Broken shells, oyster shells, or other debris used as a bed for oyster larvae in oyster cultivation.
Possibly from Dutch 'cultch' or related to Middle Low German sources. Used in oyster farming since at least the 19th century, the term refers to the physical substrate that provides settling surface for spat (young oysters). The exact origin is debated among etymologists.
Oyster farmers discovered that oyster larvae need something rough to cling to—and crushed shells from previous harvests work perfectly, so a waste product became essential infrastructure, and got its own specialized word!
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