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NEMERTEA

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(Schultze, 1851)

Sections
  • Background
  • General Description
  • Development
  • Ecology
  • Zoogeography
  • Background

    Nemertines are unsegmented, worm-like organisms, inhabiting marine, freshwater and even moist terrestrial habitats. They are not a well known group, as most live hidden away. Approximately 1000 species have been described worldwide, of which around 100 marine species are found in European waters. They are commonly called ‘ribbon-worms’, as some are very long and capable of extreme contraction and elongation. They range in length from 0.5 cm to over 30 m, making them one of the longest animals in the world. However, most are less than 30 cm in length. They are also called ‘proboscis worms’, because they can evert a proboscis to capture their food. Some are brightly coloured with patterns of yellow, orange, red and green, but most are drab.

    Adult marine nemertines are generally found on the sea bottom, beneath rocks or thick algal growth, or burrowing into soft mud or sand. They are most abundant in coastal areas, but some species have been collected as deep as 4000m. When necessary, many can swim freely over the bottom, but a number of genera are adapted to live permanently swimming at sea. Other species live commensally in the mantle cavity of bivalve molluscs, or in the pharyngeal cavity of tunicates, and some even parasitise crabs. They are mainly carnivores and can be key predators, but some also scavenge on animal remains. Larval nemerteans often occur, usually in small numbers, in finer mesh shallow water plankton samples.

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    General Description

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    Development

    Depending on species, direct developing larvae may hatch at any stage up to an immature juvenile. The appearance and shape of these larvae is very variable, but they can resemble a coelenterate planula or flatworm larva, being flattened and usually elongated and can have single or one or two pairs of ocelli on the anterior body. These larvae could probably only be identified with certainty from unpreserved material, so are not figured.

    Figure 1. Nemertean larvae

    Eggs of species with a pilidium larva hatch at a very early stage of development. The pilidium larva, which develops from an initial simple ciliated larva, is variable in shape depending on species, and has a domed body with a tuft of sensory cilia sprouting from the top of the apical plate (Figure 1A). The lower body is formed from a series of four lobes, single anterior and posterior lobes (Figure 1B) that can differ in shape and an identical lateral pair (Figure 1C). The edges of the lobes are ciliated and are used in locomotion and to generate a current of water carrying small phytoplankton food towards the mouth, which is situated internal to the lobes. The epidermis may also contain groups of pigmented cells (Figure 1D). Ocelli may be present, but only develop in species that have them as adults. After a short feeding period the pilidium commences metamorphosis. The young worm develops within the pilidium (Figure 1D), which may continue to feed. Metamorphosis is completed when the worm breaks out, often feeding on the remnants of the pilidium. Size: Direct larvae ~0.2-1.0 mm total length. Pilidium larvae ~0.5-0.8 mm from apical tuft to distal edge of lateral lobe.

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    Ecology

    A few nemertean species can reproduce asexually by forming several individuals from broken sections, produced by spontaneous fragmentation of the body as part of their typical life cycle. However, reproduction is normally sexual and in marine species the sexes are usually separate. Some species breeding throughout the year, while others have particular seasons. The reproductive process is extremely variable between groups, but fertilisation is most commonly external. In some species the adults associate in a gelatinous sheath in which the eggs and sperm are discharged. The eggs, enclosed in capsules containing several eggs, remain in the sheath attached to some suitable substrate until they hatch. In other species the gametes are shed freely into the sea. Development of the eggs can be direct, adults releasing young individuals with the adult body plan, or the eggs hatch and a miniature adult emerges. Indirect development also occurs, where a distinct intermediate larva emerges from the egg and subsequently undergoes a metamorphosis to the adult form. There are different types of indirect larvae, but the only pelagic one occurring in European waters is the pilidium (Figure 1). The pilidium and some of the direct developing larvae spend days or weeks feeding in the plankton.

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    Zoogeography

    Particularly coastal waters, but widely distributed.

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