Krill are not fish; they are small, marine invertebrates. Despite inhabiting the ocean and swimming, their biological classification places them in a completely different group of animals than fish.
What Krill Are
Krill are small crustaceans belonging to the order Euphausiacea, found in all oceans. Their name comes from a Norwegian word meaning “small fry of fish,” but they are more closely related to shrimp, lobsters, and crabs. These invertebrates play a significant role as zooplankton, forming a crucial link in marine food webs.
Most krill species measure 1 to 2 centimeters as adults, though some deep-sea species reach up to 15 centimeters. They have translucent bodies, often with a reddish-orange tint, and their internal organs may be visible through their outer shell. Krill are found from the surface down to deep waters, undertaking daily vertical migrations to feed.
Krill are filter feeders, using specialized appendages to strain microscopic phytoplankton from the water. This converts the energy from tiny plants into a form digestible by larger animals. They are a primary food source for many marine predators, including whales, seals, penguins, and various fish species.
Antarctic krill ( Euphausia superba) form massive swarms in the Southern Ocean, sometimes reaching densities of tens of thousands of individuals per cubic meter. This species alone is estimated to have one of the largest biomasses of any animal on Earth, highlighting its immense ecological importance.
How Krill Differ from Fish
A primary difference between krill and fish lies in their skeletal structures. Krill, like all crustaceans, possess a rigid outer exoskeleton made primarily of chitin that protects their soft internal tissues. This external skeleton does not grow, so krill must shed it periodically through molting to increase in size.
In contrast, fish have an endoskeleton, an internal skeleton made of bone or cartilage. This internal framework provides support and allows for muscle attachment, enabling movement. The main component of a fish’s skeleton is its vertebral column, or backbone, along with a skull and ribs.
Respiration also differs between these two groups. Krill breathe using gills that are typically exposed, located under their carapace but not fully covered. This allows water to pass over them for oxygen exchange.
Fish have internal gills located within gill slits, usually covered by a protective bony flap called an operculum in bony fish. Water flows over these internal gill filaments, facilitating the uptake of dissolved oxygen.
Their body plans and appendages further distinguish them. Krill bodies are segmented, typically divided into a fused head and thorax (cephalothorax) and an abdomen. They possess multiple pairs of legs, including swimming legs (pleopods) on their abdomen and specialized feeding legs (thoracopods) near their mouth.
Fish have a streamlined body shape designed for efficient movement through water, with fins rather than multiple legs. These fins, such as dorsal, pectoral, pelvic, and caudal (tail) fins, are supported internally by bony or cartilaginous rays and are used for propulsion, steering, and stability.
These differences in anatomy reflect their distinct biological classifications. Krill are arthropods, specifically crustaceans, a phylum characterized by segmented bodies, exoskeletons, and jointed appendages. Fish are vertebrates, belonging to the phylum Chordata, defined by the presence of a vertebral column and an internal skeleton.