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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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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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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.
  • 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).
  • G. E. MacGinitie, Ecological aspects of a California marine estuary, Am. Midland Nat. 16(5):629-765, from p. 652 (1935).
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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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