodies, which are generally thicker than their flatter sand dollar cousins.
The typical size of a sea biscuit ranges from about 0.8 to 4.7 inches (2 to 12 cm) in length, with many averaging around 3 inches (7.6 cm). When alive, their external surface is covered in short, bristle-like spines and can display colors such as dark brown, light tan, reddish-brown, green, or red. Sea biscuits are slow-moving detritivores, primarily feeding on organic matter mixed within the sediment of the seafloor. They use a specialized jaw apparatus, sometimes called Aristotle’s lantern, to grind sand particles and extract nutrients.
Sea biscuits inhabit sandy or muddy seafloors, often found in shallow waters but capable of living at depths exceeding 1,000 feet (300 meters). They are distributed in tropical and subtropical marine waters worldwide, including the Western Atlantic Ocean from the Carolinas to the Caribbean. They prefer areas with minimal vegetation, though some species can be found in seagrass beds.
Are Sea Biscuits Truly Rare?
While finding intact sea biscuit skeletons, also known as tests, washed ashore can be less common, the living organisms themselves are not considered rare within their preferred habitats. Several factors contribute to the perception of their rarity on beaches. Their habitat preference is a significant reason; sea biscuits reside in slightly deeper waters, often below the low tide line, unlike some marine creatures that frequent the intertidal zone.
Sea biscuits exhibit a burrowing behavior, burying themselves several inches deep in the sand, making them difficult to spot when alive. They possess the ability to rebury themselves quickly, within 6 to 12 minutes, if disturbed. This habit keeps them concealed from predators and strong currents, further limiting their visibility to humans. Their tests are also quite fragile. After the animal dies, their delicate skeletons can easily break apart due to wave action, currents, or impacts from marine debris, reducing the number of whole specimens that survive the journey to the shoreline.
Occasional events, such as strong ocean currents, storms, or human activities like dredging, can bring large numbers of sea biscuit tests or fossils to beaches. This can create temporary abundance in specific areas. However, this localized and temporary increase in beach finds does not reflect a change in the overall population of live sea biscuits in the ocean. The abundance of sea biscuits can vary by geographical location, with some regions naturally supporting larger populations than others.
Sea Biscuits vs. Sand Dollars
Sea biscuits and sand dollars are often confused due to their similar appearances, but they have distinct differences. Both are marine animals belonging to the echinoderm phylum and are classified as irregular urchins within the class Echinoidea. Their forms diverge notably.
Sea biscuits are characterized by their more inflated, rounded, or disc-shaped bodies, giving them a thicker appearance. In contrast, sand dollars are much flatter and more circular, often resembling a coin or a thin cookie. While live sea biscuits have short spines that may be sparse, sand dollars are covered in thousands of tiny, hair-like spines that give them a velvety texture when alive. Both types of echinoids exhibit a five-petal flower-like pattern on their upper surface, a common feature of their taxonomic group. Sea biscuits inhabit slightly deeper offshore waters, whereas sand dollars are more commonly found in shallower, nearshore environments.