Editing Sand dollar

{{short description|Order of sea urchins}}
{{Redirect|Sand Dollars|the 2014 film|Sand Dollars (film)}}
{{Automatic taxobox
| fossil_range = {{fossilrange|56|0}} <small>Late [[Paleocene]] to Recent</small><ref name="TPBDB">[https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=33109 The Paleobiology Database]</ref>
| name = Sand dollars
| image = Clypeaster reticulatus.jpg
| image_caption = A live individual of ''[[Clypeaster reticulatus]]'' ([[Mayotte]])
|taxon = Clypeasteroida
| subdivision_ranks = Suborders and families
| subdivision = See text.
}}

'''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 urchin]]s belonging to the [[Order (biology)|order]] '''Clypeasteroida'''. Some species within the order, not quite as flat, are known as [[Sea biscuit (echinoderm)|sea biscuit]]s. Sand dollars can also be called "sand cakes" or "cake urchins".<ref>{{cite news |last1=Kennedy |first1=Jennifer |title=Sand Dollar Facts |url=https://www.thoughtco.com/sand-dollar-profile-2291807 |work=ThoughtCo |date=9 October 2019 }}</ref>

==Names==
The term "sand dollar" derives from the appearance of the [[Test (biology)|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]]. To beachcombers of the past, this suggested a large, silver coin, such as the old [[Spanish dollar]], which had a diameter of 38–40&nbsp;mm.

Other names for the sand dollar include ''sand cakes, pansy shells, snapper biscuits, cake urchins'', and ''sea cookies''.<ref>{{cite web |title=Everything You Need to Know About the Sand Dollar |url=https://sanddollarshelling.com/shelling/everything-you-need-to-know-about-the-sand-dollar/ |website=Sand Dollar Shelling |date=29 November 2021 }}</ref> In South Africa, they are known as ''pansy shells'' from their suggestion of a five-petaled [[pansy|garden flower]]. The ''inflated sea biscuit'' or Caribbean sand dollar, ''[[Clypeaster rosaceus]]'', is thicker in height than most. In Spanish-speaking areas of the Americas, the sand dollar is most often known as ''{{lang|es|galleta de mar}}'' ("sea [[cookie]]"); the translated term is often encountered in English.

In the folklore of Georgia in the United States, sand dollars were believed to represent coins lost by [[mermaid]]s.<ref>{{cite book |last1=Bryant |first1=David |last2=Davidson |first2=George |title=Georgia's Amazing Coast: Natural Wonders from Alligators to Zoeas |date=2003 |publisher=University of Georgia Press |isbn=978-0-8203-2533-0 }}{{pn|date=April 2024}}</ref>

==Description==
[[File:Leodia sexiesperforata derivada 2013.jpg|thumb|''[[Leodia sexiesperforata]]'' by [[Louis Agassiz]] (1841)]]
[[File:Rotulidae.JPG|thumb|Examples of [[Rotulidae]]]]
[[Image:Encope emarginata (Leske, 1778) derivate 2013.jpg|thumb|''[[Encope emarginata]]'' (aboral and oral faces) by [[Ernst Haeckel]] (1904) ]]
[[Image:Clypeaster rosaceus (Linnaeus, 1758) derivate 2013.jpg|thumb|''[[Clypeaster rosaceus]]'' (aboral and oral faces) by [[Ernst Haeckel]] (1904)]]

Sand dollars diverged from the other irregular echinoids, namely the [[Cassiduloida|cassiduloids]], during the early [[Jurassic]],<ref>{{cite journal |last1=Kier |first1=Porter M. |title=Rapid evolution in echinoids |journal=Palaeontology |date=January 1982 |volume=25 |issue=1 |pages=1–9 |id={{BHL page|49707713}} |url=https://www.palass.org/publications/palaeontology-journal/archive/25/1/article_pp1-9 }}</ref> with the first true sand dollar genus, ''[[Togocyamus]]'', arising during the [[Paleocene]]. Soon after ''Togocyamus'', more modern-looking groups emerged during the [[Eocene]].<ref name="TPBDB"/>

