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PHORONIDA

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(Hatschek, 1888 )

Sections
  • Background
  • Development
  • Ecology
  • Zoogeography
  • Background

    Phoronids are an exclusively marine group of benthic, worm-like organisms, mainly found in water depths <50 m. Worldwide, there are only 2 genera and 13 described species, but they all have a wide geographical range. They secrete a chitinous tube in which they live freely and can be found singly or in large aggregations, encrusting or among rocks or shells, but more usually embedded in sand, mud or fine gravel. They have a terminal array of tentacles called a lophophore, used in feeding, protection and reproduction. Possession of this organ is considered to link them to bryozoans and brachiopods. Their larvae can occur in the meroplankton.

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    Development

    Hatched larvae, from species with pelagic larvae, develop anteriorly a large hood-like lobe with a fringe of cilia, the pre-oral lobe, below which is the mouth (Figure 1A). The lobe is involved in the feeding process. In the mid-region of the body is a ring of tentacles that increases in number with development, ~10-40 in fully grown larvae, depending on species. Two species have a second ring of tentacles behind the first. The tentacles are ciliated and are used in locomotion as well as feeding. The posterior part of the body contains most of the digestive system and terminates in the anus. The anus is surrounded by a ciliated band called the telotroch, which also contributes to locomotion.

    Figure 1. Phoronid larvae.

    Larvae may be pigmented to varying degrees. The fully developed, free-swimming larva is called an actinotrocha or actinotroch and larger ones are one of the largest invertebrate larvae sampled. Identification is a specialised task, as larvae are continually developing, so only examples of morphology are given (Figure 1). Because the adults are mainly restricted to shallow water, larvae will tend only to be sampled in coastal plankton samples and mainly in summer and autumn. Size: Eggs ~50-60 µm diameter, fully developed larvae ~0.6-2.5 mm total length.

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    Ecology

    Different phoronid species may be hermaphrodite or dioecious, although at least some can also reproduce asexually, either by transverse fission or by budding. There are a range of strategies by which larvae are released. One species retains its hatched larvae, so has no pelagic phase. Six species release larvae with a short pelagic phase, while a further six shed eggs directly into the surrounding seawater and have larvae with an extended pelagic phase, typically 2-4 weeks but sometimes longer. Their pelagic existence ends with an extremely rapid metamorphosis (10-15 min) and settlement to the sea bed, triggered by bacteria in the sediments.

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    Zoogeography

    Coastal waters.

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    [PHORONIDA on the taxonomic tree]

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