- abdomen
- abyssal
- acuspidate
- acute
- adherent
- adipose
- adnate
- adolescent
- adult
- aggressive behavior
- Agnatha
- air bladder
- akula
- allopatric
- AMNH
- amphidromous
- amphipods
- AMS
- anal fin
- anatomy
- anemones
- angular
- animal
- annelids
- ANSP
- Antarctic
- anterior
- anterior margin
- anteroposterior
- Antilles Islands
- antrose
- anus
- aorta
- aperture
- apex
- aphakic gap
- appendage
- aquatic
- Arctic
- arcuate
- artery
- articulation
- Ascension Island
- Atlantic
- atrophy
- attenuate
- autotrophic
- auxiliary process
- auxiliary scales
- awl-shaped
- axil
- axillary scale
- axis
- Azorean, Azores Islands