Unicellular:
* Amoeba: These are single-celled organisms with a flexible body and pseudopodia (false feet) used for movement and feeding. They are found in both fresh and saltwater environments.
* Amphidinium: A type of dinoflagellate, a single-celled organism with two flagella, often found in marine environments.
* Aphanizomenon: A genus of cyanobacteria, sometimes found in freshwater blooms.
Multicellular:
* Acoelomorpha: A group of simple, flatworms found in marine environments.
* Algae: This is a broad category, but many types of algae (like red algae, green algae) are multicellular and found in both freshwater and saltwater.
* Amphioxus: Also known as lancelets, these are small, fish-shaped marine invertebrates that are considered to be the closest living relatives of vertebrates.
* Anemones: These are predatory, flower-like animals found in marine environments, often attached to rocks or coral reefs.
* Anthozoa: A class of cnidarians that includes corals, sea anemones, and jellyfish. Many species are multicellular and found in marine environments.
This is not an exhaustive list, and there are many other organisms beginning with "A" that could be found in fresh or saltwater environments.
