Echinarachnius parma

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Geographic Range

This species is the common dollar of the North American east coast from New Jersey north. It is circumpolar and also occurs in Alaska, British Columbia, Siberia and Japan. (Fox, 2007)

Habitat

Sand dollars are found in the intertidal zones and a little deeper. Often their skeletons will wash ashore after a storm. They burrow into the sand for protection and for food (Banister and Campbell 1985).

Physical Description

This animal lacks the the five arms that are characteristic of the phylum but does posess the same five-part radial symmetry (Raven and Johnson 1999). It is generally about 5-10 cm in diameter when fully grown.

The shell has many small perforations that form a symmetrical petal-like design. The entire shell (or "test," since it is not truely a shell as it is covered by skin) is penetrated by many small, brown spines that give the shell a velvety appearance and enable the animal to move about. Since these creatures have found very effective hiding places in the sand, the spines no longer are needed for protection and have been modified ("Sand Dollar" 1997). The spines on the somewhat flattened underside of the animal allow it to burrow or to slowly creep through the sand. Fine, hair-like cilia cover the tiny spines. These cilia, in combination with a mucous coating, move food to the mouth opening which is in the center of the star shaped grooves on the underside of the animal (Page 2000). The opening for the anus is on the posterior edge of the test.

The holes in the test also allow for the tube feet of the characteristic water vascular system to protrude. These tube feet also aid in moving food to the mouth, as well as in burrowing. The test is divided into sections, each characterized by a calcerous plate that is fused to the next. The plates are either ambulacral or non-ambulacral, either with holes for the tube feet or without. This corresponds to the ambulacral grooves found in the sea stars (Anonymous 1998).

In addition to the small spines on the surface, there are small organs called pedicellariae. These organs are small jawlike structures that were once thought to be parasites.However, upon further observation, it was understood that these organs function in grooming the sand dollar and keeping would-be parasites away. This species posesses the smallest of these organs which functions mainly in grooming (Banister and Campbell 1985).

The calcareous test is what is commonly found washed up on the shore, sans the velvety spines and bleached by the sun. (Fox, 2007)

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    10 to 25 g
    0.35 to 0.88 oz

Reproduction

The sexes are separate, though there is little distinction between male and female. The possession of either gonad is all that separates the two. Gametes are released into the water column as in most echinoids, and most generally when the water is warm. The free-swimming larvae join the populations of plankton and metamorphose through several stages before the test begins to form and they become bottom dwellers (Page 2000).

Behavior

Due to their diminutive edible parts and relatively hard skeleton, few animals bother sand dollars. One animal found to enjoy them on occasion is the thick-lipped, eel-like ocean pout, Zoarces americanus (Page 2000). Other predators include several species of bony fish and sea stars (Grzimek 1972).

On the ocean bottom, sand dollars are frequently found together. This is due in part to their preference of soft bottom areas as well as convenience for reproduction (Page 2000). Not only do the sand dollars prefer to be at the bottom, they more specifically prefer to be burrowed in the sand. They accomplish this feat by pushing the anterior margin of their body through the substrate at a slight angle. Though many species of sand dollar can completely burrow into the sand in only a few minutes, the common sand dollar takes about 10 minutes to accomplish its task (Grzimek 1972). (Grzimek, 1972; Page, 2000)

Food Habits

This sand dollar burrows in the sand at the sea bottom feeding on algae and fragments of organic material found in the substrate. They scrape off substrate with large, triangular teeth that ring their mouth. The teeth in the center of the mouth are continually growing while being worn away at their free ends. Therefore, although they are held firmly in place by ligaments and other ossicles of the jaws, the teeth must be periodicaly shifted toward the mouth. This shifting is apparently accomplished by tiny muscles (Telford & Ellers 1997).

While burrowing they use their cilia-covered spines to move substrate to their mouth. Their tube feet aid in this process as well. Sand dollars generally feed on the detritus found in the substrate, but they will also feed on small plankton and algae (Grzimek 1972). (Grzimek, 1972; Telford and Ellers, 1997)

Economic Importance for Humans: Negative

There is no known adverse affect to humans.

Conservation Status

This species had relatively few predators and is quite hardy. Human intervention does not seem to have a great impact on the numbers of this sand dollar. However, there have been times when oil spills have threatened large populations.

Other Comments

For animals relatively high on the evolutionary scale, it is remarkable that a head has never been developed. While five-pointed symmetry or pentamerism is largely displayed by the adult sand dollar, larvae are bilaterally symmetrical (Banister and Campbell 1985). This particular sand dollar displays a somewhat bilateral tendency since it is an "irregular" sea urchin. This simply means that it is flattened and somewhat oval, tending toward a posterior and anterior end.

Contributors

Jessica Ables (author), Southwestern University, Stephanie Fabritius (editor), Southwestern University.

Glossary

Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

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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.

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coastal

the nearshore aquatic habitats near a coast, or shoreline.

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).

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

filter-feeding

a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.

fossorial

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

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.

holarctic

a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.

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Found in northern North America and northern Europe or Asia.

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).

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.

phytoplankton

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

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).

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

zooplankton

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

References

1997. "Sand Dollar" (On-line). Accessed April 15, 2000 at http://encarta.msn.com/.

Anonymous, 1998. Echinoderms. Encyclopedia Britannica. Chicago, Illinois: Encyclopedia Britannica Inc..

Banister, K., A. Campbell. 1985. The Encyclopedia of Aquatic Life. New York: Facts on File.

Fox, R. 2007. "Mellita quinquiesperforata -- Sand Dollar" (On-line). Invertebrate Anatomy OnLine. Accessed 07/28/09 at http://webs.lander.edu/rsfox/invertebrates/mellita.html.

Grzimek, B. 1972. Grzimek's Animal Life Encyclopedia: Volume 3: Mollusks and Echinoderms. Van Nostrand Reinhold.

Page, C. 2000. "The Common Sand Dollar" (On-line). Accessed April 9, 2000 at http://octopus.gma.org/Tidings/sanddollar.html.

Raven, P., G. Johnson. 1999. Biology, Fifth Ed.. Boston: WCB/McGraw-Hill.

Telford, M., O. Ellers. 1997. "Tooth-advancement Muscles in the Sand Dollar Echinarachnius parma" (On-line). Accessed April 10, 2000 at http://www.umesci.maine.edu/ams/IB1163.htm.