Ribbon worms (phylum Nemertea) are marine invertebrates known for their unusual hunting method. These soft-bodied creatures, which can range from a few millimeters to many meters in length, employ a specialized internal organ to capture prey. When feeding or defending, the worm executes a sudden action that appears as though it is “spitting out” a white, sticky filament. This unique strategy relies on a remarkable internal anatomy and a powerful physical mechanism.
Anatomy of the Proboscis
The structure the ribbon worm ejects is the proboscis, a long, muscular tube unique to the Nemertea phylum. This eversible organ is housed within the rhynchocoel, a separate, fluid-filled cavity located above the digestive tract.
The proboscis functions solely as a weapon for capturing prey. When retracted, it is coiled within the rhynchocoel. Its muscular structure allows it to rapidly extend and contract, pulling captured prey back toward the worm’s mouth.
The Hydraulic Ejection Mechanism
The ribbon worm utilizes a hydraulic system, not just muscle contraction, to extend the proboscis. Deployment occurs through the rapid compression of fluid contained within the rhynchocoel. This compression generates internal pressure, forcing the proboscis out of an opening near the head.
This rapid eversion allows the organ to reach lengths many times the worm’s body size. This deployment is crucial for ensnaring smaller invertebrates. Once the target is secured, the proboscis muscles contract, pulling the captured meal back toward the mouth.
Stylets and Neurotoxins
The material ejected is a weapon armed with both mechanical and chemical components. Many species, particularly those in the class Hoplonemertea, possess a sharp, harpoon-like stylet at the tip of the proboscis. The stylet pierces the prey’s exoskeleton or skin, creating a channel for toxin delivery.
Accessory stylet sacs constantly produce replacements, as the stylet can be lost or broken during a strike. The ejected material is a potent cocktail of neurotoxins and enzymes secreted onto or through the stylet. One well-studied toxin is anabaseine, which quickly paralyzes the prey’s nervous system. Other species utilize toxins like tetrodotoxin (TTX) to immobilize small invertebrates such as crabs, clams, and other worms.
Habitat and Relevance to Humans
Ribbon worms are found globally, with approximately 1,300 known species. Most inhabit marine environments, often in benthic zones nestled in sand, mud, or under rocks. They play a significant ecological role as predators, controlling populations of smaller invertebrates. A few species are also found in freshwater or damp terrestrial environments.
The neurotoxins they produce pose no danger to humans. The toxin concentration is too low, and the delivery mechanism cannot penetrate the skin of a large mammal. Some ribbon worms secrete a toxic mucous for defense, but accidental human contact typically results only in minor irritation.