- Ichthyologists
- zoologists who study fishes
- Ichthyoplankton
- planktonic fish larvae and suspended fish eggs
- Incurrent
- pertaining to a current that flows inward
- Indicator species
- a species whose presence, absence, or relative well-being in a given environment is indicative of the health of its ecosystem as a whole
- Indigenous
- native to a particular region or environment
- Indirect development
- having a larval stage of development
- Inert
- unable to move or act
- Infanticide
- killing of young
- Inorganic
- being or composed of matter that is not plant or animal
- Insectivore
- animal that primarily eats insects
- Intertidal
- the zone between high and low tides, exposed to and flooded periodically by tides.
- Invertebrate
- animals with no dorsal column of vertebrae; without a backbone
- Iridescent
- having or displaying rainbow-like colors
- Iridocyte
- cells or tissue containing reflective material, e.g., reflective cells on fishes' scales
- Isopod
- any member of the Order Isopoda of aquatic crustaceans; has six pairs of legs and a flat oval body
