- Nauplii
- first free-swimming larval stage of many crustaceans including copepods, ostrapods and decapods
- Nauplius
- first free-swimming larval stage of many crustaceans including copepods, ostrapods and decapods
- Nekton
- pelagic (open ocean) animals that are able to swim against ocean currents
- Nematocyst
- specialized stinging cells; also called cnidae, in jellies, spirocysts, and plychocysts in anthrozoans (sea anemones, corals, sea pansies, sea pens). Used for capturing prey and/or defense from predators.
- Neritic zone
- nearshore area. The area of the ocean that lies along the coasts of continents and islands, less than 300 meters below the surface.
- Neuromasts
- groups of sensory cells in the lateral line of fishes
- Neuromuscular
- pertaining to both nerves and muscles
- Neurotoxin
- toxin that acts specifically on nerve cells, commonly causing paralysis that acts specifically
- Neutral buoyancy
- weightless in water, neither rising or sinking
- Niche
- ecological role of an organism and its position in the environment
- Nocturnal
- active at night
- Noxious
- harmful to living things
- Nudibranch
- sea slug; a marine gastropod without an external shell and mantle
- Nutrient
- any of a number of inorganic or organic compounds or ions that promotes growth or provides energy
