Dendraster excentricus
Encyclopedia
Eccentric sand dollar also known as the sea-cake, biscuit-urchin, western sand dollar, or Pacific sand dollar, is a member of the order Clypeasteroida, better known as sand dollar
s, a species of flattened, burrowing sea urchin
s found along the Pacific Ocean
from Alaska
to Baja California
.
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.
, small copepod
s, diatom
s, 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.
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.
s 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.
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.
and the starry flounder Platichthys stellatus as well as crabs and sea gulls. They are sometimes settled on by a small barnacle, Balanus pacificus. 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.
Ocean acidification and sea surface warming also harm populations of sand dollars.
Sand dollar
The term Sand dollar refers to species of extremely flattened, burrowing echinoids belonging to the order Clypeasteroida. Some species within the order, not quite as flat, are known as sea biscuits...
s, a species of flattened, burrowing sea urchin
Sea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...
s found along the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
from Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
to Baja California
Baja California
Baja California officially Estado Libre y Soberano de Baja California is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is both the northernmost and westernmost state of Mexico. Before becoming a state in 1953, the area was known as the North...
.
General information
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.Description
They are colored gray, brown, black or shades of purple. Their size is variable but up to 100 mm wide, and 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 aboralAboral
In biology, aboral surfaces are surfaces away from or opposite the mouth. The term is a compound of the Latin preposition ā, a, abs, meaning from or away from and the noun ōs, ōris n., meaning mouth. It is also the opposite of oral which is the end containing the mouth of a bilaterally...
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
Coelom
The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
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.
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 (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.Behavior and feeding
Like its cousins, dendraster is a suspension feeder which feeds on crustacean larvaeCrustacean larvae
Crustaceans may pass through a number of larval and immature stages between hatching from their eggs and reaching their adult form. Each of the stages is separated by a moult, in which the hard exoskeleton is shed to allow the animal to grow...
, small copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
s, diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
s, plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
, and detritus
Detritus
Detritus is a biological term used to describe dead or waste organic material.Detritus may also refer to:* Detritus , a geological term used to describe the particles of rock produced by weathering...
. 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
Pedicellariae
A pedicellaria is a small wrench or claw-shaped structure commonly found on Echinoderms, particularly in sea stars and sea urchins...
, tube feet, and spines and pass them to the mouth. Their mouth includes a small Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...
'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.
Hydrofoil design
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
Hydrofoil
A hydrofoil is a foil which operates in water. They are similar in appearance and purpose to airfoils.Hydrofoils can be artificial, such as the rudder or keel on a boat, the diving planes on a submarine, a surfboard fin, or occur naturally, as with fish fins, the flippers of aquatic mammals, the...
, 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.
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 gonoporeGonopore
A gonopore, sometimes called a gonadopore, is a genital pore in many invertebrates. Hexapods, including insects have a single common gonopore, except mayflies, which have a pair of gonopores...
s 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.
Development
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 echinusEchinus
Echinus may refer to:* Mallotus , synonym for a genus of the plant* Echinus , a genus of animals* Echinus , a window manager* Molding , similar to the ovolo molding...
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
Benthos
Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...
and stay on the sandy bottom.
Lifespan and predation
Predators include the seastar Pisaster brevispinusPisaster brevispinus
Pisaster brevispinus, commonly called the Pink Sea Star, Giant Pink Sea Star, or Short-spined Sea Star, is a Pacific Ocean sea star.-Description:...
and the starry flounder Platichthys stellatus as well as crabs and sea gulls. They are sometimes settled on by a small barnacle, Balanus pacificus. 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.
Conservation
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.
Further reading
- Rich Mooi. "Sand Dollars of the Genus Dendraster (Echinoidea:Clypeasteroida): Phylogenetic Systematics, Heterochrony, and Distribution of Extant Species". Bulletin of Marine Science, 61(2): 343–375, 1997.
- Friedrich von Hellwald - The standard natural history, Volume 1, The Standard Natural History, Elliott Coues, Editors John Sterling KingsleyJohn Sterling KingsleyJohn Sterling Kingsley ) was an American professor of biology and zoology.-Earrly Life:John Kingsley was born on 7 April, 1854 at Cincinnatus, New York son of Lewis and Julia A. Kingsley. His father, who was then County Judge and Surrogate, moved to Norwich in 1856 where Kingsley was raised and...
, Elliott Coues. Publisher S.E. Cassino and company, 1884 , Pages 171–172.