8 Frightening Facts About the Bobbit Worm

A bobbit worm extended a couple inches above the sand with its jaws open.

kanyhun / Imazins / Getty Images

The bobbit worm gained international fame after its 2017 feature in Blue Planet II. Since then, the worm has generated fear and fascination alike. Whether it's the worm's impressive length, strong, scissor-like jaws, or its ambush-style of hunting, there's plenty of reason to be afraid of — and fascinated by — the mysterious bobbit worm.

1. The Bobbit Worm Can Grow to Be Nearly 10-Feet Long

In 2009, a nearly 10-foot-long bobbit worm was discovered living within an aquaculture raft in Shirahama, Japan. At some point during the fish pen's 13-year tenure, a bobbit worm decided to make the place its home. The aquaculture raft's hidden resident was only discovered when the raft was decommissioned. Other similarly long bobbit worms have been discovered in Australia and the Iberian Peninsula, although bobbit worms of these impressive lengths are decidedly rare.

2. They've Been Around for at Least 20 Million Years

The bobbit worm's mucus secretions and iron deposits (more on those below) together have allowed some bobbit worm dens to remain preserved in the fossil record, including a 20-million-year-old bobbit worm lair in Taiwan. Bobbit worms are unique in that they are among only a few species of predatory worms ever found fossilized — most other underwater worms discovered in the fossil record are believed to have lived off of detritus or tiny particles floating in the water.

3. Bobbit Worms Build Mucus-Lined Burrows in the Seafloor

Bobbit worm in Lembeh strait
Jenhung Huang / Getty Images

It's rare to see a bobbit worm's full body. Unlike other related species, it creates an L-shaped burrow in the sand to hide away undetected. Upon reaching sexual maturity, some bobbit worms line their burrows with mucus to establish a more permanent fixture in the sand. The proteins in the mucus strengthen the burrow's walls, helping the burrow stay in place.

4. They Hunt by Ambushing Prey

From their sandy burrows, these underwater worms do what they can to remain hidden. Some bobbit worms have been seen to go as far as using an antenna to mimic a smaller ocean worm. Regardless of whether prey is attracted to the bobbit worm's lair by the antenna decoy or by sheer bad luck, the bobbit worm responds immediately. The hidden creature is said to rapidly thrust its body out of its burrow, grab its prey, and drag its prize back into its den. The ensuing fight can collapse a bobbit worm's burrow opening.

5. Microbes Deposit Iron Outside the Bobbit Worm's Den

The mucus secreted by the bobbit worm is full of nutrients microbes love. Sulfate-reducing bacteria in particular enjoy the bobbit worm's carbon-rich mucus. By snacking on bobbit worm secretions, these microbes create conditions ripe for sulfide to accumulate. When portions of the burrow are exposed to oxygen in seawater, like the burrow's lining and the burrow opening, the iron sulfide becomes iron hydroxides like hematite, limonite, or geothite. In other portions of the bobbit worm's burrow where iron concentrations are low, small collapses in the sediment create a feather-like pattern.

6. Fish Fend off Their Attacks with Water Jets

Peter's monocle bream (Scolopsis affinis)
Peter's monocle bream (Scolopsis affinis) will defend itself against bobbit worm attacks.

Rickard Zerpe / Flickr / CC BY 2.0

Tropical fish can defend themselves from bobbit worm attacks with a tactic scientists describe as 'mobbing'. When the Peters' monocle bream, a type of tropical fish, is attacked by a bobbit worm, the fish directs sharp jets of water back at its attacker. In a coordinated group attack, other nearby Peters' monocle breams join in with additional water jets. The fish's mobbing behavior can coerce the bobbit worm into abandoning its attack.

7. Bobbit Worms Can Secretly Wreak Havoc in Aquariums

Just like the nearly 10-foot undetected bobbit worm found in a Japanese aquaculture pen, bobbit worms have been found hiding out in aquariums, too. In 2009, an aquarium in the U.K. discovered a 4-foot-long bobbit worm in one of their tanks. The bobbit worm attacked a number of prized fish before its discovery. On another occasion, a home aquarist found a bobbit worming hiding in his fish tank. In both cases, the bobbit worm broke into multiple pieces when handled. Even when separated, the bobbit worm pieces appeared to still be alive.

8. Their Jaws Are Wider Than Their Body

A bobbit worm with its jaws widened.
The bobbit worm's scissor-like jaws are wider then its body.

atese / Getty Images

The bobbit worm has two pairs of scissor-like, retractable jaws that extend well past the worm's body when open. When waiting for unsuspecting prey, the bobbit worm sits with just its jaws poking out of its burrow, open and ready to trap its next meal. According to some observations, the bobbit worm's jaws are so strong, they can cut the worm's prey in half. The bobbit worm's wide jaws are also impressively durable. Scientists have discovered the jaws of bobbit worms and their relatives preserved in the fossil record.