Sand dollars are small in size, averaging from 80 to 100 mm (3 to 4 inches).<ref name=":0" /> As with all members of the order Clypeasteroida, they possess a rigid [[skeleton]] called a [[test (biology)|test]]. The test consists of calcium carbonate plates arranged in a fivefold [[Symmetry in biology|symmetric]] pattern.<ref>{{cite web | title = Sand Dollar Printout - Enchanted Learning Software | url = http://www.enchantedlearning.com/subjects/invertebrates/echinoderm/Sanddollarprintout.shtml | publisher = Enchanted Learning | year = 2000}}</ref> The tests 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.<ref name="Grzimek">{{Cite book|last=Grzimek|first=Bernhard |title=Grzimek's animal life encyclopedia |orig-date=2003|year=2004|publisher=Gale|others=Neil Schlager, Donna Olendorf, American Zoo and Aquarium Association|isbn=0-7876-5362-4|edition=2nd|location=Detroit|oclc=49260053}}{{pn|date=April 2024}}</ref> In living individuals, the test is covered by a skin of [[velvet]]-textured spines which are covered with very small hairs ([[cilia]]). 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]]s, display [[radial symmetry]]. The petal-like pattern in sand dollars consists of five paired rows of pores. The pores are perforations in the [[endoskeleton]] through which podia for [[gas exchange]] 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]] with no morphological distinguishing features between males and females. 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 [[natural selection|adaptation]] of sand dollars, in the course of their [[biological evolution|evolution]], from creatures that originally lived their lives on top of the seabed ([[Benthos#Epibenthos|epibenthos]]) to creatures that burrow beneath it ([[Benthos#Endobenthos|endobenthos]]).

<gallery style="text-align:center;" mode="packed">
Echinocyamus_pusillus.jpg|''[[Echinocyamus pusillus]]'' 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]]s (''Dendraster excentricus'') at [[Monterey Bay Aquarium]].
Echinarachnius parma (6806969731).jpg|''[[Echinarachnius parma]]'' (family [[Echinarachniidae]]).
File:Keyhole sand dollar 01.jpg|''[[Mellita quinquiesperforata]]'' test ([[Clypeasteridae]])
File:Clypeaster reticulatus both sides.jpg|''[[Clypeaster reticulatus]]'' test ([[Clypeasteridae]])
File:Echinodiscus2.jpg|''[[Echinodiscus tenuissimus]]'' test ([[Astriclypeidae]])
File:Clypeaster aegypticus (inside).JPG|''[[Clypeaster aegypticus]]'', showing internal buttresses
</gallery>

==Suborders and families==
According to [[World Register of Marine Species]]:

* sub-order [[Clypeasterina]]
** family [[Clypeasteridae]]<small> L. Agassiz, 1835</small>
** family [[Fossulasteridae]]<small> Philip & Foster, 1971</small> †
** family [[Scutellinoididae]]<small> Irwin, 1995</small> †
* family [[Conoclypidae]]<small> von Zittel, 1879</small> †
* family [[Faujasiidae]]<small> Lambert, 1905</small> †
* family [[Oligopygidae]]<small> Duncan, 1889</small> †
* family [[Plesiolampadidae]]<small> Lambert, 1905</small> †
* sub-order [[Scutellina (echinoid)|Scutellina]]
** infra-order [[Laganiformes]]
*** family [[Echinocyamidae]]<small> Lambert & Thiéry, 1914</small>
*** family [[Fibulariidae]]<small> Gray, 1855</small>
*** family [[Laganidae]]<small> Desor, 1858</small>
** infra-order [[Scutelliformes]]
*** family [[Echinarachniidae]]<small> Lambert in Lambert & Thiéry, 1914</small>
*** family [[Eoscutellidae]]<small> Durham, 1955</small> †
*** family [[Protoscutellidae]]<small> Durham, 1955</small> †
*** family [[Rotulidae]]<small> Gray, 1855</small>
** super-family [[Scutellidea]]<small> Gray, 1825</small>
**** family [[Abertellidae]]<small> Durham, 1955</small> †
**** family [[Astriclypeidae]]<small> Stefanini, 1912</small>
**** family [[Dendrasteridae]]<small> Lambert, 1900</small> -- Pacific eccentric sand dollar.
**** family [[Mellitidae]]<small> Stefanini, 1912</small> -- Keyhole sand dollars
**** family [[Monophorasteridae]]<small> Lahille, 1896</small> †
**** family [[Scutasteridae]]<small> Durham, 1955</small> †
**** family [[Scutellidae]]<small> Gray, 1825</small>
*** family [[Taiwanasteridae]]<small> Wang, 1984</small>
** family [[Scutellinidae]]<small> Pomel, 1888a</small> †

