Editing Sand dollar (section)

==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>