The estuaries act as fisheries for fry and nurseries for crustaceans. Shrimp, oysters, crabs, whelks, cockles, and snails thrive in these waters, as do primordial horseshoe crabs (Limulus polyphemus). The region supports a $59 million-a-year Tortugas pink shrimp (Farfantepenaeus duorarum) industry, and a $22 million-a-year stone crab (Menippe mercenaria) industry.[69] Between 80 and 90 percent of species that are harvested commercially in Florida are born or spend time in the shallow waters near the Everglades.[66][70] Oysters and mangroves work in tandem to build up the coastline. The sand around the coastline has minute white particles of quartz and fine shells. When currents are right, oysters grow in colonies or beds, and deposit their shells, reinforcing the bed. Mangrove seeds, called propagules, are full embryos and float in water until they reach a favorable location and take root, often on oyster beds. They shed skin and litter, ensuring other trees will not compete for space and nutrients.[71]