The species vary in types of sea bed they inhabit, with " Diadema savignyi " inhabiting sandy beds and back reef where damaged; while " Diadema setosum " can also commonly be found among seagrass.
12.
The Tuvalu National Biodiversity Strategy and Action Plan of 2009 describes the marine environment as comprising six major ecosystem types : oceanic, outer reef, lagoonal, back reef, lagoon floor, and patch reefs, plus natural channels between the ocean and lagoon.
13.
They have also been observed 20 meters below surface waters . " P . Cylindricas " are found in back reefs and lagoons located in the waters of the oceanic West Pacific, Australia, the South China Sea, Japan, South-east Asia and the Indian Ocean.
