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 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 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, 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 dollars (''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 ...
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 (
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ''
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 ...
''
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 on
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 ...
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 sou ...
, 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, ''
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 ...
'', 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 ...
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, '' Echinarachnius parma'', 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 ...
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 t ...
ic
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: ...
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 ...