The krill are tiny crustaceans living in the ocean, found everywhere around the world. The name of the species is derived from the identically sounding Norwegian word, meaning „young fry of fish”, fry being the stage of life when the animal already looks like a mature fish and is able to feed on its own, but it still has no fins and scales.
Krill vary in terms of sizing: most of them are small, about 1-2 cms long at mature age, but there are a few species that grow to about 6-15 cms. They all have an exoskeleton that consists of three distinct parts: the head, the thorax and the abdomen. Most species feature compound eyes that adapt quickly and easily to the changing light conditions of the waters. As an appendage to the head, most animals also have antennae.
Different species have different numbers of legs. All species have five pairs of legs used for swimming and other leg pairs attached to the thoracic part of the animal’s body.
These tiny animals are filter feeders. Their mouth-like organs at the front of their heads form a very dense comb used for filtering nutrients out of the water. The food they are looking for include mainly phytoplanktons. Most of these creatures are omnivorous, meaning that they feed on zooplanktons as well as very small fish.
Behavior and Life Cycles
After these creatures emerge from the eggs, they go through several stages of development as larvae. They also go through several sessions of moulting, that is, shedding their exoskeletons when they become too tight – smaller species shed more often than larger ones and the frequency of moulting decreases as the animal advances in age. The length of inter-moult periods depends on the climate, on the species and on the availability of food as well, some species being able to moult even when food is scarce and their body size shrinks, making their exoskeleton too large.
Mating and Reproduction
Male krill deposit their sperm in the female’s body through an orifice called thelycum. The female then carries the sperm sack in her ovaries until the eggs are ready to be delivered. The number of eggs carried can reach several thousand, with the sack growing until it reaches about one third of the animal’s body.
These tiny, frail-looking animals are in fact very resilient and relatively long-lived. Some species live for as long as 6 years, while the majority of krill species live for about two years.