Krill are small, ocean-dwelling crustaceans that form a foundational link in marine food webs. Found in all the world’s oceans, krill are most famous for the immense swarms they form in the Southern Ocean. There, the Antarctic krill (Euphausia superba) dominates the ecosystem. Their massive populations serve as the primary diet for numerous larger animals, including whales, seals, and penguins. Krill’s physical characteristics allow them to thrive, making them one of the most successful animal groups.
Dimensions and External Appearance
Most krill species are relatively small as adults, generally measuring between one and two centimeters in length. The most well-known species, the Antarctic krill, is a giant among its relatives, growing up to approximately six centimeters long, which is roughly the size of a large paperclip. This larger size allows an individual to weigh over one gram when fully grown.
Krill are translucent, similar to a miniature shrimp, with a transparent shell in most species. Many krill, particularly the Antarctic species, display a reddish or pinkish hue due to pigment spots. This coloration is often enhanced by the krill’s diet; their digestive tract can be seen through the shell, sometimes appearing vivid green when they have recently fed on microscopic algae. Krill also possess prominent, large black compound eyes adapted to the low-light conditions of the deep ocean.
The Krill Body Plan
As crustaceans, krill possess a body divided into three distinct regions, two of which are fused. The head and the thorax join to form a single unit called the cephalothorax, which is covered by a protective shield known as the carapace.
Beneath the carapace, krill possess exposed gills. The appendages attached to the cephalothorax are known as thoracopods, or thoracic legs. These thoracopods are primarily used for feeding, rather than walking, and are instrumental in capturing food particles from the water.
The final section of the body is the segmented, highly muscular abdomen. Attached to the underside are five pairs of paddle-like appendages called pleopods, also known as swimmerets. These pleopods beat in a smooth, rhythmic motion to propel the krill through the water, as they must swim constantly to maintain position. The abdomen ends in a tail fan, which provides the thrust needed for a rapid escape response known as a “tail flip.”
Unique Physical Adaptations
A defining characteristic of nearly all krill species is their capacity for bioluminescence, the ability to produce their own light. This is achieved through specialized organs called photophores, which are located on the eyestalks, near the mouthparts, and at the base of the abdominal swimming legs.
These photophores emit a subtle blue-green light that is not used to illuminate the surroundings but rather for camouflage. This process is called counter-illumination, where the krill adjust the intensity of their downward-facing light to match the faint sunlight filtering down from above. By eliminating their silhouette, krill become virtually invisible to predators looking up from below, helping them evade detection in the water column.
Krill also possess a filter-feeding apparatus. The thoracopods form a finely-meshed, basket-like structure pushed through the water to strain out microscopic phytoplankton. This feeding basket is capable of filtering out single-celled algae as small as one micrometer in diameter. When food is abundant, the krill use compression filtration, where the basket is compressed to squeeze water out laterally, leaving concentrated food particles to be moved toward the mouth.