Macrobenthos consists of the organisms that live at the bottom of a water column and are visible to the naked eye. In some classification schemes, these organisms are larger than 1 mm; in another, the smallest dimension must be at least 0.5 mm. They include polychaete worms, pelecypods, anthozoans, echinoderms, sponges, ascidians, crustaceans. A visual examination of macroorganisms at the bottom of an aquatic ecosystem can be a good indicator of water quality. * Seagrass. * Echinoderms * Stony corals * A sea squirt being used as a substrate for a nudibranch's egg spiral.