Invertebrates in the Plankton: Chaetognatha
Chaetognaths are a dominant part of the plankton, outnumbered only by copepods. Chaetognaths are transparent and shaped like a torpedo or arrow; hence the common name "arrow worms." These worms are predators of copepods, larval fish, crustaceans, and other chaetognaths. Using grasping spines found on either side of the head, chaetognaths can grab and ingest very large organisms. The arrow worms use mechanoreceptors that sense water movements to help detect other organisms, and some species have a specialized venom in the head region that helps to subdue captured prey. Besides being active predators themselves, chaetognaths are an important food source for fish and other marine animals. It is believed that chaetognaths use their mechanoreceptors to follow the vertical migration of prey, that is their daily movement up or down in the water column.

When chaetognaths swim they cover their grasping spines with a hood, making them more streamlined and allowing for faster movements. Swimming in chaetognaths is accomplished by bending the head region back towards the tail in a flicking movement which cause the sleek worm to glide forward. Fins are also important in the organisms locomotion. Posterior fins are used to overcome sinking and facilitate stablization of the swimming motion. The tail fin may help to rocket the organism through the water.
 Reproduction in chaetognaths is unique. All chaetognaths are hermaphroditic, meaning they have both male and female parts. Eggs are formed in the trunk region of the body, whereas the sperm develop in the tail portion. Sperm can sometimes be seen swimming inside the tail portion of  the body. One species courts by grasping the potential mate with its head spines. It is thought that several species undergo a similiar courtship in order to prevent themselves from being interpreted as a tasty meal.
During the courtship the sperm must rupture through an outpocketing of the body and travel down along the partners body into the female opening. It is not known how sperm find their way from the male into the female opening. It is thought that some species are capable of self fertilization (egg and sperm come together from the same individual), which is rare in other invertebrates.
After fertilization, the eggs are secreted in a slime that is either dropped in the water column, brooded (kept on the body until metamorphosis), or placed on vegetation. Placing the eggs on vegetation is thought to be advantagous as it keeps the eggs from drifting away from the coastal planktonic environment.

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