Dendraster excentricus

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Geographic Range

Pacific sand dollars are found along the Pacific North American coast from southern Alaska to Baja, California. (Mooi, 1997; Pilkey and Hower, 1960)

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. (Merrill and Hobson, 1970; Mooi, 1997; Tidwell, 2005)

  • Range depth
    1 to 90 m
    3.28 to 295.28 ft
  • Average depth
    8 m
    26.25 ft

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. (Cowles, 2010; Merrill and Hobson, 1970; Tidwell, 2005)

  • Sexual Dimorphism
  • sexes alike
  • Range length
    100 (high) mm
    3.94 (high) in

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. (Burke, 1984; Durham, 1978; Emlet, 1986; Grzimek, 2003)

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. (Durham, 1978; Grzimek, 2003; Merrill and Hobson, 1970; Morris, et al., 1980)

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. (Burke, 1984; Morris, et al., 1980)

  • 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

As broadcast spawners, neither males nor females exhibit any parental investment. (Morris, et al., 1980)

  • Parental Investment
  • no parental involvement

Lifespan/Longevity

The lifespan of Pacific sand dollars is estimated to be approximately 10 years on average. (Durham, 1978; Grzimek, 2003)

  • Typical lifespan
    Status: wild
    6 to 15 years

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. (Cowles, 2010; Grzimek, 2003; Merrill and Hobson, 1970)

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. (Burke, 1980; Burke, 1983; Burke, 1984)

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. (Cowles, 2010; "Sand Dollar", 2012)

  • Animal Foods
  • echinoderms
  • other marine invertebrates
  • zooplankton

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. (Cowles, 2010; Merrill and Hobson, 1970; "Sand Dollar", 2012; Vaughn and Strathmann, 2008)

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. (Merrill and Hobson, 1970; Smith, 1973)

Commensal/Parasitic Species
  • Syndesmis dendrastorum (Order Rhabdocoela, Phylum Platyhelminthes)
  • Balanus pacificus (Order Sessilia, Phylum Arthropoda)
  • Balanus tintinnabulum californicus (Order Sessilia, Phylum Arthropoda)
  • Melanella 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. (Grzimek, 2003)

  • 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. ("Dendraster excentricus", 2012; Grzimek, 2003)

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.

Glossary

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.

World Map

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.

World Map

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.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

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.

detritivore

an animal that mainly eats decomposed plants and/or animals

detritus

particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).

diurnal
  1. active during the day, 2. lasting for one day.
ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

external fertilization

fertilization takes place outside the female's body

fertilization

union of egg and spermatozoan

fossorial

Referring to a burrowing life-style or behavior, specialized for digging or burrowing.

herbivore

An animal that eats mainly plants or parts of plants.

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.

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.

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).

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.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

omnivore

an animal that mainly eats all kinds of things, including plants and animals

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

phytoplankton

photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)

planktivore

an animal that mainly eats plankton

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.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

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).

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

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).

zooplankton

animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)

References

2012. "Dendraster excentricus" (On-line). The IUCN Red List of Threatened Species. Accessed February 21, 2013 at http://www.iucnredlist.org/search.

Monterey Bay Aquarium Foundation. 2012. "Sand Dollar" (On-line). Monterey Bay Aquarium. Accessed October 31, 2012 at http://www.montereybayaquarium.org/animals/AnimalDetails.aspx?enc=Z5SIVkZ+n+VfIl75Mb+zkw==.

Burke, R. 1984. Pheromonal Control of Metamorphosis in the Pacific Sand Dollar, Dendraster excentricus. Science, 08/1984: 442-443. Accessed February 21, 2013 at https://www.researchgate.net/publication/6027690_Pheromonal_Control_of_Metamorphosis_in_the_Pacific_Sand_Dollar_Dendraster_excentricus.

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.

Cowles, D. 2010. "Dendraster excentricus (Eschscholtz, 1831)" (On-line). Accessed September 22, 2012 at http://www.wallawalla.edu/academics/departments/biology/rosario/inverts/Echinodermata/Class%20Echinoidea/Dendraster_excentricus.html.

Durham, J. 1978. Polymorphism in the Pliocene Sand Dollar Merriamaster (Echinoidea). Journal of Paleontology, 52(2): 275–286.

Emlet, R. 1986. Larval production, dispersal, and growth in a fjord: a case study on larvae of the sand dollar Dendraster excentricus. Marine Ecology: Progress Series, 31: 245-254. Accessed February 21, 2013 at http://www.int-res.com/articles/meps/31/m031p245.pdf.

Grzimek, B. 2003. Western Sand Dollar. Pp. 412 in M Hutchins, D Thoney, N Schlager, eds. Dendraster excentricus, Vol. 1, 2nd Edition. Farmington Hills, MI: Gale Group.

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.

Tidwell, M. 2005. Beyond the Beach Blanket: A Field Guide to Southern California Coastal Wildlife. Missoula, MT: Mountain Press Publishing Company.

Vaughn, D., R. Strathmann. 2008. Predators Induce Cloning in Echinoderm Larvae. Science, 319(5869): 1503.