Dendraster excentricus
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''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 Sand dollars (also known as a sea cookie or snapper biscuit in New Zealand, or pansy shell in South Africa) are species of flat, burrowing sea urchins belonging to the order Clypeasteroida. Some species within the order, not quite as flat, are k ...
in the family Dendrasteridae. It is a flattened, burrowing sea urchin found in the north-eastern Pacific Ocean from
Alaska Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S. ...
to
Baja California Baja California (; 'Lower California'), officially the Free and Sovereign State of Baja California ( es, Estado Libre y Soberano de Baja California), is a state in Mexico. It is the northernmost and westernmost of the 32 federal entities of Mex ...
.


General information

''Dendraster excentricus'' is an irregular
echinoid Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...
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, averaging 76 mm with the world's largest found measuring 120 mm wide.
Sand dollar from Olympia beach may break a record
HeraldNet, August 21, 2013
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 Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
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 Ambulacrum is an architectural word that denotes an atrium, courtyard, or parvise in front of a basilica or church that is surrounded by arcades or colonnades, or trees, and which often contains a fountain A fountain, from the Latin "f ...
—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.
File:sand_dollar_sizes.jpeg, Dendraster Excentricus Sizes (World's Largest on Right) File:D.excentricustest.JPG, Test of a dead ''D. excentricus'' File:D.excentricusoral.JPG, Oral side of ''D. excentricus'' File:Aboraldendraster.jpg, Diagram of aboral side File:Oraldendraster.jpg, Diagram of oral side File:dendrasteraggregation.JPG, ''D. excentricus'' aggregation at low tide File:dendraster_017.JPG, Comparison of test of Dendraster and sea urchin


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. 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.


Behavior and feeding

Like its cousins, ''dendraster'' is a suspension feeder which feeds on
crustacean 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. The ...
, small copepods, diatoms,
plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...
, 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 A pedicellaria (plural: pedicellariae) is a small wrench- or claw-shaped appendage with movable jaws, called valves, commonly found on echinoderms (phylum Echinodermata), particularly in sea stars (class Asteroidea) and sea urchins (class Echinoi ...
, tube feet, and spines and pass them to the mouth. Their mouth includes a small
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ph ...
'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, 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
gonopore 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. More specifically, in the unmodified female it is t ...
s and they are fertilized by the male, who protrudes his
genital papilla The genital papilla is an anatomical feature of the external genitalia of some animals. In mammals In mammals, the genital papilla is a part of female external genitalia not present in humans, which appears as a small, fleshy flab of tissue. The p ...
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 echinus or juvenile rudiment, which will become the juvenile. The
nektonic Nekton or necton (from the ) refers to the actively swimming aquatic organisms in a body of water. The term was proposed by German biologist Ernst Haeckel to differentiate between the active swimmers in a body of water, and the passive organisms t ...
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 (), also known as benthon, is the community of organisms that live on, in, or near the bottom of a sea, river, lake, or stream, also known as the benthic zone.Pisaster brevispinus ''Pisaster brevispinus'', commonly called the pink sea star, giant pink sea star, or short-spined sea star, is a species of sea star in the northeast Pacific Ocean. It was first described to science by William Stimson in 1857. The type specime ...
'' and the starry flounder '' Platichthys stellatus'' as well as crabs and sea gulls. They are sometimes settled on by a small barnacle, '' Paraconcavus 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.


References


''Dendraster excentricus''
Intertidal Marine Invertebrates of the South Puget Sound

Walla Walla

San Francisco State University
''Sand dollar''
Monterey Bay Aquarium
''The persistence of a sand dollar''
OnEarth.org *Strathrnann M. 1987. Reproduction and Development of Marine Invertebrates of the Northern Pacific Coast. University of Washington Press, Seattle, Washington. 670 pp. *Marin Jarrin, Jose R. "Embriogenesis and Larval Stages of Dendraster excentricus". 2007. University of Oregon, Eugene, Oregon. *Smith, Andrew. 1984. Echinoid Palaeobiology. London: George Allen & Unwin.


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 Kingsley, Elliott Coues. Publisher S.E. Cassino and company, 1884, Pages 171–172. {{Taxonbar, from=Q2212339 Dendrasteridae