Sand dollars (also known as a sea cookie or snapper biscuit in

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...

, or pansy shell in

South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...

) are species of flat, burrowing

sea urchin 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 ...

s belonging to the

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

Clypeasteroida. Some species within the order, not quite as flat, are known as '' sea biscuits''. Sand dollars can also be called "sand cakes" or "cake urchins".

Anatomy

Sand dollars are small in size, averaging from three to four inches. As with all members of the order Clypeasteroida, they possess a rigid

skeleton A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...

called a

test Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film), ...

. The test consists of calcium carbonate plates arranged in a fivefold

symmetric Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definiti ...

pattern. The test of certain species of sand dollar have slits called lunules that can help the animal stay embedded in the sand to stop it from being swept away by an ocean wave. In living individuals, the test is covered by a skin of

velvet Weave details visible on a purple-colored velvet fabric Velvet is a type of woven tufted fabric in which the cut threads are evenly distributed, with a short pile, giving it a distinctive soft feel. By extension, the word ''velvety'' means ...

-textured spines which are covered with very small hairs (

cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...

). Coordinated movements of the spines enable sand dollars to move across the seabed. The velvety spines of live sand dollars appear in a variety of colors—green, blue, violet, or purple—depending on the species. Individuals which are very recently dead or dying (moribund) are sometimes found on beaches with much of the external morphology still intact. Dead individuals are commonly found with their empty test devoid of all surface material and bleached white by sunlight. The bodies of adult sand dollars, like those of other

echinoid Sea urchins () are spine (zoology), 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 s ...

s, display

radial symmetry Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a pla ...

. The petal-like pattern in sand dollars consists of five paired rows of pores. The pores are perforations in the

endoskeleton An endoskeleton (From Greek ἔνδον, éndon = "within", "inner" + σκελετός, skeletos = "skeleton") is an internal support structure of an animal, composed of mineralized tissue. Overview An endoskeleton is a skeleton that is on the i ...

through which podia for

gas exchange Gas exchange is the physical process by which gases move passively by Diffusion#Diffusion vs. bulk flow, diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liqui ...

project from the body. The mouth of the sand dollar is located on the bottom of its body at the center of the petal-like pattern. Unlike other urchins, the bodies of sand dollars also display secondary front-to-back

bilateral symmetry Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a pla ...

. The anus of sand dollars is located at the back rather than at the top as in most urchins, with many more bilateral features appearing in some species. These result from the

adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...

of sand dollars, in the course of their

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

, from creatures that originally lived their lives on top of the seabed (

epibenthos 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.endobenthos). Echinocyamus_pusillus.jpg, ''

Echinocyamus pusillus ''Echinocyamus pusillus'', commonly known as the pea urchin or green urchin, is a species of sand dollar, a sea urchin in the family Fibulariidae, native to the northeastern Atlantic Ocean and the Mediterranean Sea. It buries itself in gravel or ...

'' alive. Live Sand Dollar trying to bury itself in beach sand.jpg, Living sand dollar. Group of Sand dollars in Monterey Bay Aquarium.jpg,

Eccentric sand dollar ''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 in the family Dendrasteridae. It is a flattened, burrowing sea urchin found i ...

s (''Dendraster excentricus'') at

Monterey Bay Aquarium Monterey Bay Aquarium is a nonprofit public aquarium in Monterey, California. Known for its regional focus on the marine habitats of Monterey Bay, it was the first to exhibit a living kelp forest when it opened in October 1984. Its biologists ...

. Echinarachnius parma (6806969731).jpg, ''

Echinarachnius parma ''Echinarachnius parma'', the common sand dollar, is a species of sand dollar native to the Northern Hemisphere. Kroh, A.; Mooi, R. (2020). World Echinoidea Database. Echinarachnius parma (Lamarck, 1816). Accessed through: World Register of Mari ...

'' (family

Echinarachniidae Echinarachniidae is a family of echinoderms belonging to the order Clypeasteroida 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 ...

