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Overview

Brief Summary

The Nemertea, also called ribbon worms or proboscis worms, (and sometimes referred to as Rhynchocoela or Nemertini,) are a distinctive group of 1150 known species of mostly marine invertebrates, found world-wide (Gibson, 1995).

Nemerteans have an unsegmented body, thin and elongated with no differentiated head. Unlike flatworms, which they resemble in many ways, they have a separate mouth and anus, and a digestive tract that runs the full length of their body. Very distinctive is their eversible proboscis, which is stored in the body-length long fluid-filled rhyncocoel when not in use. When activated, it is forced inside out with hydrostatic pressure from body muscle contractions to wrap around its prey , often administering venom through rhabdites (holes). Some species (members of the order Hoplonemertea) are armed with a pointed stylet at the tip of the proboscis, which the animal uses to puncture and kill its prey. Since stylets are frequently lost or broken in hunting, and growing worms require larger stylets, the stylets are continually formed (in large epithelial cells) and stored so there is always one on hand to replace an old or lost one.

Nemerteans have more developed muscle than flatworms, and can contract their body to up to a tenth of their extended length. They use body muscles to locomote (and have cilia covering their epidermis and lots of gland cells for mucus production so they glide across surfaces). They also use body musculature to move food through their gut and blood through their circulatory system.

(Kozloff 1990; Brusca and Brusca 2003)

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 General literature on nemerteans with ID-keys

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Comprehensive Description

General Description

Nemerteans are unsegmented, dorsoventrally flattened predatory marine worms occurring at all ocean depths, freshwater, and terrestrial habitats. Approximately 1300 valid described species of the phylum Nemertea, or ribbonworms, are known worldwide. Current fieldwork suggests that at least several times this number remain to be named or discovered. Nemerteans are unsegmented worms characterized by a unique and remarkable eversible proboscis. Some are very colorful, while others are drab. They range from one millimeter to more than 30 meters long. They can be voracious predators, some are highly specialized while others are more eclectic with diets that favor other worms, crustaceans, and molluscs. They are poorly known to non-specialists because most nemerteans live in concealment, are difficult to collect, and because traditional taxonomy focuses significantly on internal anatomy based on histological study. However, many are common, abundant, and can be key predators, while the phylum itself is important to understanding evolution of early invertebrate body plans.

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The smallest of these soft, elongated, mostly marine worms may be threadlike and only a fraction of an inch long. The giants of the group, however, are the longest, though certainly not among the largest, of invertebrates. Exactly how long it is difficult to say, for all the ribbon worms are highly elastic, and the really long ones stretch out, threadlike, for yards and yards — some say much more than 30 yards in Lineus longissimus, the blackish brown worm of the North Sea. The English call it the "bootlace worm." Modest length, not more than about 8 inches, is more usual. The body may be cylindrical, as in Lineus, though more often flattened on both sides or flattened below and convex above.

Bright colorings of orange, red, purple, or green, these mostly on the upper surfaces, may betray the worms to the eyes of naturalists scanning rocky crevices or overturned stones at low tide. More often the colors blend with red or green algae or other colorful growths among which the worms live. To find small nemerteans, collectors place masses of seaweed or of bryozoan colonies that resemble delicate seaweed in dishes of sea water and let the small worms creep out on the walls of the dishes, where they can easily be seen. Some worms are white or yellowish, others somber grays or browns, but many are handsomely patterned with strongly contrasting rings or longitudinal stripes or both. The front end is not set off as a distinct head, though the tip may be expanded and have colored markings, several or numerous eyes, and sensory grooves, which make it look superficially like a head. The rear end is more or less pointed.

Another common name, proboscis worm, less widely used, calls attention to the most distinctive feature of nemerteans. This is a long, extensible, tubular proboscis that can be shot out the front end with explosive force to grasp prey or discourage enemies. The proboscis coils about the prey, holding it firmly and entangling it in sticky mucus which may be irritating or even poisonous. The proboscis is also everted as a device for burrowing in sand or mud or for attaching to objects as an aid in creeping about. It can be made to evert by irritating the animal, by plunging it into fresh water, or by placing it in a small dish of sea water and cautiously adding alcohol, drop by drop. The accurate aim of the proboscis receives recognition in the technical name of the phylum, Nemertea, from a Greek word that means "unerring." In some of the commonest worms the tip of the proboscis is armed with a sharp spike or stylet, which pierces the prey, sometimes several times, before a toxic secretion is poured on. Worms may have two or more pouches with a reserve supply of stylets, so that replacement can be made quickly if the main one is damaged. When not in use the proboscis is sheathed in a muscular tube that lies above the digestive tract.

As it goes on mostly at night, feeding is not often observed. The favored food seems to be annelids, and these have been seen to be swallowed whole, making a prominent bulge in the thin, elastic body of the nemertean. Mollusks, crustaceans, and fishes are also eaten, though bigger prey may be sucked at, not downed in one piece. Undigested residues do not have to be cast out the mouth, for the nemerteans are the lowest animals that have an anus, a second open- ing to the digestive tract, which voids materials from the rear end of the animal. The ribbon worms are built much like flatworms, but aside from the anus they can boast another important improvement. They have contractile blood vessels. Waves of contraction in the strong muscles of the body wall also help to push blood and food along their respective tubes, and in a worm at rest it is these powerful muscular waves that are seen to pass along the body.

