Krill, Euphausia superba
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Description & Behavior
Krill, which means "whale food" in Norwegian, are eucarid crustaceans divided into 2 families. The Bentheuphausiidae family consists solely of Bentheuphausia amblyops (G. O. Sars, 1883), a deep water krill species that differs from members of the Family Euphausiidae in that Bentheuphausia amblyops is not bioluminescent. The Euphausiidae family includes the other 89 known krill species, including one of the most common, Euphausia superba (Dana, 1852), which is the most frequent species associated with krill.
Krill in the Euphausiidae family are shrimp-like crustaceans that swarm in dense shoals, particularly in Antarctic waters. Krill swarms may be as dense as 10,000 krill/cubic meter of water, and can stretch for kilometers. Individuals range in length from 8-70 mm, the largest up to 14 cm long. The bioluminescence of krill species in the Euphausiidae family is a strong blue-green light that may be used for communication to help them congregate and spawn.
Like other crustaceans, krill have a hard calcified exoskeleton, which is divided into three tagmata, or segments: the cephalon, thorax, and abdomen. The head and thorax are fused into a cephalothorax and are sometimes difficult to distinguish. Generally, their head has five segments, their thorax has eight, and their tail has six. In most krill, each segment has a pair of appendages, but occasionally variations occur. The anterior-most five abdominal appendages are biramous, meaning they have two branches. The last few abdominal appendages are flattened and form the tail fin also called the telson.
World Range & Habitat
Krill, Bentheuphausia amblyops, is a bathypelagic species found in the southern part of the North Atlantic Ocean near 40°N in deep waters of at least 1,000 m. Euphausia superba are found in Antarctic waters between the continent and the polar front generally within depths of a 100 m or less. Euphausia crystallorophias is also common in Antarctic waters, but tends to inhabit pack and floating ice as well as pelagic waters.
Other krill species are found worldwide in open seas such as: the Pacific Ocean between 55°N and 55°S, in the Indian Ocean between 10°N and 10°S, the Gulf of Oman and east of Sri Lanka. Most are found near the surface, but some have been found as deep as 2,000 m.
Feeding Behavior (Ecology)
Most krill species are filter feeders and consume diatoms (phytoplankton), small one-celled or colonial algae in the Class Bacillariophyceae and occasionally zooplankton. They filter food with 1-3 thoracic (throat) appendages that have been modified into feather-like maxillipeds (see more on their morphology here). Their mouth leads to a 2-chambered stomach that contains a gastric mill to aid digestion.
Krill tend to rise to the surface at night to feed, and retreat to deeper waters during the day.
Krill reproduce during the spring by spawning eggs in several "broods" that contain as many as 8,000 eggs. Krill can release eggs multiple times per season, and the spawning season can last as long as 5 months.
Conservation Status & Comments
Krill are heavily fished commercially. The krill fishery has been the largest fishery in the Southern Ocean for the last 25 years, particularly by Ukraine, Poland and Japan. Because krill are at the center of the Antarctic food web, these countries have signed an agreement limiting the size of krill catches to hopefully keep a large enough population for the larger animals that feed on them such as baleen whales, penguins, and seals.
Visit Krill Facts - centre of information on Krill and Antarctica - KrillFacts.org for more information about krill in the Antarctic Region.
References & Further Research
Research Euphausia superba » Barcode of Life ~ BioOne ~ Biodiversity Heritage Library ~ CITES ~ Cornell Macaulay Library [audio / video] ~ Encyclopedia of Life (EOL) ~ ESA Online Journals ~ FishBase ~ Florida Museum of Natural History Ichthyology Department ~ GBIF ~ Google Scholar ~ ITIS ~ IUCN RedList (Threatened Status) ~ Marine Species Identification Portal ~ NCBI (PubMed, GenBank, etc.) ~ Ocean Biogeographic Information System ~ PLOS ~ SIRIS ~ Tree of Life Web Project ~ UNEP-WCMC Species Database ~ WoRMS
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