Editing Seashell (section)

== Atypical shells ==
A few other categories of marine animals leave remains which might be considered "seashells" in the widest possible sense of the word.

=== Chelonians ===
[[Sea turtle]]s have a [[carapace]] and [[plastron]] of bone and [[cartilage]] which is developed from their [[rib]]s. Infrequently a turtle "shell" will wash up on a beach.

=== Hard corals ===
[[Image:Shells stones and other curios.JPG|thumb|Dish with beachworn coral pieces, marine gastropod shells, and echinoderm tests, from the Caribbean and the Mediterranean]]

Pieces of the hard skeleton of [[coral]]s commonly wash up on beaches in areas where corals grow.

The construction of the shell-like structures of corals are aided by a [[symbiotic]] relationship with a class of [[algae]], [[zooxanthella]]e. Typically a coral polyp will harbor particular species of algae, which will [[photosynthesis]]e and thereby provide energy for the coral and aid in calcification,<ref name=MilneBay>
{{cite web
 |author1       = Madl, P.
 |author2       = Yip, M.
 |name-list-style = amp
 |year          = 2000
 |url           = http://biophysics.sbg.ac.at/png/png3.htm
 |work = Field Excursion to Milne Bay Province – Papua New Guinea Working Group: Scleractinian Corals (incl. Porifera)
|title=        PART-III Cnidaria (Gk. cnidos, stinging nettle) reproduction and growth of Scleractinia
|publisher=biophysics.sbg.ac.at
 |access-date    = 31 March 2006
 |url-status       = dead
 |archive-url    = https://web.archive.org/web/20120128205517/http://biophysics.sbg.ac.at/png/png3.htm
 |archive-date   = 28 January 2012}}</ref> while living in a safe environment and using the carbon dioxide and nitrogenous waste produced by the polyp.  [[Coral bleaching]] is a disruption of the balance between polyps and algae, and can lead to the breakdown and death of coral reefs.

=== Soft corals ===
[[File:1-Gorgone.jpg|thumb|An x-ray photograph of a [[gorgonian]]]]
The skeletons of soft corals such as [[gorgonian]]s, also known as [[sea fan]]s and [[sea whip]]s, commonly wash ashore in the [[tropics]] after storms.

=== Plankton and protists ===
{{further|Protist shells}}
[[File:Diatoms through the microscope.jpg|thumb|Marine [[diatom]]s form hard silicate shells]]

Plant-like [[diatom]]s and animal-like [[radiolarian]]s are two forms of [[plankton]] which form hard [[silica]]te shells. [[Foraminifera]] and [[coccolithophore]]s create shells known as "[[test (biology)|tests]]" which are made of calcium carbonate. These shells and tests are usually microscopic in size, though in the case of foraminifera, they are sometimes visible to the naked eye, often resembling miniature mollusk shells.