Pacific sand dollars are found along the Pacific North American coast from southern Alaska to Baja, California.
Biogeographic Regions: nearctic (Native ); pacific ocean (Native )
Pacific sand dollars will bury themselves in the sand to avoid predation by fish and invertebrates. In response to the presence of fish predators, larvae may asexually reproduce via budding and fission, creating smaller individuals and thereby reducing the chances of being consumed by predators due to their insignificance.
Known Predators:
Sand dollars were given their common name due to their resemblance in shape to silver dollars. A Pacific sand dollar's exoskeleton, or test, is a flat, bilaterally symmetrical disk with pale gray-lavender to purplish-black movable spines; southern specimens tend to be lighter in color than those from northern areas. Each spine is covered with tiny hair-like cilia, making live sand dollars fuzzy or velvety to the touch. A diagnostic trait is a unique, petal-like design superimposed on the aboral test surface. These designs, called petaloid ambulacra, reflect internal structure. These animals have tube feet; those located along the petalidium are used for respiration while those elsewhere are used for feeding and locomotion. The mouth is located on the underside of the test, also known as the oral surface. It contains five structures commonly referred to as “doves"; these can be seen after the test is broken. Doves are composed of teeth and jaws, forming a feeding structure unique to echinoids, known as Aristotle's lantern. Pacific sand dollars can grow larger than 100 mm in test length, with diameters up to 80 mm. Size is highly variable. In addition to size differences, adult Pacific sand dollars are distinguished from juveniles by four genital pores (gonopores), one in the middle of each petaloid ambulacrum.
Range length: 100 (high) mm.
Other Physical Features: ectothermic ; heterothermic ; radial symmetry
Sexual Dimorphism: sexes alike
The lifespan of Pacific sand dollars is estimated to be approximately 10 years on average.
Typical lifespan
Status: wild: 6 to 15 years.
Pacific sand dollar colonies inhabit nearshore, fine sand bottoms on level terrain. They can live on open coasts and in bays, tidal channels, and sheltered inlets with moderate water movement. Beds of numerous individuals run parallel to the shore in dense bands, which reform if interrupted by rough seas. Juveniles are found closer to the shore, but will gradually move seaward with age. The greatest population densities and largest individuals, measuring up to 9 cm in diameter, live in waters 4–12 deep. Deeper than 10–15 meters, individuals tend to be smaller, and size continues to decrease progressively with increasing depth. Pacific sand dollars are found as deep as 90 meters.
Range depth: 1 to 90 m.
Average depth: 8 m.
Habitat Regions: temperate ; polar ; saltwater or marine
Aquatic Biomes: benthic ; coastal
Other Habitat Features: intertidal or littoral
Pacific sand dollars suspension feed on algae, crustacean larvae, detritus, small copepods, and diatoms. Individuals capture and pass larger food items to their mouths using their pedicellariae (tiny pincer-like structures), tube feet, and spines. They may eat the larvae of their own species. Smaller food items such as detritus and diatoms are carried by cilia lining numerous food grooves on the oral surface to the mouth. Individuals can take up to 15 minutes to swallow their prey, and an additional 48 hours to completely digest their food.
Animal Foods: echinoderms; other marine invertebrates; zooplankton
Plant Foods: algae; phytoplankton
Other Foods: detritus
Primary Diet: carnivore (Eats non-insect arthropods, Eats other marine invertebrates); herbivore (Algivore); omnivore ; planktivore ; detritivore
Pacific sand dollars consume the planktonic larvae of other benthic invertebrates, influencing the species composition and diversity of the area surrounding their beds. These animals are hosts to parasitic flatworms and can be affected by the growth of barnacles on their tests (causing them to be more likely to be washed ashore). Kelp bass have been observed using the sharp edges of Pacific sand dollars to scrape off their own ectoparasites.
Commensal/Parasitic Species:
Though Pacific sand dollar tests are valued aesthetically, they have no significant economic importance to humans. There is an extensive literature on the molecular biology of sand dollar gametes and the chemical pathways involved in fertilization and cell division; these areas have been investigated to discover fundamental processes involved in cell differentiation, embryology and early stages of deuterostome development. This has greatly contributed to advances in medical research.
