Geographic Range
Pacific sand dollars are found along the Pacific North American coast from southern
Alaska to Baja, California.
- Biogeographic Regions
- nearctic
- pacific ocean
Habitat
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.
- Habitat Regions
- temperate
- polar
- saltwater or marine
- Other Habitat Features
- intertidal or littoral
Physical Description
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.
- Other Physical Features
- ectothermic
- heterothermic
- radial symmetry
- Sexual Dimorphism
- sexes alike
Development
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
Reproduction
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.
- Key Reproductive Features
- iteroparous
- seasonal breeding
- gonochoric/gonochoristic/dioecious (sexes separate)
- sexual
- fertilization
- broadcast (group) spawning
As broadcast spawners, neither males nor females exhibit any parental investment.
- Parental Investment
- no parental involvement
Lifespan/Longevity
The lifespan of Pacific sand dollars is estimated to be approximately 10 years on
average.
Behavior
Individuals aggregate in large populations on the seabed floor to exploit near-bed
currents in a manner that facilitates feeding as well as spawning for the entire group.
Individuals position themselves in rows, burying the anterior ends of their bodies
in the sand. This upright stance enables them to catch prey as it passes by on the
currents and increases water flow to all individuals in the bed. Juveniles are known
to swallow sand grains to weigh themselves down and avoid being tossed around by currents.
When lying flat, individuals tend to be active and crawl on the substrate.
Home Range
Individuals do not defend a territory or occupy a well-defined home range.
Communication and Perception
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
Food Habits
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.
- Primary Diet
-
carnivore
- eats non-insect arthropods
- eats other marine invertebrates
-
herbivore
- algivore
- omnivore
- planktivore
- detritivore
- Animal Foods
- echinoderms
- other marine invertebrates
- zooplankton
- Plant Foods
- algae
- phytoplankton
- Other Foods
- detritus
Predation
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.
Ecosystem Roles
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.
- Syndesmis dendrastorum (Order Rhabdocoela, Phylum Platyhelminthes)
- Balanus pacificus (Order Sessilia, Phylum Arthropoda)
- Balanus tintinnabulum californicus (Order Sessilia, Phylum Arthropoda)
- Balcis rutila (Order Sorbeoconcha, Phylum Mollusca)
- Kelp bass ( Paralabrax clathratus )
Economic Importance for Humans: Positive
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
Economic Importance for Humans: Negative
This species has no negative economic effects on humans.
Conservation Status
This species is abundant and there are no current conservation efforts on its behalf.
Additional Links
Contributors
Skye Allman (author), San Diego Mesa College, Sina Rubio (author), San Diego Mesa College, Paul Detwiler (editor), San Diego Mesa College, Jeremy Wright (editor), University of Michigan-Ann Arbor.
- Nearctic
-
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Pacific Ocean
-
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- temperate
-
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
- polar
-
the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.
- saltwater or marine
-
mainly lives in oceans, seas, or other bodies of salt water.
- benthic
-
Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.
- coastal
-
the nearshore aquatic habitats near a coast, or shoreline.
- intertidal or littoral
-
the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.
- ectothermic
-
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
- heterothermic
-
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
- radial symmetry
-
a form of body symmetry in which the parts of an animal are arranged concentrically around a central oral/aboral axis and more than one imaginary plane through this axis results in halves that are mirror-images of each other. Examples are cnidarians (Phylum Cnidaria, jellyfish, anemones, and corals).
- metamorphosis
-
A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.
- polygynandrous
-
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
- iteroparous
-
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
- seasonal breeding
-
breeding is confined to a particular season
- sexual
-
reproduction that includes combining the genetic contribution of two individuals, a male and a female
- fertilization
-
union of egg and spermatozoan
- external fertilization
-
fertilization takes place outside the female's body
- fossorial
-
Referring to a burrowing life-style or behavior, specialized for digging or burrowing.
- diurnal
-
- active during the day, 2. lasting for one day.
- nocturnal
-
active during the night
- motile
-
having the capacity to move from one place to another.
- sedentary
-
remains in the same area
- colonial
-
used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.
- chemical
-
uses smells or other chemicals to communicate
- pheromones
-
chemicals released into air or water that are detected by and responded to by other animals of the same species
- tactile
-
uses touch to communicate
- chemical
-
uses smells or other chemicals to communicate
- zooplankton
-
animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)
- phytoplankton
-
photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)
- detritus
-
particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).
- carnivore
-
an animal that mainly eats meat
- herbivore
-
An animal that eats mainly plants or parts of plants.
- omnivore
-
an animal that mainly eats all kinds of things, including plants and animals
- planktivore
-
an animal that mainly eats plankton
- detritivore
-
an animal that mainly eats decomposed plants and/or animals
References
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Burke, R. 1983. Neural Control of Metamporphosis in Dendraster excentricus . Biology Bulletin , 164: 176-188.
Burke, R. 1980. Podial Sensory Receptors and the Induction of Metamorphosis in Echinoids. Journal of Experimental Marine Biology and Ecology , 47/3: 223-234.
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Merrill, R., E. Hobson. 1970. Field Observations of Dendraster excentricus , a Sand Dollar of Western North America. American Midland Naturalist , 83(2): 595–624.
Mooi, R. 1997. Sand Dollars of the Genus Dendraster (Echinoidea: Clypeasteroida): Phylogenetic Systematics, Heterochrony, and Distribution of Extant Species. Bulletin of Marine Science , 61/2: 343-375. Accessed February 21, 2013 at http://www.ingentaconnect.com/content/umrsmas/bullmar/1997/00000061/00000002/art00009?crawler=true .
Morris, R., D. Abbott, E. Haderlie. 1980. Intertidal Invertebrates of California . Stanford California: Stanford University Press.
Pilkey, O., J. Hower. 1960. The Effect of Environment on the Concentration of Skeletal Magnesium and Strontium in Dendraster excentricus . The Journal of Geology , 68(2): 203–214.
Smith, N. 1973. A New Description of Syndesmis dendrastrorum (Platyhelminthes, Turbellaria), an Intestinal Rhabdocoel Inhabiting the Sand Dollar Dendraster excentricus . Biological Bulletin , 145/3: 598-606.
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Vaughn, D., R. Strathmann. 2008. Predators Induce Cloning in Echinoderm Larvae. Science , 319(5869): 1503.
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