Marine Life: Krill Resource and Research Guide
Krill are tiny shrimp-like invertebrates which are also called euphausiids. Krill are located near the bottom of the food chain as they feed on phytoplankton and zooplankton, and serve as food for many larger animals such as whales, seals, and sea birds. In this sense, they act as a chain between the plankton and mammals, birds and fish. Antarctica Krill is the species mostly found in the Southern Ocean and it makes for more than 500 million tonnes of biomass which is double that of human beings. Krill are commercially fished in the Southern Ocean around Japan and are used for human consumption in Japan and Russia. Additionally, krill are used in the pharmaceutical industry, aquarium (feeds), and aquaculture.
Krill are classified as crustaceans and their chitinous exoskeleton has three segments:
- The cephalon (head)
- The thorax
- The abdomen
The initial two segments together make the cephalothorax. In most species, the external shell is transparent. Krill have compound eyes which can adapt to different lighting conditions due to the presence of screening pigments. Krill have 2 antennae as well as several pairs of thoracic legs, which are called pereiopods. The number of legs varies among species and genera. Krill have externally visible gills and organs called photophones. Photophones emit light which is generated during the catalysis of enzyme luciferase released by pigment luciferin. An adult krill can grow up to 1-2 cm long but some species are known to grow up to 6-15 cm. Krill have a fine comb-like structure at the front of their mouth which filters out water from food before it is consumed. They are mostly omnivores but some species are carnivorous and feed on fish larvae and zooplankton.
Krill are swarming animals varying in shape and size. Swarming is the mechanism used to confuse smaller predators. Krill follow the diurnal vertical migration. The satiated animals swim less while the krill with empty stomachs swim more actively. Krill swim at the speed of 5 to 10 cm per second and when they are in danger, they escape through a mechanism called lobstering. Krill in larva form are considered to be zooplankton.
Krill reproduce when they have a good amount of food. The male krill produce sperm which are transferred to the reproductive organs of female krill and the females keep the sperm until they are ready to lay eggs. The eggs leave the body of krill after formation and the eggs are fertilized externally.
Krill are an essential part of the food chain. They convert primary production energy into a form which is suitable for larger animals to consume because large animals cannot directly consume the minuscule algae. The disturbance of krill can produce an adverse impact on the ecosystem and it can even result in a decline of krill population. Climatic changes resulting in the melting of ice in Antarctica also pose a risk to the krill population because krill feed on algae found below the ice.
The krill life cycle starts as an egg. Then the egg goes through various larval stages. The larva grows and changes multiple times. The various stages of larva growth are:
- Nauplius – This is the first stage of growth.
- Pseudometanauplius – The stage when krill lay eggs in a sac spawner.
- Metanauplius – The larva grows and gets nourishment through the yolk.
- Calyptopsis – The mouth and digestive tract is developed.
- Furcilia stages – The segments and the pair of swimmerets are added.
After this stage, the krill comes out as an immature juvenile and later develops gonads to become an adult.
Habitats around the Globe
Krill species belonging to the genus Thysanoessa are found in the Pacific and Atlantic Ocean. Euphausia pacifica are found widely in the Pacific ocean. On the other hand, Northern krill is found in the north of the Mediterranean Sea. Krill are also found in high numbers in Humboldt, California, Benguela, and Canarias.
Krill belong to the Crustacea and class Malacostraca. The class includes the superorder Eucarida which is made up of three orders, namely, the Euphausiacea (krill), minuscule Amphionides, and Decapoda (lobsters, shrimp, crabs). In addition, the order Euphausiacea has the Euphausiidae family which includes 10 genera belonging to 85 species and the Euphausia family which contains 31 species. The most famous species of krill is the Northern krill or Meganyctiphanes norvegica, Pacific krill or Euphausia pacifica, and the Antarctic krill or Euphausia superba.
Miscellaneous Krill Resources