That strange, tree-like web you’ve seen a ribbon worm shoot out is actually a body part called a proboscis, and the worm uses it to catch prey. It’s not silk, not venom spray, and not a defensive display. It’s a muscular, sticky hunting organ that the worm fires outward to net food and drag it back to its mouth.
What the “Web” Actually Is
Ribbon worms (phylum Nemertea) are mostly marine worms with a unique organ no other animal group has: an eversible proboscis stored inside a fluid-filled internal chamber called the rhynchocoel. Think of it like a finger of a rubber glove being turned inside out. The proboscis sits folded inward inside this chamber, which can run up to half the length of the worm’s body. When the worm is ready to strike, it rapidly turns the proboscis outward through an opening near its head.
In most ribbon worm species, the proboscis comes out as a single tube. But the viral videos that sparked this question typically show a species from the genus Gorgonorhynchus, which has something far more dramatic: a branching proboscis. Instead of one strand, this proboscis splits into dozens of tree-like filaments that spread across a surface like a web or net. That milky, pulsing mass you see in the videos is this branching organ unfurling in real time.
How the Worm Fires It
The proboscis launches through hydrostatic pressure, the same basic principle behind a water balloon being squeezed. Muscles in the wall of the rhynchocoel contract, pressurizing the fluid inside the chamber and forcing the proboscis to evert (flip inside out) through the opening at the front of the body. The whole process happens in a fraction of a second.
The proboscis itself is a long muscular tube formed by an inward folding of the worm’s body wall at its front end. When it fires, the surface that was previously on the inside is now on the outside, exposing a layer coated in sticky mucus. In some species, the outer surface of the everted proboscis also carries tiny rod-shaped structures called pseudocnidae. These contain hollow, thread-like cores that can puncture the skin of prey on contact, driven by mechanical stimulation when they press against a target. During attacks, researchers have observed the surface layer of the proboscis partially peeling off and sticking to prey, with numerous extruded pseudocnidae embedded in the mucus. It’s a combination of physical grip and chemical adhesion.
Why They Do It: Catching Prey
Ribbon worms are predators. They hunt mollusks, snails, and other marine worms. The proboscis is their primary weapon. A species like Gorgonorhynchus fires its branching proboscis outward like a net, tangling around the prey and holding it in place with sticky secretions. Once the target is immobilized, the worm retracts the proboscis and drags the prey toward its mouth, swallowing it whole.
For species with a non-branching proboscis, the strategy is more like a lasso than a net. The single tube wraps around the prey and pins it down. Some ribbon worm species in a group called Hoplonemertea have an even more specialized version: their proboscis tip carries a hard, pointed structure called a stylet, which they use to stab prey and inject toxic secretions directly.
Toxins in the Mucus
The sticky coating on the proboscis isn’t just glue. Ribbon worms produce a cocktail of biologically active compounds. Across different species, researchers have identified pyridine-based toxins, tetrodotoxin (the same paralytic compound found in pufferfish), and a range of peptide toxins including both cytotoxins that damage cells and neurotoxins that interfere with nerve signaling. The specific mix varies by species. Some groups carry all known toxin types, while others, like the more heavily armed Hoplonemertea (the stylet-bearing species), actually lack neurotoxins entirely, relying more on their physical stabbing apparatus.
These toxins serve double duty. Offensively, they help subdue prey during capture. Defensively, the mucus that ribbon worms secrete across their body surface also contains toxic compounds, discouraging fish and other predators from eating them.
Can the Worm Regrow Its Proboscis?
Ribbon worms are among the most regenerative animals on Earth. If a ribbon worm is cut or damaged, it can regrow lost body parts. In some species, entire new worms can form from fragments of the original. While losing or damaging the proboscis during a strike is a real cost, the worm’s regenerative capacity means it’s not a permanent loss. The proboscis can be rebuilt over time, making even an aggressive, all-out hunting strike a recoverable gamble.
Why the Videos Look So Strange
Most footage of ribbon worms “shooting webs” shows the animal out of water, often on someone’s hand or a boat deck. This is important context. In their natural shallow marine habitat, the branching proboscis would spread through water and wrap around prey on the seafloor. On a dry surface, the sticky mucus-coated filaments splatter and spread in ways that look alien and chaotic, which is why the videos tend to go viral. The worm is almost certainly stressed and firing its proboscis defensively rather than hunting. What you’re watching is a predatory organ being deployed in the wrong environment, which is exactly what makes it look so bizarre.