Sand Dollar

Let’s go through sand dollar meaning. Sand dollar, any of the invertebrate marine animals with a flat, disk-shaped body that belong to the order Clypeasteroida (class Echinoidea, phylum Echinodermata). They are cousins to sea urchins and heart urchins. The sand dollar is uniquely adapted to burrow in sandy substrates. Tiny spines used during digging as well as crawling span the complete body and then are pressed backwards toward the posterior anus. The mouth is situated near the center of the underside of the body.

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The upper surface is pentaradially symmetrical, with a pattern of five "petals" radiating from the centre. Through the test, a few species discovered stranded on North American shores have five or six slots, as well as lunules (external skeleton). The majority of sand dollars range in size from 5 to 10 cm (2 to 4 inches). Except in Europe and Antarctica, species of comparable size can be found in shallow coastal waters around the world.

Purple sand dollar

Purple sand dollar — or "sea biscuit sand dollar," or "sand cake," in other parts of the world — is purple and hairy in its prime. It belongs to the Clypeasteroida order and lives in tropical and temperate bodies of water throughout the Northern Hemisphere.

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Black sand dollar

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Image of black sand dollar 

Black sand dollars can vary from a deep brown to a purplish-red color when the sea dollar alive. The sun causes the animal's colour to fade once it dies, or the skeleton soon turns silvery-white. 

Fast Facts about Sand Dollar

Species

Sand dollars are echinoderms, that also means they have a radiating arrangement of parts as well as a body wall stiffened by bony pieces such as spines, similar to sea stars, sea cucumbers, and sea urchins. They are, in fact, flat sea urchins and belong to the same Echinoidea class as sea urchins.

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There are two types of echinoids in this class: regular echinoids (sea urchins and pencil urchins) and irregular echinoids (heart urchins, sea biscuits, and sand dollars). On top of the "normal" pentameral symmetry (five parts around a centre) that regular echinoids have, irregular echinoids have a front, a back, and basic bilateral symmetry.

Sand dollars come in a variety of species. Aside from E. parma, the following are commonly found in the United States:

Dendraster excentricus (Eccentric sand dollar, Western sand dollar, or Pacific sand dollar) can be found from Alaska to Baja, California in the Pacific Ocean. These sand dollars have grey, purple, as well as blackish spines and grow to be about 4 inches across.

Clypeaster subdepressus (sea biscuit sand dollar) can be found from the Carolinas to Brazil in the Atlantic and Caribbean seas.

Melitta sp. (keyhole sand dollars or keyhole urchins) live in tropical waters such as the Atlantic, Pacific, and Caribbean Seas. There are about 11 different species of keyhole sand dollars.

The following are the different types of sand dollars:

Habitat and Distribution

Sand dollars have been discovered in the North Pacific and eastern North Atlantic oceans, at depths ranging from just below the intertidal zone to over 7,000 feet. Sand dollars, as the name implies, prefer to live in the sand, with densities ranging from.5 to 215 per 10.7 square foot. They burrow into the sand with their spines in search of protection and food. Adult sand dollars (those measuring more than 2 inches in diameter) live in the intertidal zone.

The majority of sand dollars live in seawater (saline environments), though some species can be found in estuarine habitats that are chemically distinct from saline or freshwater environments. According to research, sand dollars need a certain level of salinity to fertilise their eggs.

Anatomy

Sand dollars are small, ranging in size from three to four inches. Sand dollars, as with all members of the Clypeasteroida order, have a rigid skeleton known as a test. The calcium carbonate plates are arranged inside a fivefold symmetric pattern for the test. Certain sand dollar species' tests have slits called lunules that assist the animal stay integrated in the sand and avoid being swept away via an ocean wave. The test is encompassed by a skin of velvet-textured spines covered with very small hairs in living individuals (cilia).

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Sand dollars can move across the seabed thanks to coordinated movements of their spines. Depending on the species, the velvety spines of live sand dollars can be green, blue, violet, or purple. Individuals who have recently died or are dying (moribund) are occasionally found on beaches with much of their external morphology intact. Dead people are frequently discovered with empty tests that have been bleached white by sunlight and are devoid of all surface material.

The bodies of adult sand dollars, like those of other echinoids, have radial symmetry. The petal-like pattern of sand dollars is made up of five paired rows of pores. The pores are endoskeleton perforations through which podia for gas exchange project from the body. The sand dollar's mouth is located on the bottom of its body, in the centre of the petal-like pattern. Sand dollars, unlike other urchins, have secondary front-to-back bilateral symmetry on their bodies. The anus of sand dollars is located at the back instead of the top, as it is in most urchins, and some species have many more bilateral features. These are the results of sand dollars evolving from creatures which lived on top of the seabed (epibenthos) to life forms that burrow beneath it (endobenthos).