Positive Impacts: body parts are source of valuable material; research and education
This species has no negative economic effects on humans.
Eggs are light orange in color and have a protective coating which prevents adults from feeding on them. Fertilized eggs undergo a number of divisions. After reaching the first larval stage ("prism"), they develop two arms, achieving the main planktonic stage, called an echinopluteus. The echinopluteus continues to grow arms, eight in total, first postoral, then antereolateral and posterodorsal, and finally preoral arms. Larvae are transparent and develop calcareous skeletons while losing their protective jelly coats. After several weeks as an echinopluteus, the larvae develops an internal echinus, which is a rudimentary juvenile version of the adult, benthic body. When the echinus is large enough that it impedes swimming, the animal settles to the ocean floor where they complete metamorphosis into their adult form; this may take anywhere from three weeks to two months This long period of larval development means that juveniles can potentially widely disperse. There is evidence that this final metamorphosis is triggered by the release of a pheromone by adults and may be delayed if it is not present, ensuring that juveniles settle on substrate that is suitable for adult sand dollars.
Development - Life Cycle: metamorphosis
This species is abundant and there are no current conservation efforts on its behalf.
US Federal List: no special status
CITES: no special status
State of Michigan List: no special status
Juveniles and mature adults have sensory receptive cells on the rims of their primary podia and on a conical projection at the center of their podial sucker. The tip of each tube foot contains a nerve that receives tactile stimuli. Larvae are able to sense pheromones released by adults, which trigger their final metamorphosis.
Communication Channels: chemical
Other Communication Modes: pheromones
Perception Channels: tactile ; chemical
Pacific sand dollars spawning season occurs from July to August, occasionally extending into the late summer and early fall. Females and males each release gametes into the water, exhibiting broadcast spawning. The dense aggregations formed by this species encourage breeding success. This species is occasionally (but rarely) hermaphroditic.
Mating System: polygynandrous (promiscuous)
Pacific sand dollars are broadcast spawners and breeding season occurs during the summer and early fall. Hundreds of thousands of eggs are released and fertilized at a time. Larvae hatch just over a day after fertilization; they are free swimming and develop independently of adults, although chemicals released by adults trigger their final metamorphosis. On average, individuals reach sexual maturity by 4 years of age.
Breeding interval: Pacific sand dollars spawn once during their breeding season.
Breeding season: Breeding season is typically May through July, sometimes extending to the early fall, depending on locality.
Range number of offspring: 356,000 to 379,000.
Average number of offspring: 367,500.
Average gestation period: 28 hours.
Average age at sexual or reproductive maturity (female): 4 years.
Average age at sexual or reproductive maturity (male): 4 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (External ); broadcast (group) spawning
As broadcast spawners, neither males nor females exhibit any parental investment.
Parental Investment: no parental involvement
"Few predators are known, except for the Glaucous-winged Gull (Larus glaucescens), which can break the tests (shell) of exposed individuals and eat the soft parts...The flatworm Syndesmis dendrastrorum occurs in the intestine anywhere from the esophagus to the rectum." (Lambert, Austin 2007)
"A flat, round type of urchin up to 10 cm in diameter with tiny spines like fuzz, covering the surface... Blackism-purple, dark brown or grey. The posterior half is usually darker than the anterior which is usually buried in the sand. Dead animals that have lost their spines are white." (Lambert, Austin 2007)
"Juneau, Alaska, to northern Baja California; 0-90 metres." (Lambert, Austin 2007)
"The growth rate is fairly steady until about the fifth year, when it slows greatly." (Lambert, Austin 2007)
"Usually found along sheltered shores on sandy beaches near the low tide mark." (Lambert, Austin 2007)
"Few live longer than nine years." (Lambert, Austin 2007)
"The only other sand dollar in this region is Echinarachnius parma, which has the petal-like pattern in the centre of the test rather than toward the posterior edge." (Lambert, Austin 2007)
"The mouth is central and the anus is near the posterior edge on the oral side. The petal-like pattern on the dorsal surface corresponds to the five ambulacra of regular urchins. In this species it is offset from the centre toward the posterior. The anterior petal is longer than the others. The food grooves are concentrated on the posterior half of the test; they branch out from the mouth distally and have smaller side branches" (Lambert, Austin 2007)
"In the San Juan Islands, spawning usually occurs from mid April to July, but potentially from late March to late summer." (Lambert, Austin 2007)
"Dendraster excentricus is primarily a suspension feeder, rather than a deposit feeder like most other species of sand dollars. With the body vertical in the sand, they can capture larger suspended particles and some active prey. Barrel-tipped tupe feet extend outside the spines to capture dinoflagellates, small crustaceans, diatoms and pieces of algae. Tube feet beside and in the food grooves move the particles toward the mouth with the aid of mucus; cilia are not involved in moving the food. Near the mouth, oral tube feet and oral spines push the food into the mouth where it is collected and macerated by the jaws." (Lambert, Austin 2007)
Dendraster excentricus, also known as the eccentric sand dollar, sea-cake, biscuit-urchin, western sand dollar, or Pacific sand dollar, is a species of sand dollar in the family Dendrasteridae. It is a flattened, burrowing sea urchin found in the north-eastern Pacific Ocean from Alaska to Baja California.
