The Order Euphausiacea comprises small, shrimp-like crustaceans commonly known as krill. They are found in oceans worldwide, often forming large swarms that serve as a critical food source for many marine animals, including whales, seals, penguins, and fish. Euphausiaceans are mostly filter-feeders, consuming phytoplankton and microscopic zooplankton, and play a key role in the marine food web and nutrient cycling. They possess transparent bodies, thoracic appendages for swimming, and bioluminescent organs in many species, which help in communication, predator avoidance, and mating. Their ecological importance makes them a cornerstone of ocean ecosystems, particularly in polar regions.