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Distribution in scandinavian waters
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Maximum length: One specie (Sagitta maxima) can be as
long as 9cm. Compare with a finger.
Appearance: Uncoloured and transparent. On the side of the head
are a pair of rows of curved grasping spines. From the anterior (on
the middle of the body) to the tail are a pair of lateral fins. The
eyes of Sagitta are black and evident. Compare Sagitta
with the arrow worms of the group Eukrohnia.
Depth: From the surface to a depth of several hundred metres.
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 there after inject poison. Besides broods, they eat many
of the other creatures that inhabit pelagic waters, such as krill,
ghost shrimps, copepods,
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 larva stage) into arrow worms.
Classification: The family Sagitta is part of the group
chaetognatha. The two specie that are most common in Swedish waters
are Sagitta setosa and Sagitta elegans. Sagitta elegans
is shown in the picture above.
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