J-O

Macrofauna
Macrofauna are benthic or soil organisms, which are normally > 0.5 mm, but in this case, also include animals at least 0.25 mm in length.

Magma
Molten rock under Earth's surface.

Meiofauna
Microscopic, multicellular organisms that live in both marine and fresh water environments defined by size, being those that can pass through a 0.5 mm mesh but be retained by a 45 µm mesh.

Mesopelagic Zone
The region of an ocean extending from 200 - 1,000 metres below sea level.  Some light penetrates this zone, however, it is insufficient for photosynthesis.

Metazoa
Multicellular organisms (i.e. not bacteria or protozoa).

Meteorological
The science that deals with the phenomena of the atmosphere, especially weather and weather conditions.

Mid-ocean ridge
One of three major topographic divisions of an ocean basin. Mid-ocean ridges are the central belts of submarine topography with a characteristic axial rift which marks a spreading plate boundary.

Mollusc
Belonging to the phylum of Mollusca, molluscs are soft-bodied animals with a head, a foot, a visceral body mass and a mantle, often (but not always) protected by an external shell.

Nekton
Organisms with swimming abilities that permit them to move actively through the water column, and to move against currents.

NGO
Non-Governmental Organization.