A few ribbon worms swim by undulations of the long body. The young and the smaller forms glide along, by means of beating cilia on the body surface, over a lubricating bed of secreted slime. In larger worms more use is made of muscular contractions for creeping. Some even spiral ahead at times by agile body contortions.

One may grasp several inches of a delicate, slimy nemertean and pull cautiously lest it break, yet have it slip from one's fingers and disappear down a crack in the rock. Worms that do break in escaping from would-be captors, human or animal, almost always replace a missing rear end; and certain species can regenerate a whole worm from any fragment that contains a portion of one of the lateral nerve cords. As in flatworms, the capacity for regeneration goes with the natural capacity of certain species for reproducing asexually by fragmentation of the body, especially during warm months. A large specimen of Lineus socialis, which lives gregariously under stones on the American Atlantic coast (or of Lineus vegetus on the west coast), may fragment into six to twenty or more pieces. After transforming into complete worms of smaller size, these grow again and later reproduce sexually. Most though not all ribbon worms are of separate sexes. The eggs are usually laid in gelatinous strings or masses, and the young hatch as juvenile worms. In some species of Lineus, in Cerebratulus, and in some of their relatives, the egg hatches as a gelatinous, helmet-shaped, free-swimming little larva, called a pilidium. It must feed on microscopic organisms and develop further before it takes on the structure of the adult.

For the most part, ribbon worms are bottom dwellers on temperate marine shores, where they burrow in mud or sand or creep about among rocks and seaweeds between tide marks or in shallow waters. Only a few burrow into the deep-sea bottom, sometimes at depths of forty-five hundred feet or more. Of some 570 described species, nearly 200 are found along the Atlantic or Mediterranean shores of Europe. About 100 live on the Pacific coast of North America, at least 18 of them identical or very similar to European species. The Atlantic coast of North America has few more than 50 known species, and W. R. Coe, the American authority on nemerteans, thought this was due to the cold arctic current that comes close to the coast as far south as Cape Hatteras, for many of the missing genera are warm-temperate forms. Almost 30 species are described from Japanese shores. In the open seas, chiefly the southern parts of the North Atlantic, there are nearly 60 gelatinous species that drift or swim slowly far below the surface. They have been brought up from depths ranging from six hundred to nine thousand feet, most from below three thousand feet. Nemerteans are less common in tropical or subtropical seas, but well rep- resented in arctic and antarctic waters, often by the familiar temperate genera: Lineus, Amphiporus, Cerebratulus, and Tetrastemma.

Perhaps the most cosmopolitan species is Lineus ruber, found from Siberia to South Africa. The slender, rounded body is 3 to 9 inches long; and different varieties are colored red, green, or brown, any of them difficult to see in natural surroundings, even when one has lifted the stone under which the worm lives. Fresh waters, especially in northern latitudes, harbor species of the genus Prostoma. What seems to be a single species, Prostoma rubrum, a slender reddish worm less than 1 inch long, can be found in pools and quiet streams in nearly all parts of the United States. It clings to the leaves of aquatic plants and feeds on minute crustaceans, nematodes, and turbellarians. In Europe this genus has also an eyeless variant that lives in caves.

Land nemerteans are all of the genus Geonemertes. The two best-known species are slender, pale in color, and not more than 2 inches long. By exploiting the nemertean talent for copious secretion of slime, land nemerteans manage to live along marine shores, in moist earth, or under foliage and fallen logs, in such places as Bermuda, Australia, New Zealand, and many South Pacific islands. In the Seychelles, Geonemertes arboricola occupies the leaf bases of a screw pine (Pandanus) tree, often living high in the tree.

Only Carcinonemertes has been classed as a parasite. It lives on the gills of various crabs when it is young, and then moves to the egg masses, both feeding on the eggs and living as a commensal by eating any small animals it can find as it clings to its host. Adults of Carcinonemertes carcinophila are about 1 inch long and orange- or brick-red.

Commensal nemerteans live mostly in tunicates, sponges, or bivalves, sharing the food in the host's feeding currents. Common in the mantle cavity of various clams on European and both American coasts is Malacobdella grossa, a short, white, thick worm, with an adhesive disk at the rear. It creeps in leechlike fashion. The genus to which it belongs constitutes a separate order of nemerteans.

