We are independent & ad-supported. We may earn a commission for purchases made through our links.
Advertiser Disclosure
Our website is an independent, advertising-supported platform. We provide our content free of charge to our readers, and to keep it that way, we rely on revenue generated through advertisements and affiliate partnerships. This means that when you click on certain links on our site and make a purchase, we may earn a commission. Learn more.
How We Make Money
We sustain our operations through affiliate commissions and advertising. If you click on an affiliate link and make a purchase, we may receive a commission from the merchant at no additional cost to you. We also display advertisements on our website, which help generate revenue to support our work and keep our content free for readers. Our editorial team operates independently of our advertising and affiliate partnerships to ensure that our content remains unbiased and focused on providing you with the best information and recommendations based on thorough research and honest evaluations. To remain transparent, we’ve provided a list of our current affiliate partners here.

What are Arrow Worms?

Michael Anissimov
Updated May 21, 2024
Our promise to you
All The Science is dedicated to creating trustworthy, high-quality content that always prioritizes transparency, integrity, and inclusivity above all else. Our ensure that our content creation and review process includes rigorous fact-checking, evidence-based, and continual updates to ensure accuracy and reliability.

Our Promise to you

Founded in 2002, our company has been a trusted resource for readers seeking informative and engaging content. Our dedication to quality remains unwavering—and will never change. We follow a strict editorial policy, ensuring that our content is authored by highly qualified professionals and edited by subject matter experts. This guarantees that everything we publish is objective, accurate, and trustworthy.

Over the years, we've refined our approach to cover a wide range of topics, providing readers with reliable and practical advice to enhance their knowledge and skills. That's why millions of readers turn to us each year. Join us in celebrating the joy of learning, guided by standards you can trust.

Editorial Standards

At All The Science, we are committed to creating content that you can trust. Our editorial process is designed to ensure that every piece of content we publish is accurate, reliable, and informative.

Our team of experienced writers and editors follows a strict set of guidelines to ensure the highest quality content. We conduct thorough research, fact-check all information, and rely on credible sources to back up our claims. Our content is reviewed by subject-matter experts to ensure accuracy and clarity.

We believe in transparency and maintain editorial independence from our advertisers. Our team does not receive direct compensation from advertisers, allowing us to create unbiased content that prioritizes your interests.

Arrow worms are a highly unusual group of "worms" not obviously related to any other worm or any other animal. They make up their own phylum, Chaetognatha, one animal phylum among about 37 total. In terms of numbers, arrow worms are among the most abundant phyla on the planet -- only arthropods, nematodes, and a few others even come close. Arrow worms inhabit the water column, usually at the pelagic zone, within 200 m (656 ft), where most plankton lives, though they can be found as deep as the lower mesopelagic zone, 1000 m (3,280 ft). Alongside copepods, the crustacean "insects of the sea", chaetognaths are the most common form of plankton.

Their name -- Chaetognatha -- comes from the Greek chaite for long hair and gnathos for jaw. The long hair is a reference to the hooked, chitinous grasping spines found in pairs on their head, their primary hunting tool. The spines are held in a hood while they are swimming. Arrow worms are carnivores, eating up any plankton that they can find. Their size is roughly 3-5 cm (1-2 in), about the length of a fingernail, though some individuals may be as large as 10 cm (4 in). There are only about 120 species of chaetognaths known in 20 genera, but despite the low species count, their abundance is difficult for the human mind to imagine.

We can give a rough estimate for the number of arrow worms worldwide. Measured densities range from about 1 to 30 individuals per cubic meter of illuminated water, with an average of five. As the World Ocean has an area of about 340 million square kilometers, not including Arctic and Antarctic waters, and the pelagic zone is 200 m deep, we can estimate a world total of approximately 340 billion, which is probably within an order of magnitude of being correct. Interestingly, this also allows us to estimate that the human biomass exceeds the arrow worm biomass.

Because chaetognaths are transparent, scientists can put them under a microscope and observe the entire process of their digestion. They estimate that arrow worms eat between 3 and 50 prey items per day, including numerous types of larvae, copepods and other small crustaceans, and other arrow worms. They pierce the thin protective tissue layers of these organisms with their chitin spines, then inject them with toxins, such as the neurotoxin tetrodotoxin, to cause death.

Arrow worms are considered a good model for an early bilaterian. They are thought to be basal protostomes (one of the two major divisions of animals), even though elements of their embryological development are reminiscent of deuterostomes. This is thought to be because they represent a very early branching off of protostomes from deuterostomes. This event likely occurred an extremely long time ago, in the Ediacaran period. Molecular and morphological studies have indicated that chaetognaths are most closely related to nematodes, and may in fact be related the common ancestor of ecdysozoans (the group that includes every animal that sheds its cuticle).

All The Science is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Michael Anissimov
By Michael Anissimov
Michael Anissimov is a dedicated All The Science contributor and brings his expertise in paleontology, physics, biology, astronomy, chemistry, and futurism to his articles. An avid blogger, Michael is deeply passionate about stem cell research, regenerative medicine, and life extension therapies. His professional experience includes work with the Methuselah Foundation, Singularity Institute for Artificial Intelligence, and Lifeboat Foundation, further showcasing his commitment to scientific advancement.
Discussion Comments
Michael Anissimov
Michael Anissimov
Michael Anissimov is a dedicated All The Science contributor and brings his expertise in paleontology, physics, biology...
Learn more
All The Science, in your inbox

Our latest articles, guides, and more, delivered daily.

All The Science, in your inbox

Our latest articles, guides, and more, delivered daily.