An aquatic organism is any living thing that lives in water, whether freshwater or saltwater. This includes a vast diversity of life forms, from microscopic bacteria and algae to enormous whales and giant squid.

Here's a breakdown:

* "Aquatic" means "relating to water".

* "Organism" refers to any living individual, plant or animal.

Examples of aquatic organisms:

* Plants: Seagrass, seaweed, kelp, water lilies, phytoplankton

* Animals: Fish, whales, dolphins, jellyfish, crabs, lobsters, starfish, sea turtles, amphibians, freshwater snails

* Microorganisms: Bacteria, algae, zooplankton

Key characteristics of aquatic organisms:

* Adaptations for life in water: Aquatic organisms have evolved specific traits to survive in their watery environment, such as gills for breathing, fins for swimming, streamlined bodies, and the ability to regulate their buoyancy.

* Importance to ecosystems: Aquatic organisms play a crucial role in maintaining the balance of their respective ecosystems, providing food sources, cleaning the water, and contributing to the overall health of the planet.

Note: The term "aquatic organism" encompasses all life forms that inhabit water, including those that spend only part of their lives in water, like amphibians and some insects.