Euphasiids is the scientific name for many species of tiny crustaceans known colloquially as krill. The name “krill” comes from the Norwegian word krill, meaning “small fry of fish”, which is also often attributed to species of fish. These small planktonic animals form the base of the ocean’s food chain worldwide.
The food chain begins with tiny floating plants known as phytoplankton that store the sun’s energy during photosynthesis. Euphasiids convert the plant sugars in the phytoplankton into animal tissue, and they then are eaten by fish, birds and marine mammals.
Significant krill predators in the north Pacific are salmon, herring and hake and the baleen whales. The hake is so closely allied to the krill as a food species that the two are almost inseparable. The Cassin’s auklet and ancient murrelet eat krill directly, and many other seabirds eat herring, sandlance and other small fish that eat krill.
The red colouration of krill is derived from carotenoid pigments.
Types Of Krill
There are about 85 species of euphasiids in the world’s oceans of which 15 species exist in all oceans. About twenty species of euphausiids have been recorded off British Columbia, with five species being the most numerous: Euphausia pacifica, Thysanoessa spinifera, Thysanoessa inspinata, Thysanoessa longipes and Thysanoessa rashii.
About 25 species are abundant off California. Unlike most zooplankton, euphausiids can live at depth in the ocean. They move between the ocean depths at night and surface waters during the day.
Red krill can turn surface waters a reddish hue in high density.
The average krill is about 16 mm long and can live one or two years. Spawning occurs in spring and summer to coincide with periods of high phytoplankton abundance. Euphausiids are harvested as a feed supplement for fish farms to give salmon flesh its pink colour, as fish food for aquarists, and as a fishery for human consumption.
The predominant euphausiids in the Gulf of Alaska are Thysanoessa inermis and Euphausia pacifica. Gravid females of T. inermis release eggs in April and May continually over a three day period. Gravid female E. pacifica are numerous from July to October and release eggs only for one day.
Large females release the most eggs. Eggs hatch simultaneously over a few hours depending on incubation temperature creating ‘blooms’ of plankton in the ocean.
We don’t usually think of plankton as being capable of migration but each evening, these tiny crustaceans make an extraordinary journey of 500 or more meters between the depths of the ocean and the surface waters. During a 24-hour period, these animals may pass through a range of temperatures of 16°C.
Except for Bentheuphausia amblyops, krill are bioluminescent animals having organs called photophores that can emit light. The light is generated by an enzyme-catalysed chemiluminescence reaction, wherein a luciferin (a kind of pigment) is activated by a luciferase enzyme.
Studies indicate that the luciferin of many krill species is a fluorescent tetrapyrrole similar but not identical to dinoflagellate luciferin and that the krill probably do not produce this substance themselves but acquire it as part of their diet, which contains dinoflagellates.
Krill photophores are complex organs with lenses and focusing abilities, and can be rotated by muscles. The precise function of these organs is as yet unknown; possibilities include mating, social interaction or orientation and as a form of counter-illumination camouflage to compensate for their shadow against overhead ambient light.
Abraham, C.L., G. S. Wolfe, J. M. Hipfner and W.J. Sydeman. The seasonal cycle of euphausiid zooplankton in the California Current system: A predator’s perspective. PISCES
McFarlane, G.A., and R.J. Beamish. 1985. Biology and fishery of Pacific Whiting, Merluccius productus, in the Strait of Georgia. Marine Fisheries Review 47:23-34
P. J. Herring; E. A. Widder (2001). Bioluminescence in Plankton and Nekton. In J. H. Steele; S. A. Thorpe; K. K. Turekian (eds.). Encyclopedia of Ocean Science. Vol. 1. Academic Press, San Diego.
Yin, K. P. J. Harrison, R. H. Goldblatt and R. J. Beamish. 1996. Spring bloom in the central Strait of Georgia: interactions of river discharge, winds and grazing. Marine Ecology Progress Series 138: 255-263.