There is a diverse range of benthic (seabed) habitats in the HIMI region, caused by the varying submarine topography and the action of oceanic currents.
The sediments on the sea floor mainly comprise silica-rich diatom mud or ‘ooze’, with some areas containing quantities of calcareous sediments (calcium-rich shells of dead small marine animals).
A range of species are present on the sea bottom, with echinoderms (radially symmetrical marine invertebrates with an internal calcium-rich skeleton, such as sea urchins) being the most common, including seven species that possibly live only in the HIMI region. Other slow-growing and vulnerable benthic species, such as corals, sponges and barnacles are also found.
An analysis of the available information about benthic assemblages, substrate and the physical characteristics of the marine environment indicated that areas shallower than 1000 m can be divided into thirteen potentially unique units based on a combination of their biological and physical characteristics.
A more thorough description of the biophysical units and benthic communities, including lists of benthic species found at HIMI, is given in the Australian Antarctic Division Report Conservation of marine habitats in the region of Heard Island and McDonald Islands [PDF]