Gastropods are mollusks with only one shell (also called univalves) or no shell, such as snails, slugs, limpets, and abalone. These creatures are commonly found in the intertidal zone where they are exposed to extremes in temperature, light, and moisture and must be able to withstand being frequently battered by strong waves. Most gastropods are grazers, but there are a few exceptions. Some are predatory, including the invasive eastern oyster drill (Urosalpinx cinerea) and whelk snail (Baccinum undatum), which are found in Tomales Bay at Point Reyes National Seashore and are thought to have devastating effects on native oyster populations. Native species include the black turban (Chlorostoma funebralis), one of the most abundant intertidal snails of the Pacific Coast and owl limpets (Lottia gigantea), which are solitary gastropods. The red abalone (Haliotis rufescens) and federally listed black abalone (Haliotis cracherodii) were formerly abundant species of rocky intertidal habitats. They are now a rarity, however, likely as a result of over harvesting.