''Echinarachnius parma'', the common sand dollar, is a species of
sand dollar
Sand dollars (also known as sea cookies or snapper biscuits in New Zealand and Brazil, or pansy shells in South Africa) are species of flat, burrowing sea urchins belonging to the order Clypeasteroida. Some species within the order, not quite a ...
native to the
Northern Hemisphere
The Northern Hemisphere is the half of Earth that is north of the equator. For other planets in the Solar System, north is defined by humans as being in the same celestial sphere, celestial hemisphere relative to the invariable plane of the Solar ...
.Kroh, A.; Mooi, R. (2020). World Echinoidea Database. Echinarachnius parma (Lamarck, 1816). Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=158062 on 2020-12-31
;Subspecies:
* ''Echinarachnius parma obesus'' H.L. Clark, 1914
* ''Echinarachnius parma parma'' (Lamarck, 1816)
* ''Echinarachnius parma sakkalinensis'' Argamakowa, 1934
Distribution
It is found in the North Pacific and Northwest Atlantic, on the North American east coast from
New Jersey
New Jersey is a U.S. state, state located in both the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeastern regions of the United States. Located at the geographic hub of the urban area, heavily urbanized Northeas ...
north, as well as in
Alaska
Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...
,
Siberia
Siberia ( ; , ) is an extensive geographical region comprising all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has formed a part of the sovereign territory of Russia and its predecessor states ...
,
British Columbia
British Columbia is the westernmost Provinces and territories of Canada, province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that ...
, and
Japan
Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
. It inhabits isolated areas on sandy bottoms below the low
tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another.
Tide tables ...
level down to a depth of .
Description
The tests (shells) of these sand dollars are round, flat and disc-like, typically measuring in diameter. The growth rate for this animal is between 3.5 and 6 mm/yr in the latter 5 years of their lifespan, and the lifespan is typically around 8 years. The entire shell is also covered with maroon-colored moveable spines. The color is a purplish brown, becoming bleached white when deceased and washed ashore. This color occurs due to only the hard, calcified exoskeleton remaining, with further bleaching by sun and saltwater as it reaches the shore. As in other
echinoderm
An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...
s, they have five-fold radial symmetry, with a petal-shaped structure, or petalidium, on the aboral surface. The feet containing small hair-like structures (cilia) are located on this surface. The mouth, food grooves, and anus are located on the bottom side of the animal.
This and other species of ''Echinarachnius'' have been around since the
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58
Reproduction
The reproductive strategy of the common sand dollar includes both sexual and asexual methods, with the latter being particularly fascinating. Sand dollars typically reproduce sexually through external fertilization, where eggs and sperm are released into the water column. However, under certain conditions, particularly in the larval stage, sand dollars exhibit a form of asexual reproduction.
When faced with predation threats, sand dollar larvae can reproduce asexually through budding or fission. This response allows the larvae to split into smaller clones, which are less detectable by predators. The process not only increases the larvae's chances of survival but also enhances the population's ability to persist in predator-rich environments. This strategy is thought to be an evolutionary adaptation to reduce predation pressure on vulnerable larvae.
References
* Say, T. (1826). On the species of the Linnean genus Echinus, inhabiting the coast of the United States. Journal of the Academy of Natural Sciences of Philadelphia. 5, 225–229
* Gosner, K. L. (1971). Guide to identification of marine and estuarine invertebrates: Cape Hatteras to the Bay of Fundy. John Wiley & Sons, Inc. 693 p.
* Linkletter, L. E. (1977). A checklist of marine fauna and flora of the Bay of Fundy. Huntsman Marine Laboratory, St. Andrews, N.B. 68: p
* Mortensen, T. (1948). A Monograph of the Echinoidea. IV, 2. Clypeasteroida. Clypeasteridæ, Arachnoidæ, Fibulariidæ, Laganidæ and Scutellidæ. 471 pp., C. A. Reitzel, Copenhagen.
* Bromley, J.E.C., and J.S. Bleakney. (1984). Keys to the fauna and flora of Minas Basin. National Research Council of Canada Report 24119. 366 p
''Echinarachnius parma'' Animal Diversity Web