<gallery widths="200px" heights="180px">
Image:SandDollar2.jpg|Underside of live ''[[Mellita quinquiesperforata]]''
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, South Carolina|Hilton Head Island]]
File:Key Biscayne FL Sea Biscuit Clypeaster rosaceus.jpg|Live sea biscuit, ''[[Clypeaster rosaceus]]'', commonly found off [[Key Biscayne]], Florida
</gallery>

==Behavior and habitat==
[[File:Sand Dollar digging in.ogv|thumb|right|A sand dollar digging into the sand on the [[Playa Novillero]] beach at low tide on the Pacific coast of Mexico]]
[[File:Sand Dollar Cilia.webm|thumb|right|Spines on the underside of a sand dollar on the beach at [[Hilton Head Island, South Carolina]]]]

Sand dollars can be found in [[temperate zone|temperate]] and [[tropical zone]]s along all continents.<ref name=":0">{{Cite book|title=Encyclopedia of Animals|publisher=Great Neck Publishing|year=2017}}</ref> Sand dollars live in waters below the mean [[low tide]] line, on or just beneath the surface of [[sand]]y and [[mud]]dy areas. The common sand dollar, ''[[Echinarachnius parma]]'', can be found in the [[Northern Hemisphere]] from the [[intertidal]] zone to the depths of the ocean, while the [[keyhole sand dollar]]s (three species of the genus ''[[Mellita]]'') can be found on many a wide range of coasts in and around the [[Caribbean Sea]].

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.<ref name="Sayre 1996">{{Cite book|last=Sayre|first=April Pulley |title=Seashore|date=1996|publisher=Twenty-First Century Books|isbn=0-8050-4085-4|edition=1st|location=New York|oclc=34516888}}{{pn|date=April 2024}}</ref> 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.<ref name="Grzimek"/>

Sand dollars frequently gather on the ocean floor, in part to their preference for soft bottom areas, which are convenient for their [[reproduction]].{{why|date=February 2022}} The sexes are separate and, as with most echinoids, [[gametes]] are released into the water column and go through [[external fertilization]]. The [[nekton]]ic [[larva]]e metamorphose through several stages before the skeleton or test begins to form, at which point they become [[benthos|benthic]].

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 their original length which can help conceal them from the predator.<ref>{{cite journal |last1=Choi |first1=Charles Q. |title=Split Defense |journal=Scientific American |date=May 2008 |volume=298 |issue=5 |pages=38 |doi=10.1038/scientificamerican0508-38b |bibcode=2008SciAm.298e..38C }}</ref> 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.<ref name="Grzimek"/> 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.<ref name="Sayre 1996"/>

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.<ref>{{cite news |last1=Nelson |first1=Angela |title=9 Fascinating Facts About Sand Dollars |url=https://www.treehugger.com/things-you-dont-know-about-sand-dollars-4864334 |work=Treehugger |date=10 June 2022 }}</ref>

== Sex and reproduction ==
During the month of August, the sand dollar will have had its highest gonad index, and in November and December is the time when sand dollars will spawn.<ref>{{Cite journal |last1=Cocanour |first1=Barbara |last2=Allen |first2=Kenneth |date=1967-01-01 |title=The breeding cycles of a sand dollar and a sea urchin |url=https://www.sciencedirect.com/science/article/abs/pii/0010406X67907499 |journal=Comparative Biochemistry and Physiology |volume=20 |issue=1 |pages=327–331 |doi=10.1016/0010-406X(67)90749-9 |issn=0010-406X}}</ref> Broadcast spawning is how sand dollars reproduce.

For animals like the sand dollar (an [[echinoderm]]) their main mode of reproduction is [[Spawning|broadcast spawning]].<ref>{{Cite journal |last1=Leuchtenberger |first1=Sara Grace |last2=Daleo |first2=Maris |last3=Gullickson |first3=Peter |last4=Delgado |first4=Andi |last5=Lo |first5=Carly |last6=Nishizaki |first6=Michael T. |date=2022-12-01 |title=The effects of temperature and pH on the reproductive ecology of sand dollars and sea urchins: Impacts on sperm swimming and fertilization |journal=PLOS ONE |language=en |volume=17 |issue=12 |pages=e0276134 |doi=10.1371/journal.pone.0276134 |doi-access=free |issn=1932-6203 |pmc=9714736 |pmid=36454769|bibcode=2022PLoSO..1776134L }}</ref> An adult sand dollar's gender cannot be determined directly by just looking at it. The only way to determine the gender of the animal is by the color of its gamete exudate. The gamete exudate is the fluid released into the water during the spawning of sand dollars. Yellow exudate will contain eggs and white exudate will contain sperm.<ref>{{Cite web |title=Inducing Fertilization and Development in Sand Dollars {{!}} Embryo Project Encyclopedia |url=https://embryo.asu.edu/pages/inducing-fertilization-and-development-sand-dollars |access-date=2025-04-13 |website=embryo.asu.edu}}</ref>

