Ribbon Worm

Reviewed by: BD Editors

Two nemertean species isolated against a white background
A Ribbon Worm

Kingdom Animalia
Phylum Nemertea
Class Multiple (roughly 900 species)
Niche Benthic predator
Length Most species between 0.1-2.040 in (0.1-100 cm)
Lifespan Up to 24 months
Social Structure Solitary
Conservation Status Not endangered
Preferred Habitat Benthic marine habitats
Main Food Items and Prey Annelid worms, clams, crabs, fish
Predators Fish, crabs, sea birds, other ribbon worm species

The Basics

The ribbon worm, or nemerteans, is a group of segmented marine worms found throughout the world’s oceans. Ranging in size depending on the species, some ribbon worms grow very long. Indeed, the bootlace worm (Lineus longissimus), can grow longer than a blue whale, possibly making it the longest animal in the world.

A terrestrial ribbon worm moving against a green backdrop
Most ribbon worms are marine organisms while some are aquatic and fewer still are terrestrial, often living in damp environments such as under rotting logs.

Description

Nemerteans are a unique group of organisms that have a significant ecological role. The typical ribbon work is slim and long. The smallest species measure just a few millimeters in length and most are less than 8 inches long. However, some species are much larger, with many growing over a meter in length while the largest ever officially recorded was a specimen of Lineus longissimus. Although only a few millimeters wide, this specimen was measured to be more than 175 feet long. Longer than the blue whale, this may make L. longissimus the longest animal on earth.

The outside layer of nemertean’s bodies is made up of cilia that help them move, as well as glandular epithelium with rhabdites, specialized mucus-producing cells. The mucus produced by these rhabdites allows the cilia to move, providing mobility for the organism. Indeed, a trail of ‘slime’ can be observed behind most species. Ribbon worms have ventral mouths set back slightly from the front of the body. Its digestive system consists of a foregut, stomach, and intestine in the middle section of the body and its anus at the very tip of its tail.

A striped ribbon worm moving along a leaf
There are over 900 species of ribbon worms ranging in size from just a few millimeters to hundreds of feet.

The Proboscis Worm

All ribbon worms also have a proboscis – an infolding of the body wall – that can ‘unfold’ rapidly and is used to attack its prey. In the class Anopla, it exits from an orifice that is separate from the mouth and coils around its prey, immobilizing it with adhesive and toxic secretions. Some members of this class have a highly-branched proboscis, effectively forming a net that it throws over its prey before retracting it and drawing the prey into its mouth. In other classes, such as Enopla, the proboscis has a hard, calcareous (calcium-based) barb that it uses to stab its prey repeatedly, also injecting toxins and digestive enzymes in the process. The prey is then swallowed or partially digested first, depending on its size and the specific species’ feeding behaviors.

Distribution and Habitat

The majority of nemertean species are found in marine habitats, distributed throughout the world’s oceans. Some species, however, are aquatic, living in freshwater environments such as streams and ponds. Fewer still are entirely terrestrial, living their lives outside of the water.

Diet and Predators

Most ribbon worm species are carnivorous, feeding largely on annelids, clams, and various crustaceans such as crabs. Some species will also eat fish and many will scavenge on virtually anything they can find. The few terrestrial species of Argonemertes feed solely on insects and myriapods. Some species live inside the mantle cavity of various mollusks, feeding on small organisms filtered out by their host. In some cases, this relationship is parasitic, such as  Carcinonmetes errans, which has reduced the egg production of its host the Dungeness crab, a commercially important species.

A ribbon worm consuming a small fish.
Ribbon worms are capable of consuming prey relatively large for their size, using their proboscis to attack and often poison their prey.

Due to their ability to fill small cracks, crevices, or even live inside other organisms, most nemerteans have few predators. In addition, most species secrete toxins that deter many would-be predators. Bottom-feeding fish and horseshoe crabs are known to feed on some species, while seabirds likely feed on those accessible in the intertidal zone.

Reproduction

Ribbon worms possess some unique and fascinating reproduction and regeneration strategies. Larger species often break into segments when disturbed, with each fragment normally growing into full individuals. In some species, this also occurs routinely and in the absence of disturbance or stimulation. Ribbon worms also reproduce sexually and are gonochoric – with distinct male and female members in most species. Fascinatingly, all freshwater species are hermaphroditic, in addition to some of the hundreds of marine species.

In most species, temporary gonads will form in a row down each side of their bodies. These are ejected through gonoducts, which are also formed temporarily, with fertilization typically occurring externally. In some cases, eggs and sperm are broadcasted into the water while in others they may be laid in a burrow or tube. Some species even build cocoon-like structures or gelatinous masses to protect them during development. The number of eggs produced by females of each species varies greatly and is not well understood in many species. Similarly, the lifespan of many species remains unknown. However, it is believed to be on the order of months, with Paranemertes peregrina having been observed to have a lifespan of approximately 18 months.

Fun Facts about the Ribbon Worm!

Besides being one of the longest animals in the world, ribbon worms are a fascinating group for many reasons. From their circulatory system lacking a central heart and their ability to respire through their skin, these lesser-known animals offer many opportunities to explore various biological concepts.

Heartless

Nemerteans have unique circulatory systems lacking a central heart. Instead, they have two or more lateral vessels that run along their body and are joined at the end to form a loop. These are filled with fluid analogous to blood that is circulated by the contraction of muscles in the vessels and the body wall. This circulatory system is considered a rudimentary version of more complex systems. In fact, ribbon worms are the simplest animals to possess a circulatory system of any form.

A ribbon worm moving across a grey substrate
Ribbon worms use cilia lining their bodies to move within a mucus that they produce.

The Longest Animals

The largest species of ribbon worm is the bootlace worm, Lineus longissimus. This species is native to the North Sea where it can be found among rocks. Not only is it the largest species of ribbon worm, but it may be longer than the blue whale, making it the longest animal in the world.

Despite being only a few centimeters wide, this species has been regularly measured at lengths of over 100 feet and in some cases has been thought to grow to nearly 200 feet. However, due to the unique morphology that allows them to stretch and contract down to 10% of their maximum length, uncertainty remains around the accuracy of these measurements.

The Parasitic Worm

Not all ribbon worms are predatory. Instead, many live inside the mantle cavity of other species such as mollusks. In many cases, these are parasitic relationships. For example, a genus of ribbon worms known as Carcinonemertes is known to parasitize crabs such as the Dungeness crab. This ribbon worm lives on the crab, subsisting primarily on its eggs. This significantly reduces the reproductive capacity of the host individual which, in the case of this commercially fished species, has economic consequences for humans as well.

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Biologydictionary.net Editors. “Ribbon Worm.” Biology Dictionary, Biologydictionary.net, 23 Nov. 2020, https://biologydictionary.net/ribbon-worm/.
Biologydictionary.net Editors. (2020, November 23). Ribbon Worm. Retrieved from https://biologydictionary.net/ribbon-worm/
Biologydictionary.net Editors. “Ribbon Worm.” Biology Dictionary. Biologydictionary.net, November 23, 2020. https://biologydictionary.net/ribbon-worm/.

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