- Radial symmetry
- symmetrical arrangement of an animal's body around a central axis
- Radula
- tongue-like band of teeth used by many mollusks to scrape, tear or bore
- Raft
- large number of animals together in the water; usually applied to birds and pinnepeds
- Rasp
- to scrape or rub as with a file
- Red tide
- concentrations of certain microscopic organisms, particularly dinoflagellates, that discolor surface waters, usually in coastal regions and reduce oxygen supplies to animals deeper in the water column; some may be toxic
- Regeneration
- ability of some organisms to renew or replace an injured or lost organ or body part
- Regurgitation
- casting up of incompletely digested food, usually for the feeding of young
- Reptile
- vertebrate animal characterized as cold-blooded and having scales
- Respiration
- breathing; the act of assimilating oxygen and releasing carbon dioxide and other products
- Rhizomes
- horizontal plants capable of producing the upward shoot and downward root of a new plant
- Rhopalia
- concentration of neurons and sensory organs around a jelly's bell and between lobes; contains statocysts and ocelli
- Rostrum
- in fish - forward projection of snout; in cetaceans - upper jaw forming part of snout or beak; in crustaceans - forward pointing spine on the carapace
- Rufous
- reddish; tinged with red; brownish-red
