Invertebrates
Arrow Worm
Chaetognatha · also called arrow worms, chaetognaths
An arrow worm drifts through open water almost invisibly, a nearly transparent, dart-shaped body giving away nothing until the moment it strikes, snapping curved, hooked spines around the mouth fast enough to catch prey built for quick escapes. It never even needs to see its target coming, relying instead on sensing tiny vibrations in the water around it.
Overview
Arrow worms, phylum Chaetognatha, are small, transparent, torpedo-shaped predators found throughout the world’s oceans as a major part of the zooplankton community. They hunt copepods and other small prey using a rapid grabbing strike, guided by sensory bristles that detect water movement rather than sight.
Small, transparent, and surprisingly important
Their near-invisible, streamlined bodies help them avoid detection by both predators and prey. Despite receiving little public attention, arrow worms are abundant enough to be significant predators of copepods, forming an important link in marine food webs worldwide.
Status
Least Concern — abundant and widespread across every ocean basin, a stable and foundational part of global zooplankton communities.
Arrow Worm: common questions
How fast can an arrow worm actually catch its prey?
Arrow worms are capable of an extremely rapid grabbing strike, using curved, hooked spines surrounding the mouth to seize nearby prey such as copepods in a fraction of a second, a strike quick enough to successfully capture prey animals that are themselves adapted for fast escape responses, making the arrow worm a surprisingly effective and quick predator despite its small, delicate-looking body.
How does an arrow worm find prey without good eyesight?
Rather than relying primarily on vision, arrow worms detect prey mainly through sensory bristles distributed across the body surface that pick up subtle vibrations and water movement created by nearby swimming animals, allowing them to sense the direction and location of potential prey even in low-light or completely dark deeper water where vision would be of little use anyway.
Why are arrow worms nearly transparent?
A largely transparent body helps an arrow worm remain inconspicuous while drifting and swimming through open water, making it considerably harder for both predators looking to eat the arrow worm and prey it is hunting to visually detect it against the featureless backdrop of open ocean water, an effective camouflage strategy commonly seen among small, open-water zooplankton species generally.
Are arrow worms considered important in the ocean food web?
Yes, despite being small and often overlooked compared with more famous zooplankton groups, arrow worms are extremely abundant throughout the world's oceans and function as significant predators of copepods and other small zooplankton, while arrow worms themselves are in turn eaten by various larger predators, making them a genuinely important, if underappreciated, link connecting different levels of marine food webs worldwide.