What is a primary feature of amphipods?

Amphipods are a diverse group of crustaceans characterized by their ability to rapidly lose and gain water from their surroundings. This is primarily due to their habitat, which often includes aquatic environments where osmotic pressure changes can occur. Because they can tolerate varying salinity levels and can adjust their internal osmotic balance, it's crucial for their survival in dynamic environments.

The other options do not accurately describe amphipods. They do not possess a waxy outer layer like some terrestrial insects that helps reduce water loss. While amphipods are often confused with insects due to their segmented bodies, they are not insects; they belong to the class Malacostraca, which is a part of the larger phylum Arthropoda. Additionally, while some amphipods might display variations in coloration, they are not particularly noted for bright colors as a defining characteristic. Understanding these aspects helps clarify the unique adaptations amphipods have developed for their environments.