Physical Characteristics

Sand dollars and sea urchins are spine-covered sea creatures with five arms and prickly skin. The majority of sea urchins are nearly spherical and have five-way symmetry. Sand dollars are typically flat and have two-way symmetry. Sea urchins and sand dollars have rows of tube feet that run from the anus (AY-nuhs) on the pinnacle of the animal to the mouth (AY-nuhs) on the bottom. A small plate with holes at the top of the shell allows seawater to enter a system of tubes running via the body to the tube feet. Muscles contract to draw seawater into the tubes as well as transport it to the tube feet, which then expand due to the force of the water pressure.

Sea urchin and sand dollar shells are made up of tightly packed plates. If the shell is damaged, this design prevents cracks from spreading. The smallest urchin's shell is 0.2 inch (5 millimetres) across, while the largest is about 15 inch (38 centimetres) across.

All sea urchins have moveable spines, but the structure of the spines varies between species. Some spines are thick and blunt, while others are long, venomous, and pointed. Spines are used for movement, fighting predators, camouflaging from predators, and creating shade to protect from direct sunlight.

Types of Sand Dollars

Let’s go through the types of sand dollars.

Arrow head
Sea Gopher
Pancake
Flat Round

Diet and Behavior

Sand dollars eat small food particles in the sand, typically microscopically sized algae, but they also eat animal fragments and are classified as carnivores by the World Register of Marine Species. The particles settle on the spines and are carried to the sand dollar's mouth by its tube feet, pedicellaria (pincers), as well as mucous-coated cilia. Some sea urchins rest on the sand's edges to maximise their ability to catch passing prey.

The mouth of a sand dollar, like that of other sea urchins, is known as Aristotle's lantern and is composed of five jaws. If you gently shake a sand dollar test, you may hear the pieces of the mouth rattling inside.

Reproduction and Offspring

Male and female sand dollars exist, but it is difficult to tell which is which from the outside. Sexual reproduction occurs when sand dollars release eggs and sperm into the water.

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The fertilised eggs are yellow and covered in a protective jelly, with an average diameter of about 135 microns, or 1/500th of an inch. They grow into tiny larvae that feed and move with the help of cilia. After a few weeks, the larva sinks to the bottom and metamorphoses.

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Juveniles (less than 2 inches in diameter) are found in subtidal zones and gradually migrate to exposed beach areas as they mature; the smallest are discovered at the highest beach elevations. They are very small, that's why sometimes it's called baby sand dollar. They can borrow up to two inches deep in the sand, and in dense populations, they can stack up to three animals deep.

Threats

Fishing, particularly bottom trawling, could have an effect on sand dollars, as may ocean acidification, which may reduce the ability of the test to form; climate change, which may reduce available habitat; and collection. Fertilization rates decrease as salinity decreases. Although there is a wealth of information on preserving sand dollars, you should only collect dead sand dollars and never live ones.

Sand dollars are not eaten by humans, and they are also prey for sea stars, fish, as well as crabs.

Conservation Status

The sand dollar is not even an endangered species at the moment.

Sand Dollars and Humans

Sand dollar tests are sold for decorative purposes or as souvenirs in shell shops and on the internet, and are frequently accompanied by a card or inscription referencing the Legend of the Sand Dollar. Such references are associated with Christian mythology, sand dollar meaning non religious, implying that the five-pointed "star" in the centre of the sand dollar's test is a representation of the Star of Bethlehem, which leads the wise to the baby Jesus.

The five openings in the test are said to represent Jesus' crucifixion wounds: four in his hands and feet and one in his side. There is said to be an outline of a Christmas poinsettia on the underside of the sand dollar test, and if you break it open, you would then find five small bones that represent "doves of peace." The five jaws of the sand dollar's mouth (Aristotle's lantern) are represented by these doves. It means the sand dollar bones are present.

Other lore about sand dollars refers to the washed-up tests as mermaid coins or Atlantis coins.

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How do sand dollars reproduce?

These disk-shaped animals breed in colonies by releasing eggs and sperm into the water. When one individual begins to spawn, all of the others follow suit. Simultaneous spawning greatly increases the chances of fertilisation and species survival.

Spawning typically occurs from spring to August and is likely to occur multiple times. Females can lay over 350,000 eggs per year. The fertilised eggs float in ocean currents for several weeks as they go through a series of developmental changes, none of which we would recognise as a sand dollar.

Thousands of people are eaten by a wide variety of sea life during this time. Those who reach full maturity settle on the seafloor to become exact replicas of their parents.

Echinoderms, which means "spiny-skinned," are a type of animal that includes sand dollars. Sand dollars, like urchins, starfish, sea cucumbers, and other members of this group, feed primarily on organic matter mixed among sand grains. The mouth is located on the underside of the body and contains five grinding teeth.

Because of their burrowing habits, these flat, round animals are difficult to spot in the shallow waters where they live.

Most beachgoers are familiar with the smooth, white, and delicate sand dollars that wash up on shore. These are the skeletons of once-living sand dollars and are highly prized finds. Their white colour is the result of sun bleaching. You can often hear their five teeth rattling inside by gently shaking the fragile shell.

Live sand dollars, on the other hand, were also dark brown, sometimes purplish or greenish in colour, and covered in short, nearly invisible bristly spines.