Profile of Gray's Reef Geological cross-section Profile of Gray's Reef Habitat's Waters above the Reef Planktonic (drifting) and nektonic (swimming) invertebrates are found in the waters above the reef. Planktonic species include large medusae such as the sea nettle and the cannonball jelly. Squid exemplify a nektonic species that actively pursue prey in these waters. Flat Top Ridge Horizontal reef tops provide habitat for sessile (attached) benthic (bottom dwelling) invertebrates which rely upon ocean currents for food, gas exchange, waste removal, and egg dispersal. Examples include hard corals, soft corals, sponges, and hydroids. Scarps Little sediment accumulates on the vertical reef surfaces known as scarps. As a result, the bare substrate supports colonies of encrusting sponges and bryozoans (mossy animals). Barnacles, tunicates, and sea anemones are also found on these exposed surfaces. Ledges and Crevices Both predators and prey seek out good hiding places within the reef. Diurnal (active at day) species find refuge within the reef during the night. Nocturnal (active at night) species seek to conceal themselves within the reef during the daylight hours. Lobsters, crabs, sea urchins, and octopods prefer this type of habitat. Slopes and Sandy Areas Shifting sands and a lack of firm substrate preclude most sessile forms from settling along slopes and in sandy areas of the reef. Burrowing clams, mobile snails, sea stars, and burrowing polycheate worms are better adapted to life in these loose sediments. Flat Bottom Troughs Among the rock rubble, coarse sands, and shell fragments of the flat bottom troughs are found brittle stars, fan and tube worms, burrowing crustaceans, small crabs, and snails. This habitat is also preferred by certain burrowing echinoderms such as the sea cucumber. Invertebrates of the reef have adapted to the specific resource characteristics of habitats described above. This ability to adapt to different, specific resources helps to decrease competition for food and space among the residents of the reef, thus allowing for a greater diversity of species within the community.