Dendraster excentricus is an irregular echinoid that is flattened and burrows into the sand, unlike the regular echinoids, or sea urchins. It can be found living in the Pacific Ocean from Alaska to Baja California. The range for Dendraster excentricus is larger and includes the range of the other two extant species of Dendraster: D. vizcainoensis and D. terminalis. The flower pattern in this species is off-center, giving it the species name excentricus. Its test (skeleton) is compared to that of a sea urchin below.
They are colored gray, brown, black or shades of purple. Their size is variable, averaging 76 mm with the world's largest found measuring 120 mm wide.[2] 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 (oral) 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.
Dendraster Excentricus Sizes (World's Largest on Right)
Test of a dead D. excentricus
Oral side of D. excentricus
Diagram of aboral side
Diagram of oral side
D. excentricus aggregation at low tide
Comparison of test of Dendraster and sea urchin
They are either found subtidally in bays or open coastal areas or in the low intertidal zone on sandy on the Northeast Pacific coast. They 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 (0.85 sq m). It is the only sand dollar found in Oregon and Washington. It has been found on Burfoot Beach in the South Puget sound.
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.
This particular species of sand dollar is known for its curious behavior:
When exposed to a steady flow of water, they gather in groups, forming aligned rows in the sand, while digging their front edge in and raising their back edge into the flow of water, lined up so it passes from right to left across their bodies. Because the shape of a sand dollar is a hydrofoil, this draws particles of food closer in to their mouths during feeding, a benefit enhanced by the alignment of many individuals together into a communal feeding group.
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.
The first larval stage is called a prism. After this stage the embryo will develop two arms transforming itself into an echinopluteus larva. This is followed by the development of arms, until it reaches 8 arms all together. After this the larva develops an echinus or juvenile rudiment, which will become the juvenile. The nektonic larvae are pelagic and travel away from the parent group with the current. The developed larvae will receive a chemical cue from adults to settle down into a bed of sand dollars and begin to undergo metamorphosis to their adult sand dollar form. As adults they are benthos and stay on the sandy bottom.
Predators include the seastar Pisaster brevispinus and the starry flounder Platichthys stellatus as well as crabs and sea gulls. They are sometimes settled on by a small barnacle, Paraconcavus pacificus.[3] Large storms or high temperatures and desiccation can cause mass mortality if low tide coincides with a hot midday and the animals are exposed to air for just 2 to 3 hours or washed up and buried in the sand. Old age is thought to be the main cause of death of Dendraster excentricus. They may live up to 13 years and can be aged by counting growth rings on the plates of the test or by counting the pores in a petal of the petalidium.
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.
Dendraster excentricus, also known as the eccentric sand dollar, sea-cake, biscuit-urchin, western sand dollar, or Pacific sand dollar, is a species of sand dollar in the family Dendrasteridae. It is a flattened, burrowing sea urchin found in the north-eastern Pacific Ocean from Alaska to Baja California.