The other three orders contain all the more typical elongated worms; they are distinguished from each other mostly by internal characters, such as the arrangement of the muscle layers. The paleonemerteans, with an unarmed proboscis, include such forms as Tubulanus.  Also with unarmed proboscis are the heteronemerteans, among them Lineus and Cerebratulus. The latter is a very large, firm, and flattened worm which lives in burrows in sand or mud and swims actively through the water. The hoplonemerteans, with an armed proboscis, are divided into two suborders. In one the members have at the tip of the proboscis a single stylet, a straight or curved thorn which pierces and holds prey. These include many quite common shore forms such as Amphiporus; the very slender Emplectonema, found among mussels and barnacles on pilings; and Paranemertes peregrina of the American west coast, often a rich purple on the upper surface. The parasitic or commensal Carcinonemertes belongs here, as do various commensal species, the fresh-water forms, and also the land nemerteans. In the second suborder, members have on the tip of the proboscis not one large stylet but a large number of minute ones. These worms include some shore species, but most float or swim in the open sea far below the surface. Many are broad, flattened worms, of yellow, orange, pink, or red hues. The drifting types are quite gelatinous, the swimming ones equipped with tail and sometimes also with side fins.

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Etymology

Greek: "Νεμερτής " - Nemertes, sea nymph, a daughter of Nereus and Doris.

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Physical Description

Size

Most nemerteans are between 5mm and 20 cms, but one Norwegian species, Lineus longissimus gets to a length of 30 meters, and classic literature describes a report of a nemertean 60 meters long – longer than Blue whales, which are considered the world’s longest animal species (Brusca and Brusca 2003)

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Ecology

Habitat

While most nemerteans have a benthic marine lifestyle, they are also found in fresh water and brackish waters, and there are several terrestrial species.

(Kozloff 1990; Brusca and Brusca 2003)

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Nemerteans hide in rocky crevices, beneath stones, algal holdfasts, or burrow into substrate.

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Trophic Strategy

Mostly ferocious predators or scavengers, nemerteans hunt their prey using their eversible proboscis, a specialized organ unique to the group. While far less common, there are also filter-feedering nemerteans (that form associations with clams, tunicates and muscles, feeding within the mantles), and some that are parasites on crab species. (Brusca and Brusca 2003)

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Associations

Known Prey Organisms

Annelid worms, crustaceans

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Known prey organisms

Nemertea preys on:
Lumbrinereis
Notomastus
detritus

Based on studies in:
USA: California (Estuarine, Intertidal, Littoral)
South Africa (Desert or dune)

This list may not be complete but is based on published studies.
  • G. E. MacGinitie, Ecological aspects of a California marine estuary, Am. Midland Nat. 16(5):629-765, from p. 652 (1935).
  • A. C. Brown, Food relationships on the intertidal sandy beaches of the Cape Peninsula, S. Afr. J. Sci. 60:35-41, from p. 39 (1964).
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Known predators

Nemertea is prey of:
Calidris ferruginea

Based on studies in:
South Africa (Desert or dune)

This list may not be complete but is based on published studies.
  • A. C. Brown, Food relationships on the intertidal sandy beaches of the Cape Peninsula, S. Afr. J. Sci. 60:35-41, from p. 39 (1964).
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Life History and Behavior

Life Cycle

Nemerteans have simple reproduction, with many testes or ovaries built into the body wall so eggs and sperm are released directly to the outside via pores, or by breakages in the body wall. Most marine species have separate sexes, but freshwater and terrestrial species are often hermaphroditic. Usually fertilization occurs externally, with spawning induced by mating behaviors and pheremones. Mating balls can be seen containing many individuals responding to chemical cues and often coordinated spawning. Eggs develop either individually or in clumps protected in egg masses, or in a few species carried by the females in the ovaries until they hatch. In most nemertean classes development is direct, yielding oval shaped, cilia covered juveniles, which may have a pelagic lifestyle temporarily before settling into a more benthic one. There is some diversity in developmental strategies among the order Heteronemertea. Many of these species develop indirectly through a feeding, swimming larval stage called the pilidium, which looks like a swimming helmet with ciliated lobes. After swimming and feeding freely, the larval ectoderm separates to form a protective skin housing the metamorphosed juvenile inside, which lives planktonically before shedding the skin and settling on the benthos. A few species, including Micrura akkeshiensis goes through the Iwaka larval stage (similar to a pilidium larva but without the lobes). Those heteronemerteans that hatch from a benthic egg case undergo development through the Desor larval stage, which although usually classified as direct, does go through a metamorphosis. Some nemerteans can also reproduce asexually by splitting, but this is not usually a regular reproductive strategy. Many species can regenerate to various degrees, including regeneration of the proboscis.

(Kozloff 2003; Brusca and Brusca 1990)

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Evolution and Systematics

Evolution

Systematics and Taxonomy

Nermertea have traditionally been considered acoelomates, and as the sister taxon to the acoelomate flatworms, and have solid bodies (Brusca and Brusca 2003). However, new morphological and molecular analyses now suggest that their body cavities, the rhynchoceol and circulatory system, may in fact be homologous to the coelomic cavities of animals within the Lophotrochozoa Halanych 2004, Tuberville et al 1992). The relationships within the phylum have also been investigated with molecular markers, and in most cases support traditional classifications (Thollesson and Norenburg 2003). There is no fossil record for the Nemerteans. This is not suprising since they have soft bodies, but even their mineralized stylets have not been found (Waggoner and Collins 2001).

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Life History

Direct or indirect development with a pilidium larva.

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Molecular Biology and Genetics

Molecular Biology

16S rDNA and cytochrome oxidase (CO1)

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