Aquascope Facts
Eukrohnia-arrow worm
(2 specie in Swedish waters)


Distribution in scandinavian waters
Maximum length: 4,5 cm. Compare its size with a finger.
Appearance: The creature is uncoloured and transparent. On its head are two rows of bristles with which it catches food. On its torso and tail, horizontal fins are located which are difficult to see because of their transparency. The body of the Eukrohnia-arrow worm is relatively soft and sloppy and clear black eyes are absent. Compare with the sagitta-arrow worm .
Depth: Is found from the surface and down to 1 000 m.
Environment: Pelagic.
Misc: Arrow worms are very effective predators that can catch and kill broods of their own size. They are fast swimmers and are able to puncture the external skeleton of their prey with their bristles and teeth and thereafter inject poison. Besides broods, they eat many of the other creatures that inhabit pelagic waters, such as krill, amphipods, ostracods, medusae and other arrow worms. The most important ingredient in their diet are copepods. Many of the above prey are themselves predators and eat amongst other things, arrow worms.
    Every arrow worm developes both male and female sexual organs. The male organs develope first, but they seldom fertilize themselves although they carry sperm and eggs at the same time. Fertilized eggs are released directly into the water where they develope directly (without a larvae stage) into arrow worms.
Classification: Eukrohnia-arrow worms belong to the chaetognatha group. The picture above shows Eukrohnia hamata.

Eukrohnia-arrow worm     Other names

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Bo Johannesson | Martin Larsvik | Lars-Ove Loo | Helena Samuelsson