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The sand dollar

The sand dollar ( Echinarachnius parma ) is the echinoid order of the phylum echinoderms, an invertebrate organism whose dried skeletons are found on beaches around the world. Living animals are brightly colored, but dried skeletons found on beaches are often white or greyish, with a star-shaped marking in their center. The common name assigned to these animals comes from the resemblance of their dried skeletons to a silver dollar coin. When alive, the sand dollar looks very different. They have a circular shape between 5 and 10 centimeters in diameter. They are covered with short, velvety spines, ranging in color from purple to reddish-brown.

Dry sand dollar exoskeleton. Dry sand dollar exoskeleton.

The sand dollar found on beaches is its dried exoskeleton, a structure of fused calcareous plates that is covered in living animals by skin and spines. The exoskeleton of the sand dollar is different from that of other echinoderms. For example, the exoskeleton of starfish is made up of small calcareous plates that are flexible, and the exoskeleton of sea cucumbers is made up of small calcareous formations inserted into the body. The upper surface of the dry sand dollar exoskeleton is patterned to resemble five petals, as seen in the figure above. From each of the five petals extend five tubules that the animal uses to breathe. The sand dollar’s anus is located on the back of the animal, at the edge of the skeleton below the single vertical line extending from the center of the five petals. The sand dollar moves using the spikes located on its underside.

Sand Dollar Taxonomy

The sand dollar belongs to the phylum echinoderms (Echinodermata, from the Greek ekhino , spike, and derma , skin) and, along with starfish, sea cucumbers, and sea urchins, their organisms have a radial arrangement. of five elements, with a body wall, an exoskeleton, formed by calcareous structures. Echinoderms are benthic marine organisms, they live on the seabed. The sand dollar belongs to the order of the echinoids (order Echinoidea), an order that groups together the sea urchins. In a traditional classification, but currently disputed, the echinoids are divided into two subclasses, the regularia , which groups together the hedgehogs, and the irregularia, which groups together sand dollars and sea biscuits.

In addition to the common, most widespread sand dollar species, Echinarachnius parma , there are other sand dollar species. The species Dendraster excentricus , the eccentric, western or Pacific sand dollar, is found on the coasts of the Pacific Ocean, from Alaska to Baja California, reaches a size of 10 centimeters in diameter and has spikes that range in color from gray to to purple and black. The species Clypeaster subdepressus , the sand dollar, lives in waters of tropical and subtropical regions; on the coasts of the Caribbean Sea and the Atlantic Ocean, from North Carolina to Rio de Janeiro in Brazil, and on the Atlantic coasts of Central America. The Mellitas sp .., the keyhole sand dollar or keyhole hedgehog, are eleven species that inhabit the tropical coasts of the Atlantic and Pacific oceans, and in the Caribbean.

The taxonomic classification of this organism is Echinarachnius parma (Lamarck 1816); kingdom Animalia, phylum Echinodermata, class Echinoidea, order Clypeasteroida, family Echinarachniidae, genus Echinarachnius , species Echinarachnius parma . The subspecies Echinarachnius parma obesus (Clark 1914) and Echinarachnius parma sakkalinensis (Argamakowa 1934) were also identified.

The habitat and habits of the sand dollar

The common sand dollar is an organism that is distributed along the coasts of the Northern Hemisphere, in warm waters, but also in the cold waters of Alaska and Siberia. Specimens of the common sand dollar have been found on the coasts of the North Pacific Ocean, from British Columbia in Canada to Japan, and in the North Atlantic Ocean. It inhabits sandy seabeds at depths greater than low tide, up to depths of 1500 meters. The number of individuals that develop in these sites is highly variable, from less than a sand dollar per square meter to more than 200 individuals per square meter.

The sand dollar. The sand dollar.

The sand dollar uses its spikes to burrow into the sand, seeking protection and food. These echinoderms feed on crustacean larvae, small copepods, diatoms, small algae, and organic debris. They incorporate small food particles that they extract from the sand and according to this diet they have been classified as carnivores by the World Register of Marine Species (WoRMS for its acronym in English). Food particles adhere to the spines and are then transported to the sand dollar’s mouth by its tubules, pedicellariae (pincers), and mucous-coated cilia. Some specimens rest on the sand on their edges to maximize their ability to catch floating prey.

Like other sea urchins, the mouth of a sand dollar is called an Aristotle’s lantern and is made up of five jaws. If you pick up a dried sand dollar skeleton and gently shake it, you might hear the mouth pieces echoing inside.

The sand dollar, like all echinoderms, is a marine animal, but some species thrive in estuaries, where freshwater draining into the sea mixes with brackish water. The properties of these habitats are different from marine and freshwater habitats, and tend to be highly variable. However, the sand dollar does not thrive in freshwater habitats and has been shown to require a certain minimum level of salinity to reproduce.

Sand dollar reproduction

The sand dollar has sexual reproduction. There is a male and a female, although they are not easily differentiated externally. Fertilization occurs when the female deposits the ovules and the male releases the sperm into the water. Fertilized eggs are yellow in color and covered by a protective gel; they have a diameter of about 135 microns (0.135 millimeters). When the eggs hatch, they develop into small larvae that feed and move using cilia. After several weeks, the larva settles to the bottom and undergoes metamorphosis.

Sand dollar juveniles are less than two inches in diameter and develop in the deeper areas at low tide. Then, as they mature, they slowly migrate to exposed areas of the beach. Juveniles can bury themselves in sand up to two inches deep, and where sand dollar populations are very dense, up to three animals may nestle at different depths.

Threats to the sand dollar

The sand dollar can be affected by fishing, especially that using bottom trawls. The acidification of the areas where its habitat is found affects the formation of its exoskeleton, and the decrease in salinity reduces the rate of fertilization. The sand dollar is not eaten by humans, but it can be preyed on by other organisms, such as starfish, fish, and crabs. We must remember to only collect dry sand dollar skeletons, never a living organism. The sand dollar is not currently listed as an endangered species.

Dried sand dollar skeletons are sold in shell and shell shops for decorative purposes or tourist souvenirs, sometimes accompanied by a card or inscription referencing the legend of the sand dollar. Reference to this legend is associated with Christian mythology, which mentions that the five-pointed star drawn in the center of the upper part of the dry skeleton of the sand dollar is a representation of the star of Bethlehem that guided the wise men of Orient, the so-called “Wise Men”, towards the baby Jesus. The five openings in the dried skeleton are said to represent Jesus’ wounds during his crucifixion, four on his hands and feet and the fifth on his side. It is also said that at the bottom of the dried skeleton of the sand dollar is drawn the outline of a Christmas poinsettia; and if you open it you will find five small calcareous formations that represent doves of peace. These dove figurations are actually the five jaws in the mouth of the sand dollar, Aristotle’s lantern. Another sand dollar lore relates its dried skeletons to mermaid coins or coins from Atlantis.

Sources

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