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Natural selection: The process whereby some organisms in a species have certain inherited variations that give them an advantage over others.

Nauplius: The first planktonic larval stage of barnacles and some other crustaceans. The nauplius larva metamorphoses into the cypris larva.

Nautilus: A soft-bodied marine animal with many arms and a spiral shell. Nautiluses are related to octopuses and squid.

Neap tide: Small or moderate tide, occurring every two weeks when the Moon is in its quarter phase.

Nematocyst: A stinging cell found on the tentacles of cnidarians.

Neoprene: Oil-resistant synthetic rubber.

Neutral buoyancy: Snorkelling term referring to maintaining a position in water where you go neither up or down.

Newton: Unit of force.

Nitrogen: Chemical dissolved in blood and found in air. Also found in fertilisers.

Notochord: A long, flexible rod which runs the length of the back in some kinds of animals (animals that belong to the phylum Chordata). In vertebrates, the notochord develops into part of the backbone.

Nudibranch: A sea slug; member of a group of snails without shells that breathe through long, feathery gills on their backs.

Nutrients: Elements or compounds essential as raw material for organic growth and development such as carbon, oxygen, nitrogen and phosphorus.