A zooplankton sample from the North Sea full of copepods and several of arrowworms.

  • Taxonomy

    Kingdom: Animals

    Phylum: Arrow worms (Chaetognatha)

  • Location

    Marine waters, worldwide.

  • Size

    2 – 120 mm


Arrow worms are a separate strain of the animal kingdom. Arrow worms are usually fairly large, transparent, streamlined animals with fins on both sides of the body and a tail fin, which makes them look like fish. On their heads they have a set of hooks to grab prey, and many species can inject poison into their prey to paralyse it.

Biology and ecology

Arrow worms are an important part of plankton biomass in the sea. They are real predators that can detect vibrations in the water with a system similar to the sidelines of fish and strike quickly, after which they grab the prey with hook-shaped bristles on the head. They are very fast swimmers and most will have likely escaped quickly when sampling with a plankton net. 

Arrow worms are hermaphroditic, and have both ovaries and testes in their bodies. There is no larval stage, young arrow worms can look for food almost immediately.

An arrow worm from the North Sea with a parasitic nematode and a just eaten copepod visible.

Two arrow worms from the North Sea with again parasitic nematodes.