page_title: Seashell
section_title: Hard corals
hierarchical_section_title: Seashell / Other more atypical kinds / Hard corals
caption_attribution_description: Beach drift gastropod shells, echinoderm tests and coral pieces from various areas including the Mediterranean
context_page_description: A seashell or sea shell, also known simply as a shell, is a hard, protective outer layer created by an animal that lives in the sea. The shell is part of the body of the animal. Empty seashells are often found washed up on beaches by beachcombers. The shells are empty because the animal has died and the soft parts have been eaten by another animal or have decomposed.
  A seashell is usually the exoskeleton of an invertebrate, and is typically composed of calcium carbonate or chitin. Most shells that are found on beaches are the shells of marine mollusks, partly because these shells are usually made of calcium carbonate, and endure better than shells made of chitin.
  Apart from mollusk shells, other shells that can be found on beaches are those of barnacles, horseshoe crabs and brachiopods. Marine annelid worms in the family Serpulidae create shells which are tubes made of calcium carbonate cemented onto other surfaces. The shells of sea urchins are called "tests", and the moulted shells of crabs and lobsters are exuviae. While most seashells are external, some cephalopods have internal shells.
context_section_description: Pieces of the hard skeleton of corals 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, zooxanthellae. Typically a coral polyp will harbor particular species of algae, which will photosynthesise and thereby provide energy for the coral and aid in calcification, 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.
caption: Dish with beachworn coral pieces, marine gastropod shells, and echinoderm tests, from the Caribbean and the Mediterranean
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