). File:Keyhole sand dollar 01.jpg, ''

Mellita quinquiesperforata ''Mellita quinquiesperforata'' (Leske, 1778) is a tropical species of sand dollar, a flat, round marine animal related to sea urchins, starfish, and other echinoderms. They can be found along the eastern coast of the United States and the coast o ...

'' test (

Clypeasteridae Clypeasteridae is a family of sea urchins in the order Clypeasteroida. This family was first scientifically described in 1835 by the Swiss-American biologist Louis Agassiz. Genera The World Register of Marine Species list the following genera a ...

) File:Clypeaster reticulatus both sides.jpg, ''

Clypeaster reticulatus ''Clypeaster reticulatus'', the reticulated sea biscuit, is a species of sea urchin in the Family Clypeasteridae. This species was first scientifically described in 1758 by Carl Linnaeus. It lives on the sandy seabed of shallow seas, semi-immers ...

'' test (

) File:Echinodiscus2.jpg, ''

Echinodiscus tenuissimus ''Echinodiscus tenuissimus'' is a species of sea urchin. References Astriclypeidae {{echinoidea-stub ...

'' test ( Astriclypeidae) File:Clypeaster aegypticus (inside).JPG, '' Clypeaster aegypticus'', showing internal buttresses

Suborders and families

According to

World Register of Marine Species The World Register of Marine Species (WoRMS) is a taxonomic database that aims to provide an authoritative and comprehensive list of names of marine organisms. Content The content of the registry is edited and maintained by scientific specialist ...

: * sub-order Clypeasterina ** family

L. Agassiz, 1835 ** family Fossulasteridae Philip & Foster, 1971 † ** family Scutellinoididae Irwin, 1995 † * family Conoclypidae von Zittel, 1879 † * family

Faujasiidae Faujasiidae is a family of echinoderms belonging to the order Clypeasteroida 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 t ...

Lambert, 1905 † * family Oligopygidae Duncan, 1889 † * family Plesiolampadidae Lambert, 1905 † * sub-order Scutellina ** infra-order Laganiformes *** family

Echinocyamidae Echinocyamidae is a family of sand dollars. They are found mostly off the coast of Britain and Ireland and the North Sea, with scattered populations in the tropics. Description They are Laganiformes with: *simple radial internal buttresses alo ...

Lambert & Thiéry, 1914 *** family

Fibulariidae Fibulariidae is a family of echinoderms belonging to the order Clypeasteroida. Genera: * '' Cyamidia'' Lambert & Thiery, 1914 * ''Fibularia ''Fibularia'' is a genus of echinoderms belonging to the family Fibulariidae. The genus has almost a cos ...

Gray, 1855 *** family

Laganidae Laganidae is a family of echinoderms belonging to the order Clypeasteroida 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 ...

Desor, 1858 ** infra-order Scutelliformes *** family

Lambert in Lambert & Thiéry, 1914 *** family Eoscutellidae Durham, 1955 † *** family Protoscutellidae Durham, 1955 † *** family Rotulidae Gray, 1855 ** super-family Scutellidea Gray, 1825 **** family Abertellidae Durham, 1955 † **** family Astriclypeidae Stefanini, 1912 **** family Dendrasteridae Lambert, 1900 -- Pacific eccentric sand dollar. **** family Mellitidae Stefanini, 1912 -- Keyhole sand dollars **** family Monophorasteridae Lahille, 1896 † **** family Scutasteridae Durham, 1955 † **** family Scutellidae Gray, 1825 *** family

Taiwanasteridae Taiwanasteridae is a family of echinoderms belonging to the order Clypeasteroida 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 ...

Wang, 1984 ** family Scutellinidae Pomel, 1888a † Image:SandDollar2.jpg, Underside of live ''

'' Image:SandDollarsOnSeabed.JPG, A number of sand dollars on the seabed Image:Mitchelville Beach Park 021008 Sand Dollar.jpg, Sand dollar beneath the sand at

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 can ...

Hilton Head Island Hilton Head Island, sometimes referred to as simply Hilton Head, is a South Carolina Lowcountry, Lowcountry resort town and barrier island in Beaufort County, South Carolina, United States. It is northeast of Savannah, Georgia, and southwest of C ...

