Habitat: Rock or coral reef, seagrass and algae beds, mangroves, estuaries, and numerous other habitats, even brackish and hypersaline waters. Distribution: Found in all but the extreme polar regions (though not reported from every Caribbean island). Natural History Notes: This species may reach densities of 500 individuals per square meter or 500 individuals per liter in algae. These densities are maintained by negative phototaxic behavior and by the tendency of individuals to roll into a ball and sink to the bottom. Individuals feed on suspended material, bottom detritus, and tiny plants and animals. Predators of the species include gobies and wrasses. Individuals are vivparous, self-fertilizing hermaphrodites. Those in the tropics have fewer embryos than those in temperate regions, and breed year-round, as opposed to seasonally. Eggs are small (0.1 mm in diameter) and embryonic development takes 3 to 7 months. Dispersal is thought to have occurred over large geographic distances despite the lack of a planktonic larval stage. Several species of copepod parasites affect the host. At least two species of microscopic mesozoan parasites disrupt reproduction and growth. Individuals of this species sometimes cling to large brittle stars, urchins, or sea cucumbers. Depth: Intertidal to 1,330 m. Characteristics: In the tropics, this small species reaches at most 3 mm in disk diameter with arms 11mm long; in temperate waters it can grow larger than 5 mm in disk diameter. Individuals have coarse scales on top of the disk. The disk is white, yellow, orange, pale brown to reddish, or gray, and the radial shields tend to have a white distal tip. The arms are pale brown, yellowish, or white, sometimes with thin dusky, green or brown bands.