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Arrow Worms - Phylum: Chaetognatha
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"In a flash it covers a distance of some five or six times its own length; and its great jaws, opened wide, snap upon its victim."
Sir Alister Hardy (1896-1985) on Chaetognatha in 'The Open Sea', 1965.
Sir Alister Hardy (1896-1985) on Chaetognatha in 'The Open Sea', 1965.
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Arrow Worms are mostly small, often transparent, predatory animals ranging in length from 2 - 120 millimetres. With their tubular bodies, bilateral and tail fins, they resemble thin squids but, instead of arms have a circular row of spines protruding from either side of the mouth with which is latches on to its' prey. Currently the phylum consists of a mere 132 species but Chaetognatha are found in very large numbers in all marine environments. Sometimes they may account for as much as 10% of all local zooplankton mass.
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The name Chaetognatha is derivered from two Greek words: 'chaeto' (= mane or bristle) and 'gnathos' (= jaw) in reference to the two rows of spines that adorn their heads. Their bodyplan is simple with a defined head, body and tail. The mouth opening empties into a pharynx which continues into a straight intestinal cavity that empties via the anus near the tail. They have a nervous system, albeit a rudimentary one. Arrow Worms also have eyes and a line of bristles running the length of the body on each side that serve as sensory organs.
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Image 1. The head of an Arrow Worm by Zatelmar.
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Chaetognatha were first described by Maarten Slabber, a Dutch amateur naturalist in 1778. Slabber did not know where to place it taxonomically, leaving this to 'the opinion of his readers' and it was not until 1854 that the German zoologist Rudolf Leuckart elevated them into their own phylum.
Despite the fact that Chaetognatha are soft-bodied they are known from the fossil record and first appear in rocks from the early Cambrium about 540 - 520 Mya. However, as predators it is quite possible they may have appeared onto the scene as early as late Pre-Cambrium. |
Over the past 500 million years plus it would seem that the Chaetognatha have not changed much since they first came about. Some sources do argue though that during their evolution they may well have lost original features that may have reappeared again at some later stage. Or, that new features evolved that later disappeared.
Arrow Worms have long been regarded as carnivores but recent studies have shown that many also eat Phytoplankton as well as Copepods, small fish larvae and other Zooplankton. |
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Chaetognaths are hermaphroditic, meaning they have both male and female reproductive organs. Self fertilisation seems to be prevented not just by the fact that the male and female organs are well separated from one another within the body, but also by the male and female organs maturing at different stages in the life of the animal. Fertilisation of the eggs usually occurs and eggs may be deposited onto the sea floor or just released into the water. Some species have special brooding pouches near the tail from which live young emerge after hatching of the eggs. In cold waters Arrow Worms may live for up to two years; in warm waters for as little as six weeks.
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Image 2. by Zatelmar.
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References and Links:
^ Image 1. By Zatelmar. CC BY-SA 3.0 via https://commons.wikimedia.org/w/index.php?curid=2777766
^ Image 2. By Zatelmar. CC BY-SA 3.0 via https://commons.wikimedia.org/wiki/File:Chaetoblack.png
^ Gasmi, S., Nve, G., Pech, N. et al. 'Evolutionary history of Chaetognatha inferred from molecular and morphological data: a case study for body plan simplification.' Front Zool 11, 84 (2014). https://doi.org/10.1186/s12983-014-0084-7
^ Slabber, M. 'Natuurkundige verlustigingen behelzende microscopise waarneemingen van in- en uit-landse water- en land-dieren.' (Natural Philosophical Delights involving Microscopic Observations of Domestic and Foreign Water and Land Animals), J.Bosch, Haarlem, the Netherlands, 1778.
^ Szaniawski, H. 'Cambrian chaetognaths recognized in Burgess Shale fossils' (2005). Acta Palaeontologica Polonica 50 (1): 1–8.
^ Vannier J, Steiner M, Renvoisé E, Hu SX, Casanova JP. 'Early Cambrian origin of modern food webs: evidence from predator arrow worms.' Proc Biol Sci. 2007 Mar 7;274(1610):627-33. doi: 10.1098/rspb.2006.3761. PMID: 17254986; PMCID: PMC2197202.
https://animaldiversity.org/accounts/Chaetognatha/
https://biodiversity.org.au/afd/taxa/CHAETOGNATHA
https://cronodon.com/BioTech/Chaetognaths.html
https://en.wikipedia.org/wiki/Phytoplankton
https://en.wikipedia.org/wiki/Rudolf_Leuckart
https://en.wikipedia.org/wiki/Zooplankton
https://www.lindahall.org/about/news/scientist-of-the-day/martinus-slabber
https://www.marinespecies.org/index.php
^ Image 1. By Zatelmar. CC BY-SA 3.0 via https://commons.wikimedia.org/w/index.php?curid=2777766
^ Image 2. By Zatelmar. CC BY-SA 3.0 via https://commons.wikimedia.org/wiki/File:Chaetoblack.png
^ Gasmi, S., Nve, G., Pech, N. et al. 'Evolutionary history of Chaetognatha inferred from molecular and morphological data: a case study for body plan simplification.' Front Zool 11, 84 (2014). https://doi.org/10.1186/s12983-014-0084-7
^ Slabber, M. 'Natuurkundige verlustigingen behelzende microscopise waarneemingen van in- en uit-landse water- en land-dieren.' (Natural Philosophical Delights involving Microscopic Observations of Domestic and Foreign Water and Land Animals), J.Bosch, Haarlem, the Netherlands, 1778.
^ Szaniawski, H. 'Cambrian chaetognaths recognized in Burgess Shale fossils' (2005). Acta Palaeontologica Polonica 50 (1): 1–8.
^ Vannier J, Steiner M, Renvoisé E, Hu SX, Casanova JP. 'Early Cambrian origin of modern food webs: evidence from predator arrow worms.' Proc Biol Sci. 2007 Mar 7;274(1610):627-33. doi: 10.1098/rspb.2006.3761. PMID: 17254986; PMCID: PMC2197202.
https://animaldiversity.org/accounts/Chaetognatha/
https://biodiversity.org.au/afd/taxa/CHAETOGNATHA
https://cronodon.com/BioTech/Chaetognaths.html
https://en.wikipedia.org/wiki/Phytoplankton
https://en.wikipedia.org/wiki/Rudolf_Leuckart
https://en.wikipedia.org/wiki/Zooplankton
https://www.lindahall.org/about/news/scientist-of-the-day/martinus-slabber
https://www.marinespecies.org/index.php
Text: Erik Beringen
Header Image: TNRP stock image.
Header Image: TNRP stock image.