Sand dollars and starfish, despite their differing appearances, share a common ancestry within the marine world. They both belong to the same broader group of animals, highlighting an evolutionary connection. This shared classification explains some underlying biological similarities, even as their individual characteristics allow them to thrive in varied ocean environments.
The Echinoderm Connection
Both sand dollars and starfish are part of the phylum Echinodermata, a diverse group of marine invertebrates. The name “Echinodermata” translates to “spiny skin,” a reference to the calcified plates, called ossicles, embedded in their skin that form an internal skeleton. This endoskeleton provides both rigidity and protection.
A defining feature of echinoderms is their unique water vascular system, a hydraulic network of fluid-filled canals and external tube feet. This system is primarily used for locomotion, allowing them to move by alternately contracting muscles to extend their tube feet, which then push against the ground. The water vascular system also plays a role in food and waste transportation, gas exchange, and adhesion. Adult echinoderms also exhibit radial symmetry, meaning their body parts are arranged around a central axis, often in multiples of five.
Sand Dollar Characteristics
Sand dollars are a type of flattened sea urchin, adapted for life on sandy seafloors. Their body is disc-shaped, typically 2 to 4 inches across, and covered in densely packed, short spines that give them a velvety texture. These spines, aided by cilia, facilitate their movement across the sand or help them burrow into the sediment for shelter. Young sand dollars can even ingest sand grains to weigh themselves down in turbulent water.
The top surface of a sand dollar displays a distinctive five-pointed, petal-like pattern, with rows of pores where specialized tube feet protrude for gas exchange. Their mouth is located on the underside, at the center of this pattern. Sand dollars are filter feeders, using their spines and small pincer-like structures to capture plankton, detritus, and other tiny organic particles from the water or sand. They can grind their food using a jaw with five tooth-like sections.
Starfish Characteristics
Starfish, also known as sea stars, have a central disc with five arms radiating outwards, though some species can have many more. Their upper surface can be smooth, granular, or spiny, and they come in a wide array of vibrant colors. Starfish primarily use their numerous tube feet, located in grooves on the underside of their arms, for locomotion and to grip surfaces. These tube feet operate through their hydraulic water vascular system, allowing them to move slowly but with considerable power.
Many starfish are predators, often feeding on bivalves. To consume their prey, some species can evert their stomach to digest food externally. Starfish also possess the ability to regenerate lost or damaged arms. They inhabit all the world’s oceans, from shallow intertidal zones to deep-sea floors, thriving in various marine substrates.