From a previous blurtit answer and very good also.
Seashells are one example of non-living things in a coral reef; there will be many seashells in evidence in a typical coral reef...some will be alive, and some will be non-living. For example, an oyster may be alive, but a cracked mussel shell will be non-living.
Examples Of Non Living Things In A Coral Reef
• Soil - Soil is not alive, and it will always be present on the sea-bed, and around the coral reef. Certain organisms may live in the soil, but the basic soil compound is not composed of living organisms. Soil may be soft or hard, light or dark; however, it will never, ever be alive. Soil may rest on various parts of a coral reef. Nutrients in soil may be ingested as food by certain types of sea creatures.
• Water - Water is everywhere is a coral reef, and this substance is non-living. Algae and microscopic organisms may float through water, but they are not a part of it. Water may contain minerals and salts that nourish many creatures of the sea. Water may contain bit of seaweed that are alive, or non-living, depending on the stage of the seaweed's life cycle.
• Stones - Pebbles and stone will be found in a coral reef, and these rocks are not living things. Stones may have living things clinging to them, but they are ultimately inanimate objects.
• Shells - Shells which are no longer living, such as dead barnacles, will remain in a coral reef; crushed seashells, none of which are living, may be found on the sea bed at the base, or bottom, of a coral reef.
A typical coral reef is an amazing microcosm of living organisms...from algae to fish to mollusks, and on and on. However, there will be always be non-living elements in this exciting and picturesque underwater environment.