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CHAETOGNATHA

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((Leuckart, 1854) Hyman, 1959 )

Sections
  • Background
  • Morphology
  • Development
  • Ecology
  • Zoogeography
  • Background

    Chaetognaths, commonly known as arrow worms, have a distinctive body shape that is easy to recognise and enables separation from other marine planktonic groups. Chaetognaths are a small phylum (~120 species) of carnivorous invertebrates ranging in size from 2 to 120 mm. They are fast, active swimmers and are the dominant invertebrate predators of zooplankton, playing an important role in the marine food web as the primary predators of copepods.  Since a number of species are associated with specific physical and chemical conditions of sea water they have been used as indicators of water masses (Alvariño, 1965Fraser, 1952; Meek, 1928; Russell, 1939). Chaetognaths have also been used as indicators for potentially important fishery areas (Boltovskoy, 1981).  Most are planktonic and few, three genera Spadella, Bathyspadell and Krohnitella are benthic. Depending on which characters are applied the genus Sagitta has been divided into several genera (Tokioka, 1965; Bieri, 1991; Alvariño, 1965; Boltovskoy, 1999; Papillon et al. 2006). Very little is known about their physiology, reproductive biology, population structures and systematic relationship. Their relationship to other phyla is still uncertain and after applying cladistic and molecular techniques the phylogeny of the group is still uncertain (Telford et al. 1997; Papillon et al. 2006). Therefore, the classical genus Sagitta has been assumed for this review.

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    Morphology

    The body plan is torpedo-like usually with two pairs of lateral fins (which may be joined and appear as one continuous fin) and a tail. The lateral fins are similar to the flight feathers of an arrow hence the name. The outer body is often translucent with a non-chitinous cuticle and 2 cell layers.  Internal tissues and organs include muscles and nervous tissue, a gut and reproductive structures all visible through the body wall. There are many unusual structural features, including ad retractile hood, a multi-layered epidermis and a ‘corona ciliata’ of unknown function.  There is no circulatory, gas exchange or excretory structures. The body is clearly separated into a head, trunk and tail region.

    The head is usually rounded and slightly flattened. It has a complicated muscle system for extending teeth, hooks and movement of the hood. The mouth opening, vestibule, vestibular pits and teeth are visible on the ventral surface. The hooks are much longer that the teeth but are essentially of similar structure. The number and form of hooks and teeth vary according to the species, increasing with maturity and then decreasingwith age and absent in older animals. The anterior trunk region sometimes has an extendible and visible collarette. The intestine has no appendages except a lateral and anteriorly-directed protrusion called alimentary diverticula which occur only in some species. Both the collarette and gut diverticula, are visible through the body wall.

    Figure 1. General chaetognath external anatomy.

    The anterior and posterior fins are covered by a thin, monolayered epidermis stiffened by two rows of chitinous rays and filled with an amorphous extracellular matrix (Welsch and Storch 1983; Ahnelt 1984 in Kapp, 1991). They are very fragile and easily damaged consequently, it is rare to have samples with complete fins. The shape and position of the fins with respect to the ventral ganglion are important characters for identification.  Conspicuous ciliary fanswhich are thought to be  receptor organs are present on the head, trunk, tail and even fins of living specimens (Kapp, 1991).

    As active predators, chaetognaths have a relatively complex nervous system with an elaborate assortment of vibration receptors for detection of moving prey. A pair of eyes of remarkably diverse structure is usually present on the dorsal surface of the head. Although eyes are lacking in all Heterokrohnia and some Eukrohnia species. The size and shape of pigmented spots in the eyes may vary and could be useful in species identification (Bone and Goto, 1991).

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    Development

    Development from fertilized egg to adult is continuous and little is known about chaetognath larval characters. Although chaetognaths do not undergo a metamorphosis, the early developmental changes have been sufficient for several authors to describe them as ‘larvae’. Newly hatched larvae lack anterior fins, caudal septum and anus. The benthic larvae of Spadella adhere to the surface on which they were hatched (John, 1933 in Pearre, 1991). Larvae of Sagitta are plankonic and usually remain near the water surface. (Pearre, 1991)

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    Ecology

    Chaetognaths are protandric hermaphrodites with reproductive organs visible through the body wall. The male organs are in a separated area of the tail region and consist of the testes, vas deferens and seminal vesicles. The spermatozoa mature in the testes, travel via the vas deferens to the seminal vesicles where they are stored until required. During copulation the sperm are released as a spermatophore and attached to the opposite partner.  The seminal vesicles have a species-specific structure. In some Sagitta species, for example, they are elongate with a terminal knob filled with glandular secretion, and in the Sagitta serratodentata group they are not only bipartite but provided with external structures (Pierrot-Bults 1974, 1976).
     
    The female organs develop at the posterior end of the trunk and are separated from the tail region by a septum. They consist of a pair of ovaries, seminal pouches and gonopores. During fertilization sperm from an attached spermatophore migrate via the gonopore into the seminal pouches. Fertilization occurs in the ovary and the eggs are then released into the water column. There are records of both self and cross fertilization. The shape and relative sizes of reproductive systems are useful characters for identification.
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    Zoogeography

    Distribution

     

    Coloured areas: abundance (scale is log10 (x+1)); blank areas: no sampling, or no presence data recorded; open circle: species never recorded; black diamond: only 1 presence; +: > 1 presence data.
     
    Data from Continuous Plankton Recorder Survey, 1958-1999.
     
    Reproduced from Beaugrand et al, 2004.
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