Various sandy-bottom-dwelling vascularozoans, better known as "sand wheels."
From top: Name: Buer's Larger Sprocket Species: Buer gigas Notes: The Buer's Sprockets are sediment-feeders, as are the majority of other sand wheels. They slowly "roll" by the coordinated movements of its spines, pedicellaria, and tubefeet digging into the substrate in order to allow the body of the animal to rotate. This also allows it to feed, in that, as it digs into the substrate, edible particles are captured, and moved into its mouth.
If disturbed, they bury themselves into the substrate with surprising celerity.
Name: Miniature Acorn Star Species: Microbalanaster bune Notes: It is not a sand wheel, but a starfish vascularozoan. It is a predator, and spends its time searching for buried molluscs/conchozoans. When it locates the burrow of its prey, it extends specialized tubefeet in order to inject a narcotic venom with which to sedate its victim. Once subdued, it pulls its victim out of the sand into its mouth. If faced with predators, acorn stars are capable of squirting their venom into the water, as well as quickly burying themselves in the substrate. Even so, acorn stars are eaten by a wide variety of predators, including fish, snails, and predatory vascularozoans, especially large serpent stars.
Name: Cookie Sprocket Species: Odontopita edulis Notes: Cookie sprockets are a cosmopolitan genus sand wheel with a 27 million year old pedigree. As with the Buer's sprocket, it is a wheel-like sediment feeder. In some areas, the populations are so dense that their broken tests are a major component of the sediment.
Name: Lesser Tombstone Species: Mnimakapella gryphaea Notes: This peculiar-looking species is a predatory sand wheel. Should small nekton approach it, they risk being entangled and ensnared with its hair-like tubefeet, and hidden, venomous pedicellaria. Larger, or struggling prey are subdued either by the animal falling on it, or by dragging the prey into the substrate and be smothered.
Name: Sand Tire Species: Trochotherium kykloides Notes: Sand tires are among the most wheel-like of all sand wheels. They are suspension-feeders that feed by filtering out edible particles that enter into its water-vascular system. Sand tires have lost their spines and reduced their pedicellaria, instead relying solely on their tubefeet for mobility. They resist jawed predators by relying on their tough, toxin-ladened bodies, and fast regenerative abilities.