- abdomen
- abiotic
- abscissa
- abundance
- abyssal
- abyssopelagic
- acantho-
- acclimatization
- acrocentric
- aculeate
- acuminate
- acute
- adaptation
- adaptativity
- adherent
- adhesion
- adipose
- adnate
- adolescent
- adult
- advected
- aggressive behavior
- Agnatha
- agricultural
- aigle
- air bladder
- alga
- allantois
- allele
- allopatric
- allozyme
- ambicolorate
- ammocoete
- AMNH
- amnion
- amniote egg
- amorphous
- amphidromous
- amphipods
- AMS
- anal fin
- anatomy
- anchialine
- androgenesis
- androgenic
- anemones
- angiosperms
- angular
- angulate
- animal
- aniso-
- anisogamy
- annelids
- annuli
- ANSP
- Antarctic
- Antarctic Convergence
- Antarctic Divergence
- Antarctica
- anteriomesial
- anterior
- anteroposterior
- anticyclone
- antrose
- anus
- aorta
- aperture
- apex
- aphakic gap
- aphotic
- apivorous
- apodal
- appendage
- aquatic
- arable
- arachnoid
- Arafura Sea
- arborescent
- Arctic
- arcuate
- areola
- arid
- armed
- armless
- artery
- articulation
- association
- Atlantic
- atrophy
- attenuate
- author
- autotrophic
- auxiliary process
- auxiliary scales
- awl-shaped
- axil
- axillary
- axillary scale
- axis