By completing reproduction through spawning, sand dollars will reproduce in groups which increases the chances of fertilization. Male sand dollars start spawning by releasing sperm through the accessory papillae or the [[Gonopore|gonopores]]. After roughly 10 minutes, spawning stops, and the sand dollar will cover itself with sand. After the male spawns, the sperm enters the female's egg, beginning fertilization.

After roughly 80 minutes of fertilization, cell division will begin to occur, also known as [[Cleavage (embryo)|cleavage]]. After cleavage, the cells become [[Blastulation|blastulae]] and then [[Gastrulation|gastrula]]. During the gastrula stage, the embryo's height decreases and the width slightly increases. After the gastrula stage, the larvae will begin the pluteus stage. At this point in a sand dollar's early stages of life, it will want to feed but cannot. The sand dollar [[Larva|larvae]] will have mouths that are able to open but cannot eat.

Along with wanting to feed, the larvae will have three identifiable body parts: an esophagus, stomach, and intestine. Though at the beginning of the pluteus stage, the larvae cannot feed, it will feed towards the end of the stage just before [[metamorphosis]]. The larvae will push out the forming juvenile sand dollar and begin metamorphosis. For sand dollars, this stage takes about an hour and thirty minutes for the larval tissues to regress. Since the larval tissues are in regress, the sand dollar will then start to undergo resorption of these tissues. After metamorphosis, the sand dollar will start to become a juvenile, developing a skeleton, teeth, and will begin to be able to feed themselves.<ref>{{Cite journal |last1=Vellutini |first1=Bruno C. |last2=Migotto |first2=Alvaro E. |date=2010-03-22 |title=Embryonic, Larval, and Juvenile Development of the Sea Biscuit Clypeaster subdepressus (Echinodermata: Clypeasteroida) |journal=PLOS ONE |language=en |volume=5 |issue=3 |pages=e9654 |doi=10.1371/journal.pone.0009654 |doi-access=free |issn=1932-6203 |pmc=2842294 |pmid=20339592|bibcode=2010PLoSO...5.9654V }}</ref>

==References==
{{Reflist|40em}}

== External links ==
{{Commons category|Clypeasteroida}}
* {{ITIS |id=157984 |taxon=Clypeasteroida}}
* {{cite journal |last1=Mooi |first1=Rich |title=Paedomorphosis, Aristotle's lantern, and the origin of the sand dollars (Echinodermata: Clypeasteroida) |journal=Paleobiology |date=1990 |volume=16 |issue=1 |pages=25–48 |doi=10.1017/S0094837300009714 |bibcode=1990Pbio...16...25M |jstor=2400931 }}
* {{cite journal |last1=Ellers |first1=Olaf |last2=Telford |first2=Malcolm |title=Muscles advance the teeth in sand dollars and other sea urchins |journal=Proceedings of the Royal Society of London. Series B: Biological Sciences |date=22 October 1997 |volume=264 |issue=1387 |pages=1525–1530 |doi=10.1098/rspb.1997.0211 |pmc=1688700 }}
* {{cite journal |last1=Stock |first1=Stuart R. |title=Sea urchins have teeth? A review of their microstructure, biomineralization, development and mechanical properties |journal=Connective Tissue Research |date=January 2014 |volume=55 |issue=1 |pages=41–51 |doi=10.3109/03008207.2013.867338 |pmid=24437604 |pmc=4727832 }}
* [https://web.archive.org/web/20141104065231/http://octopus.gma.org/Tidings/sand-dollar.html The Common Sand Dollar by Cheryl Page] <!-- Accessed 02/03/2006 -->
* [https://msc.brunovellutini.com/vida-de-bolacha/ Video showing the life cycle of ''Clypeaster subdepressus'']

{{Taxonbar|from=Q1071960}}

{{DEFAULTSORT:Sand Dollar}}
[[Category:Clypeasteroida| ]]
[[Category:Gnathostomata (echinoid)]]
[[Category:Articles containing video clips]]
[[Category:Extant Thanetian first appearances]]