Editing Seashell (section)

=== 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.