Range: Worldwide marine waters

Habitat: Pelagic to benthic waters

Size: A few millimeters to 12cm (4.7in)

Diet: Copepods and other crustaceans, fish and their larvae

Threats: Fish, whales, each other, and more

Lifespan: Six weeks to two years

Transcript:

Today on Animal Fact Files we’re discussing arrow worms. These animals are so unique, even today scientists aren't exactly sure where to place them in the animal kingdom. Since their description in the 1770s, arrow worms were thought to be a type of arthropod, or maybe a mollusc! Today, it's debated which group of animals arrow worms are most closely related to, though current information suggests similarities to rotifers.

There's nothing like an arrow worm. These invertebrates have a long, thin body made up of a head, a trunk, and a tail. They have one to two pairs of fins that help with stabilization while swimming and their head is adorned with sharp spines or hooks for catching prey. There are nearly 200 described arrow worm species and likely more awaiting discovery. These animals live in marine habitats from coastal tide pools and estuarine environments to the depths of the ocean. Some are benthic, meaning they spend their lives at the bottom of the water column, sometimes attaching themselves to vegetation. Others swim in the open ocean making nightly migrations from deeper water to the water's surface.

Arrow worms are predatory. Their main diet consists of copepods which are small crustaceans that live in water. Arrow worms also eat other crustaceans as well as fish and fish larvae. They get their common name because they shoot through the water like an arrow towards their prey. Though these animals have eyes, they depend more on small hair-like structures surrounding their body to detect movement in the water. They'll rocket toward a meal with hungry jaws. The spines or hooks on the sides of their head help capture and hold prey. These worms even come equipped with toxins to render prey immobile. It's the same type of toxin found in blue ring octopus venom! And, no, we couldn't find any reports of human death by arrow worm.

When their jaws aren't in use, an arrow worm has a hood they can pull over their head to make them more streamlined in the water. Their bodies are transparent to translucent depending on the species, though deep water species may appear orange and opaque, and some species are bioluminescent. There are two fascinating facts about transparent arrow worms. First of all, some arrow worms have eyes that actually face down through their body allowing them to detect light and movement both above and beneath them. Second, a see-through body helps them blend in with their environment. Arrow worm predators include fish like skates and anglerfish, mammals like whales, and invertebrates like comb jellies and mussels. They also eat each other! 

In size, arrow worms range from just a few millimeters to twelve centimeters (4.7in) long. They're hermaphrodites meaning they process both male and female organs and, unlike many hermaphroditic invertebrates, they are able to self fertilize, though they don't commonly do this. Instead, they meet up with a partner, sometimes dancing to gain their attention, and swap sperm to fertilize each other's eggs. Some arrow worms brood their eggs until they hatch about 48 hours later. Others release their eggs which may settle to the sea floor, attach to vegetation, or float in the water column depending on the species. Arrow worms have direct development meaning there isn't a larva stage. The smaller species living in warm tropical waters may develop from eggs to reproductive adults in just six weeks time, while larger species in cold Arctic waters can take up to two years! It's believed they die after reproduction.

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