File:Key Biscayne FL Sea Biscuit Clypeaster rosaceus.jpg, Live sea biscuit, ''

Clypeaster rosaceus ''Clypeaster rosaceus'', the fat sea biscuit, is a species of sea urchin in the family Clypeasteridae. It occurs in shallow water in the western Atlantic Ocean and was first scientifically described in 1758 by Carl Linnaeus. Description This is ...

'', commonly found off

Key Biscayne Key Biscayne ( es, Cayo Vizcaíno, link=no) is an island located in Miami-Dade County, Florida, located between the Atlantic Ocean and Biscayne Bay. It is the southernmost of the barrier islands along the Atlantic coast of Florida, and lies sout ...

, Florida

Common name

The term "sand dollar" derives from the appearance of the tests (skeletons) of dead individuals after being washed ashore. The test lacks its velvet-like skin of spines and has often been bleached white by

sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...

. To beachcombers of the past, this suggested a large, silver coin, such as the old

Spanish dollar The Spanish dollar, also known as the piece of eight ( es, Real de a ocho, , , or ), is a silver coin of approximately diameter worth eight Spanish reales. It was minted in the Spanish Empire following a monetary reform in 1497 with content ...

(diameter 38–40 mm). Sand dollars are named as such not for their monetary value, but because of their appearance. Other names for the sand dollar include sand cakes, pansy shells, snapper biscuits, cake urchins, and sea cookies. In South Africa, they are known as pansy shells from their suggestion of a five-petaled garden flower. The Caribbean sand dollar or inflated sea biscuit, ''

'', is thicker in height than most. In Spanish-speaking areas of the Americas, the sand dollar is most often known as ' (sea

cookie A cookie is a baked or cooked snack or dessert that is typically small, flat and sweet. It usually contains flour, sugar, egg, and some type of oil, fat, or butter. It may include other ingredients such as raisins, oats, chocolate chips, n ...

); the translated term is often encountered in English.

Behavior and habitat

Sand dollars can be found in

temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...

and

tropical zone The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...

s along the Central and South American coasts, though they have been documented as far north as the eastern coast of the United States. Sand dollars live in warm waters below the mean low

water line The waterline is the line where the hull of a ship meets the surface of the water. Specifically, it is also the name of a special marking, also known as an international load line, Plimsoll line and water line (positioned amidships), that indi ...

, on or just beneath the surface of

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...

y and

mud A MUD (; originally multi-user dungeon, with later variants multi-user dimension and multi-user domain) is a Multiplayer video game, multiplayer Time-keeping systems in games#Real-time, real-time virtual world, usually Text-based game, text-bas ...

dy areas. The common sand dollar, ''

'', can be found in 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 as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...

from the

intertidal The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of Marine habitat, habitats with var ...

zone to the depths of the ocean, while the keyhole sand dollars (three species of the genus ''Mellita'') can be found on many a wide range of coasts in and around the

Caribbean Sea The Caribbean Sea ( es, Mar Caribe; french: Mer des Caraïbes; ht, Lanmè Karayib; jam, Kiaribiyan Sii; nl, Caraïbische Zee; pap, Laman Karibe) is a sea of the Atlantic Ocean in the tropics of the Western Hemisphere. It is bounded by Mexico ...

. The spines on the somewhat flattened topside and underside of the animal allow it to burrow or creep through the sediment when looking for shelter or food. Fine, hair-like cilia cover these tiny spines. Sand dollars usually eat algae and organic matter found along the ocean floor, though some species will tip on their side to catch organic matter floating in ocean currents. Sand dollars frequently gather on the ocean floor, in part to their preference for soft bottom areas, which are convenient for their

reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual or ...

. The sexes are separate and, as with most echinoids,

gametes A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce t ...

are released into the water column and go through

external fertilization External fertilization is a mode of reproduction in which a male organism's sperm fertilizes a female organism's egg outside of the female's body. It is contrasted with internal fertilization, in which sperm are introduced via insemination and then ...

. The

nekton 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 ...

