Sand dollars are unique marine invertebrates, recognized by their flattened, disk-shaped bodies. These creatures are closely related to sea urchins and sea stars, all belonging to the echinoderm family. Live sand dollars are typically covered in tiny, dense spines, giving them a velvety texture and appearing in shades of gray, dark purple, or reddish-brown. They inhabit shallow marine environments, preferring sandy or muddy bottoms where they often gather in large groups. The distinctive five-petal pattern, commonly seen on the white shells found on beaches, is the internal skeleton, or test, that remains after the animal dies.
Reproduction
The life cycle of a sand dollar begins with sexual reproduction through broadcast spawning. During specific seasons, male and female sand dollars release their gametes—sperm and eggs—directly into the surrounding water. Sand dollars frequently aggregate in dense groups on the seafloor, which significantly increases the likelihood of successful external fertilization as gametes mix in the water column. Once a sperm fertilizes an egg, a zygote forms, marking the start of a new sand dollar’s development.
Larval Stages and Development
Following fertilization, the zygote undergoes rapid cell division. The developing embryo transforms into a free-swimming, planktonic pluteus larva. These microscopic larvae have delicate, arm-like projections, which help them move and sweep in food particles.
These larval stages are crucial for dispersal, allowing young sand dollars to drift with ocean currents and colonize new areas. For several weeks, the larvae feed on microscopic algae, plankton, and other organic matter. Some sand dollar larvae can clone themselves when sensing dissolved mucus from predatory fish, effectively doubling their numbers while reducing their individual size to evade detection.
Metamorphosis and Settlement
After weeks of drifting in the water column, the pluteus larva undergoes metamorphosis. This transitions it from a free-swimming, planktonic form to a benthic, bottom-dwelling juvenile. The larva sinks to the seafloor, where it undergoes rapid changes. During this process, larval structures are reabsorbed, and the creature develops the flattened body and internal skeleton characteristic of a miniature sand dollar.
Sand dollar larvae prefer fine sand and moderate currents for successful settlement. Chemical cues, such as pheromones released by adult sand dollars, can induce and accelerate this metamorphosis. Additionally, fluid turbulence can also prompt larvae to settle, signaling a suitable nearshore habitat.
Juvenile and Adult Life
After metamorphosis, the juvenile sand dollar begins its life on the seafloor. Initially, it lacks a fully formed skeleton, gut, or mouth, but these develop within seven days, allowing it to begin feeding independently. The juvenile grows into the familiar adult form, developing its characteristic five-fold symmetry and covering of tiny spines and cilia. Adult sand dollars use these spines and cilia to move and burrow into the sand, often burying themselves to avoid predators and currents.
They primarily feed on microscopic algae, bacteria, and organic particles in the sand, using their spines and tube feet to transport food to a central mouth on their underside. Some species also filter feed, capturing plankton from the water column. Sand dollars typically live for 6 to 10 years, though some can survive up to 15 years, and their age can be determined by counting growth rings on their skeletons. They play a role in marine ecosystems by consuming organic debris, helping to keep the ocean floor clean, and serving as a food source for various marine animals.