haemocyanin	A respiratory pigment in the haemolymph of crustaceans which contains copper prophyrin molecules. The copper binds the free oxygen of water more efficiently than the iron in haemoglobin.
half-life time	The time taken for half of the atoms in a radioactive material to undergo decay.
herbivore	An organism that feeds on plants or other autotrophic organisms. The term is used primarily to describe animals.
hermetic	Sealed so as to be airtight.
heterogeneity	Not of the same type (genetic variance).
holdfast	The organ of attachment of a seaweed or related plant.
holoplankton	Animal species which remain planktonic organisms throughout their entire life cycle.
hydrocarbon	Any organic compound containing only carbon and hydrogen
hydrodynamics	The branch of science concerned with the mechanical properties of fluids, esp. liquids.
Hydrogen sulphide (H2S)	A strong-smelling soluble gas, H2S, resulting from anaerobic decomposition; commonly found in the hypolimnion or bottom mud. Also can be released from the sedimented organic material (faeces and waste feed) that accumulates beneath fish cages or oyster trestles at sites restricted with regard to currents and depth.
hypothermia	An abnormally low body temperature.