Sand dollars are marine invertebrates with flat, disc-shaped bodies. As a type of flattened sea urchin, they belong to the broader group of echinoderms, which also includes starfish and sea cucumbers. Living sand dollars are covered in short, velvety spines and fine cilia, which can appear in various colors. Their rigid outer skeleton, called a test, often displays a star-like or petal-shaped pattern, making them a common find on sandy coastlines.
How They Move
Sand dollars move across the seafloor using thousands of tiny spines covering their body. These spines, shorter and softer than those of sea urchins, provide traction and propulsion. Their coordinated waving allows the sand dollar to slowly glide or creep across sandy substrates. This deliberate movement helps them navigate in search of food or burrowing spots. Their flattened body shape also assists in reducing resistance as they move through the sand.
Sand dollars can also right themselves if overturned. If flipped by currents, they use spines and tube feet to slowly rotate back. This self-righting mechanism aids their survival in dynamic coastal environments. Their tube feet, while primarily involved in gas exchange and feeding, also contribute to this precise maneuvering.
Their Feeding Habits
Sand dollars are detritivores, feeding on decaying organic matter, diatoms, and microscopic organisms in the sand and water column. Their feeding mechanism uses numerous small spines and cilia to sweep tiny food particles from the sand or water towards their mouth.
Their mouth is centrally located on their underside, efficiently collecting food from the substrate. Grooves on their undersurface, lined with cilia, then transport the gathered particles to the mouth. Some species tip on their side to capture organic matter suspended in ocean currents. This continuous sifting of sediment processes organic material within their habitat.
Life Underground: Burrowing Behavior
Burrowing is a fundamental behavior for sand dollars, serving multiple functions for survival in sandy marine environments. They burrow to seek protection from predators like cod, flounder, and sheepshead fish. This also provides stability against strong currents and wave action, preventing them from being washed ashore or displaced. Additionally, it allows access to food sources embedded within the sediment.
Burrowing involves a coordinated effort of their short, velvety spines and body movements. By manipulating their spines, sand dollars dig into the sand, pushing sediment aside to create space. Different species vary in burrowing depth and angle, with some lying just beneath the surface and others burying deeper. This adaptation highlights their specialized lifestyle within the sediment.
Their Role in the Marine Environment
Sand dollars contribute to the marine ecosystem through burrowing and feeding, acting as sediment churners. This process, known as bioturbation, involves mixing sandy substrates. By burrowing and sifting for food, sand dollars aerate the sediment, improving oxygen levels for other benthic organisms. This churning also facilitates the recycling of nutrients trapped within the seabed, making them available for other marine life.
In the food web, they serve as both consumers and a food source. As detritivores, they process organic debris, converting it for re-entry into the nutrient cycle. They also become food for various marine predators, linking lower trophic levels to higher ones. Their presence contributes to the health and functionality of coastal sandy habitats.