Is Krill Fish?

Krill are small, saltwater-dwelling creatures found in ocean waters all around the globe. As a dietary staple of all sorts of ocean life, they are essential to supporting Earth's marine ecosystems. They live entirely submerged lives, they breathe through gills and they get around by swimming -- but krill aren't fish. Krill are actually crustaceans. More than 90 different documented krill species exist today.

  1. Description

    • Krill range from about 1 inch to 2.5 inches long and weigh about a gram. They closely resemble shrimp, one of the more well-known crustaceans. The krill body is divided into three segments -- the abdomen, thorax and cephalon -- though the latter two are fused together and difficult to distinguish, sometimes referred to cumulatively as the cephalothorax. Each segment typically has a pair of legs. Their heads are further broken down into five segments, their thoraxes into eight segments, and their abdomen and tail sections into six segments. These invertebrates also have a protective shell known as a carapace. Many krill species are luminescent.

    Behavior

    • Krill are herbivores, feeding on algae and phytoplankton, which are single-celled marine plant life-forms. These crustaceans in turn are the primary food for many fish and birds and for baleen whales. They populate all ocean waters but are most densely found in Antarctic waters. Krill are crucial. As "National Geographic" puts it, "Without krill, most of the life forms in the Antarctic would disappear." Krill congregate in groups called swarms between the water's surface and a depth of 6,000 feet. Swarms can be as dense as 10,000 krill per cubic meter of water and can span more than a mile.

    Crustaceans

    • With more than 20,000 known species, fish are vertebrates with fins, and most have scales. Krill are invertebrates with exoskeletons, hallmarks of crustaceans and providing clear distinctions from fish. Crustaceans are grouped under the Cnidaria, or invertebrate, phylum. Other notable features of krill and all crustaceans include a segmented body, two pairs of antennae, a pair of mandibles and a pair of eyes usually set on stalks. Shrimp, crabs, lobsters, barnacles, crayfish and water fleas are other well-known crustaceans.

    Classification

    • Krill, other crustaceans and fish are all part of the animal kingdom, which is the broadest of the five categories of classification of life on Earth. The other kingdoms include plants, fungi, bacteria and protists, which are single-celled microorganisms. The animal kingdom is broken down into 40 phyla, and this level of taxonomy is where krill and fish part ways. Fish are in the Chordate phylum, which includes all vertebrates; krill are one of about 9,500 water-dwelling life-forms in the Cnidaria phylum of invertebrates. The next breakdown is class: Fish are in the bony fish class while krill are in the crustacean class.