Appearance
They are colored gray, brown, black or shades of purple. Their size is variable, averaging 76 mm with the worlds largest found measuring 120 mm wide. They have a dome shaped carapace varying in height to about 10 mm with a circular body or test.Their body is covered with fine, spiny tube-like feet with cilia, and like other echinoderms they have five-fold radial symmetry. The mouth, anus, and food grooves are on the lower surface and the aboral surface has a petalidium, or petal shaped structure, with tube feet. Dead individuals have a gray/white test, or skeleton, which is often found washed up on beaches.
It has a water-vascular system from the internal cavity or coelom that connect with tube feet. The tube feet are arranged in five paired rows and are found on the ambulacra—the five radial areas on the undersurface of the animal, and are used for locomotion, feeding, and respiration. Spines are generally club shaped in adults, and less so in juveniles. The five ambulacral rows alternate with five interambulacral areas, where calcareous plates extend into the test. At the center on the aboral side is the madreporite—a perforated platelike structure, and on the interambulacra are the four tiny genital pores. Radiating out from the genital pores are the five flower petals, which represents the ambulacral radii. The mouth is in the center on the bottom side, with the anus toward the edge.
Status
The habitat they live in on the sandy seafloor is sometimes damaged by bottom trawling, causing harm to many organisms.Ocean acidification and sea surface warming also harm populations of sand dollars.
Behavior
Like its cousins, ''dendraster'' is a suspension feeder which feeds on crustacean larvae, small copepods, diatoms, plankton, and detritus. Adult sand dollars move mainly by waving their spines, while juveniles use their tube feet. The tube feet along the petalidium are larger and are used for respiration while tube feet elsewhere on the body are smaller and are used for feeding and locomotion. They frequently move around if they are lying flat. When feeding they usually lay at an angle with their anterior end buried and catch small prey and algae with its pedicellariae, tube feet, and spines and pass them to the mouth. Their mouth includes a small Aristotle's lantern structure found in most Echinoids. In high currents adults grow heavier skeletons while juveniles swallow heavy sand grains to keep from being swept away. They will bury themselves when they are being preyed on.
Habitat
They are either found subtidally in bays or open coastal areas or in the low intertidal zone on sandy on the Northeast Pacific coast. It can live at a depth of 40 to 90 meters, but usually is found in more shallow areas. Sand dollars are usually crowded together over an area half buried in the sand. As many as 625 sand dollars can live in one square yard . It is the only sand dollar found in Oregon and Washington. It has been found on Burfoot Beach in the South Puget sound.
Reproduction
Sexes are separate, with no noticeable differences in external features of the two sexes. Reproduction is sexual and ''D. excentricus'' reaches sexual maturity between 1 and 4 years of age, spawning in late spring and early summer. Fertilization is external, the female ''Dendraster'' discharges the eggs through her gonopores and they are fertilized by the male, who protrudes his genital papilla from his body wall. This is one reason they are believed to live in large groups and tend to release gametes at the same time into the water column. Eggs are pale orange, and are covered by a thick jelly coat which keeps adults from eating the eggs.
Food
Like its cousins, ''dendraster'' is a suspension feeder which feeds on crustacean larvae, small copepods, diatoms, plankton, and detritus. Adult sand dollars move mainly by waving their spines, while juveniles use their tube feet. The tube feet along the petalidium are larger and are used for respiration while tube feet elsewhere on the body are smaller and are used for feeding and locomotion. They frequently move around if they are lying flat. When feeding they usually lay at an angle with their anterior end buried and catch small prey and algae with its pedicellariae, tube feet, and spines and pass them to the mouth. Their mouth includes a small Aristotle's lantern structure found in most Echinoids. In high currents adults grow heavier skeletons while juveniles swallow heavy sand grains to keep from being swept away. They will bury themselves when they are being preyed on.References:
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