Mollusks make shells, which are their main protection from predators and the environment. More than 300 species live in the shallow waters around Sanibel and Captiva Islands, occupying habitats such as muddy areas, mangrove stands, oyster reefs, sandy beaches, piles, and wharves. By diet, these mollusks fall into several categories: herbivores such as limpets, turbans, and top snails graze on seaweeds and microscopic marine plants called algae; predators such as crown conchs, tulips, lightning and other whelks, and the horse conch feed on marine invertebrates, including other mollusks; and most bivalves, including clams, cockles, oysters, mussels, and pen shells, are filter-feeders that take in large amounts of water and retain micro-algae as food. A small number of gastropods, such as slipper and worm snails, are also filter-feeders. In the calm waters of Pine Island Sound and San Carlos Bay near Sanibel, oysters are the main filter-feeders, and an individual oyster may filter hundreds of gallons of seawater each day. Oysters also form reefs that provide habitat, food, and shelter for many organisms, and oyster reefs indicate the health of the surrounding environment. Among local mollusks, the horse conch is the top predator, feeding mostly on other large mollusks such as tulips and lightning whelks.