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...

e metamorphose through several stages before the skeleton or test begins to form, at which point they become

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

. In 2008, biologists discovered that sand dollar larvae will clone themselves for a few different reasons. When a predator is near, certain species of sand dollar larvae will split themselves in half in a process they use to asexually clone themselves when sensing danger. The cloning process can take up to 24 hours and creates larvae that are 2/3 smaller than their original size which can help conceal them from the predator. The larvae of these sand dollars clone themselves when they sense dissolved mucus from a predatory fish. The larvae exposed to this mucus from the predatory fish respond to the threat by cloning themselves. This process doubles their population and halves their size which allows them to better escape detection by the predatory fish but may make them more vulnerable to attacks from smaller predators like crustaceans. Sand dollars will also clone themselves during normal asexual reproduction. Larvae will undergo this process when food is plentiful or temperature conditions are optimal. Cloning may also occur to make use of the tissues that are normally lost during metamorphosis. The flattened test of the sand dollar allows it to burrow into the sand and remain hidden from sight from potential predators. Predators of the sand dollar are the fish species cod, flounder, sheepshead and haddock. These fish will prey on sand dollars even through their tough exterior. Sand dollars have spines on their bodies that help them to move around the ocean floor. When a sand dollar dies, it loses the spines and becomes smooth as the exoskeleton is then exposed.

Evolution

The ancestors of sand dollars diverged from the other irregular echinoids, namely the cassiduloids, during the early

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

, with the first true sand dollar genus, '' Togocyamus'', arising during the

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...

. Soon after ''Togocyamus'', more modern-looking groups emerged during the

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

Folklore

Dead sand dollars are sometimes said to represent coins lost by

mermaid In folklore, a mermaid is an aquatic creature with the head and upper body of a female human and the tail of a fish. Mermaids appear in the folklore of many cultures worldwide, including Europe, Asia, and Africa. Mermaids are sometimes asso ...

s or the people of

Atlantis Atlantis ( grc, Ἀτλαντὶς νῆσος, , island of Atlas (mythology), Atlas) is a fictional island mentioned in an allegory on the hubris of nations in Plato's works ''Timaeus (dialogue), Timaeus'' and ''Critias (dialogue), Critias'' ...

. Some

Christian Christians () are people who follow or adhere to Christianity, a monotheistic Abrahamic religion based on the life and teachings of Jesus Christ. The words ''Christ'' and ''Christian'' derive from the Koine Greek title ''Christós'' (Χρι ...

missionaries A missionary is a member of a religious group which is sent into an area in order to promote its faith or provide services to people, such as education, literacy, social justice, health care, and economic development.Thomas Hale 'On Being a Mi ...

found

symbolism Symbolism or symbolist may refer to: Arts * Symbolism (arts), a 19th-century movement rejecting Realism ** Symbolist movement in Romania, symbolist literature and visual arts in Romania during the late 19th and early 20th centuries ** Russian sy ...

in the fivefold radial pattern and dove-shaped internal structures, comparing the holes with the crucifixion wounds of Christ, and other features with the

Star of Bethlehem The Star of Bethlehem, or Christmas Star, appears in the nativity story of the Gospel of Matthew chapter 2 where "wise men from the East" (Magi) are inspired by the star to travel to Jerusalem. There, they meet King Herod of Judea, and ask him: ...

, an Easter lily, a

poinsettia The poinsettia ( or ) (''Euphorbia pulcherrima'') is a commercially important flowering plant species of the diverse spurge family Euphorbiaceae. Indigenous to Mexico and Central America, the poinsettia was first described by Europeans in 1834 ...

, and

doves Columbidae () is a bird family consisting of doves and pigeons. It is the only family in the order Columbiformes. These are stout-bodied birds with short necks and short slender bills that in some species feature fleshy ceres. They primarily ...

References

External links

*
The Common Sand Dollar by Cheryl Page

Video showing the life cycle of ''Clypeaster subdepressus''
{{DEFAULTSORT:Sand Dollar Gnathostomata (echinoid) Articles containing video clips Extant Thanetian first appearances