The distinction between krill and plankton is a common point of confusion. The two terms are often used interchangeably, yet one describes a specific type of animal, while the other refers to an entire lifestyle shared by countless organisms. Understanding this relationship requires exploring the fundamental differences in size, anatomy, and how each organism interacts with ocean currents. Krill represents one of the largest and most complex creatures within the vast, functionally defined category of plankton.
What Exactly is Plankton?
Plankton is a collective term for any organism that floats or drifts in the water column, largely unable to propel itself against the prevailing ocean currents. The word itself comes from the Greek word for “wanderer” or “drifter,” summarizing this passive existence. This functional definition means plankton includes a diverse array of life, ranging from bacteria and viruses to large, gelatinous creatures like jellyfish.
The category is broadly divided into two trophic groups based on how they obtain energy. Phytoplankton are the plant-like producers, composed mainly of microscopic algae that perform photosynthesis to convert sunlight into energy. Zooplankton are the animal-like consumers, which feed on phytoplankton, other zooplankton, and organic debris. These organisms form the base of nearly every aquatic food web.
Krill: A Specific Crustacean
Krill are small, shrimp-like invertebrates belonging to the Order Euphausiacea within the Phylum Arthropoda. They are recognized as malacostracan crustaceans, sharing their class with animals like crabs and true shrimp. Krill possess a segmented body protected by a chitinous exoskeleton, where the head and thorax are fused into a single cephalothorax.
A key anatomical feature distinguishing them from microscopic zooplankton is the presence of externally visible gills and multiple pairs of thoracic legs, or pereiopods, used for feeding. Krill are found in all the world’s oceans, but their most notable characteristic is their tendency to aggregate in immense, dense swarms.
Comparing Size, Movement, and Anatomy
The core difference is that krill are a specific, large type of zooplankton, often categorized as macro-zooplankton. While most plankton are microscopic, typically less than one millimeter in size, adult krill are multi-centimeter organisms. For instance, the Antarctic krill (Euphausia superba) can grow up to 6 centimeters long, making them giants compared to the single-celled diatoms they consume.
The most significant contrast lies in movement: plankton, by definition, must drift with the currents, but adult krill are active swimmers. Krill use five pairs of specialized swimming legs, called pleopods or swimmerets, to propel themselves. This swimming ability allows them to migrate vertically hundreds of meters daily and to move against weak currents, leading some researchers to classify them as micro-nekton. Krill also possess complex compound eyes and light-emitting organs called photophores, which are structural features far more advanced than those found in most planktonic organisms.
Roles in the Marine Food Web
Both plankton and krill underpin the entire marine ecosystem. Phytoplankton are responsible for generating a significant percentage of the world’s oxygen through photosynthesis, functioning as the primary producers for the aquatic food chain. They convert solar energy and carbon dioxide into organic matter, making subsequent life in the ocean possible.
Krill act as a direct and highly efficient link between these microscopic plants and the largest marine predators. They are considered a keystone species, especially in the Southern Ocean, due to their enormous collective biomass. Krill serve as the primary food source for baleen whales, seals, and penguins, channeling energy from the phytoplankton base upward. Furthermore, through their dense swarms and the production of fast-sinking fecal pellets, krill play a role in the biological pump, helping transport carbon from surface waters down to the deep ocean.