About Echinarachnius parma (Lamarck, 1816)
Echinarachnius parma (Lamarck, 1816), commonly known as the common sand dollar, has round, flat, disc-like tests (shells) that typically measure 3 inches (7.6 cm) in diameter. Over the last five years of its lifespan, this species grows at a rate of 3.5 to 6 mm per year, and its typical total lifespan is around 8 years. The entire shell is covered in maroon-colored moveable spines, and the living sand dollar has a purplish brown color. When individuals die and wash ashore, their hard calcified exoskeleton remains, and it bleaches to white under exposure to sun and saltwater. Like other echinoderms, Echinarachnius parma has five-fold radial symmetry, with a petal-shaped structure called a petalidium on its aboral (upper) surface. Feet that bear small hair-like cilia are located on this upper surface, while the mouth, food grooves, and anus are located on the animal's underside. Species in the genus Echinarachnius, including this one, have existed since the Pliocene epoch. This sand dollar is distributed in the North Pacific and Northwest Atlantic. On the North American east coast, it ranges from New Jersey northward; it is also found in Alaska, Siberia, British Columbia, and Japan. It lives in isolated areas on sandy bottoms from below the low tide line down to a depth of 5,000 feet (1,500 m). The common sand dollar uses both sexual and asexual reproductive strategies. It typically reproduces sexually via external fertilization, where eggs and sperm are released into open water. However, asexual reproduction can occur under specific conditions, most often during the larval stage. When threatened by predators, sand dollar larvae can reproduce asexually through budding or fission. This process splits the original larva into smaller clones that are less easily detected by predators, increasing the individual larvae's chance of survival and improving the population's ability to persist in environments with high predator presence. This asexual reproductive response is considered an evolutionary adaptation that reduces predation pressure on vulnerable larvae.
