The ocean teems with organisms that profoundly shape marine environments. Among these are plankton and krill, often discussed together due to their foundational roles in aquatic ecosystems. While both are fundamental to ocean life, their relationship is intricate: krill are a specific group within the broader category of plankton. This article clarifies the distinctions and connections between these two marine organism groups.
Understanding Plankton
Plankton refers to a wide array of organisms that drift in water bodies, unable to swim against currents. This diverse group includes species from nearly all kingdoms of life, ranging from microscopic bacteria and viruses to larger organisms like jellyfish. Plankton are classified into two main types based on their feeding behavior: phytoplankton and zooplankton.
Phytoplankton are plant-like, autotrophic organisms that produce their own food through photosynthesis. Examples include diatoms, dinoflagellates, and cyanobacteria. Zooplankton are animal-like and heterotrophic, consuming other organisms for sustenance. This group includes small protozoans, crustaceans like copepods, and the larval stages of various marine animals and fish.
Understanding Krill
Krill are small, shrimp-like marine crustaceans belonging to the order Euphausiacea, found in all of the world’s oceans. Over 85 known species exist, with the Antarctic krill (Euphausia superba) being one of the most abundant animal species on the planet by biomass, estimated to be hundreds of millions of tons. These invertebrates measure 2.5 inches (6 cm) in length.
Krill are transparent with a reddish tint and possess large black eyes. They exhibit bioluminescence, emitting a yellow-green light from specialized organs called photophores, which assists in communication and evading predators. Krill also display strong schooling behavior, forming massive swarms that can span miles and reach densities of up to 30,000 individuals per cubic meter, sometimes visible from space. Their primary diet consists of phytoplankton, which they filter from the water.
Key Distinctions and Overlap
The fundamental distinction between krill and plankton lies in their classification: krill are a specific type of zooplankton. While all krill are plankton because they drift with currents, not all plankton are krill. Plankton is a broad ecological category defined by passive movement, whereas krill represent a distinct taxonomic order of crustaceans.
Krill are macroscopic, larger than most individual microscopic plankton. While still subject to currents, adult krill possess more developed swimming abilities compared to many other zooplankton, allowing them to undertake daily vertical migrations. These migrations involve moving towards the surface at night to feed and descending to deeper waters during the day, a coordinated movement that distinguishes them from simpler, less mobile planktonic forms. Krill are complex organisms with segmented bodies, multiple legs, and a chitinous exoskeleton, unlike the often single-celled or simpler multicellular structures of many other planktonic species. Their life cycle involves multiple stages, and they can live for up to six years, a longer lifespan than most microscopic plankton.
Their Role in Marine Ecosystems
Both plankton and krill play foundational roles in marine ecosystems, forming the base of the food web. Phytoplankton are the primary producers, converting sunlight into chemical energy through photosynthesis and generating a significant portion of the Earth’s oxygen. Zooplankton, including krill, act as primary consumers, grazing on phytoplankton and transferring this energy to higher trophic levels.
Krill are recognized as a keystone species due to their immense biomass and their position as a direct food source for a wide array of marine animals. Baleen whales consume enormous quantities of krill. Seals, various species of penguins, seabirds, and fish also rely heavily on krill for sustenance. This makes krill an important link in the transfer of energy from microscopic phytoplankton to larger marine predators, supporting the biodiversity and stability of ocean ecosystems.