Krill are small, shrimp-like marine invertebrates belonging to the order Euphausiacea, found throughout the world’s oceans. These crustaceans are recognized for their immense collective biomass, making them one of the most abundant animal groups on the planet. They form a foundational component of many aquatic food webs, converting the energy from microscopic plants into a form consumable by larger marine life.
Defining Krill Appendages
Krill possess numerous pairs of appendages, which can be primarily divided into two functional groups based on their location on the body. The posterior section of the body, the abdomen, bears five pairs of limbs specifically adapted for locomotion. These abdominal legs are known as pleopods, or swimmerets, and are primarily responsible for propelling the animal through the water column.
The anterior section, the cephalothorax (a fusion of the head and thorax), holds the other set of limbs. These are called thoracopods or pereiopods, and their number ranges from six to eight pairs depending on the species. Since the five pairs of pleopods are consistent across all krill, the total number of appendages, including the thoracopods, may be up to 13 pairs or 26 individual limbs.
Functional Roles of the Appendages
The division of krill appendages into pleopods and thoracopods reflects a specialization for two very different biological functions: movement and feeding. The five pairs of pleopods on the abdomen beat rhythmically to create a continuous, smooth swimming motion, allowing the krill to maintain its position or migrate vertically in the water. These limbs are powerful enough to enable an escape reaction, known as “lobstering,” where the animal flicks its tail to shoot backward at high speed when sensing a threat.
The thoracopods, located closer to the mouth, are highly modified to facilitate the krill’s primary method of nutrient acquisition, which is filter-feeding. These thoracic legs are covered in fine, hair-like structures called setae and interlock to form a fine, conical “feeding basket.” By slowly pushing water through this filtering mesh, the krill is able to strain out tiny particles of food, such as phytoplankton. The constant movement of the thoracopods also helps circulate water over the gills, which are located at the base of these limbs, assisting with respiration.
Krill in the Global Marine Ecosystem
Krill’s abundance and feeding efficiency establish them as a foundational link between primary producers and higher-trophic-level consumers in the ocean. The Antarctic krill, in particular, is considered a keystone species in the Southern Ocean due to its massive population, with its total biomass estimated to be in the hundreds of millions of tons. This single species supports nearly all of the region’s top predators, including baleen whales, seals, penguins, and various seabirds and fish.
The ecological importance of krill is magnified by their distinctive behaviors, such as forming incredibly dense swarms that can cover several square kilometers. These aggregations make them an easy and energy-rich meal for large predators like the blue whale, which can consume several tons of krill per day. Many krill species also exhibit diel vertical migration, moving to the surface layers at night to feed and retreating to deeper, darker waters during the day, which helps them avoid visual predators.
Krill also play a role in the ocean’s carbon cycle, often referred to as the “biological pump.” By consuming phytoplankton at the surface and then migrating to deeper waters, they excrete fecal pellets that are dense and sink rapidly. This process moves carbon from the surface layer into the deep ocean, effectively sequestering it from the atmosphere. Most krill species possess light-producing organs called photophores, which allow them to glow through bioluminescence, a feature used for communication, schooling, or counter-illumination camouflage against predators below.