Field of Science


If you've ever spent time, as I certainly did back in my undergraduate days, thumbing through textbooks of animal diversity, then you may be familiar with the so-called 'minor phyla'. These are those isolated subgroups of the animal kingdom that are phylogenetically remote from other such taxa but which, owing to low diversity and/or low exposure, are commonly not regarded as warranting more than a cursory summary in the end-papers of some other more prominent group. One such group is the arrow worms (Chaetognatha), and for this post I'm focusing on the arrow worm genus Aidanosagitta.

Arrow worms are marine micropredators, slender-bodied animals mostly growing to a bit less than a centimetre in length but generally not seen without the aid of a microscope due to being mostly transparent. The greater number of arrow worm species are planktonic and could be described as superficially fish-like with paired fins running down the side of the body. The front of the head forms a flexible hood within with the mouth is flanked by elongate spines, used for grasping prey.

Two Aidanosagitta species: A. bella (above) and A. venusta (below), from Kasatkina & Selivanova (2003).

A review of the arrow worms by Tokioka (1965) recognised fifteen genera within the phylum. Aidanosagitta is one of the planktonic genera; currently, about thirty species are recognised within this genus. Distinguishing features of the genus include a firm, muscular body, diverticula arising from the gut, and the posterior pair of fins being located on the 'tail' section of the body (behind the anus) (Kasatkina & Selivanova 2003).

The majority of Aidanosagitta species are found in tropical and subtropical waters, most commonly in inlets and lakes. An exception is provided by a number of species found in colders waters adjoining the north-west Pacific, in the Sea of Okhotsk and the Sea of Japan (Kasatkina & Selivanova 2003). Species are distinguished by features such as the sizes of the fins, the size and position of the large subenteric ganglion, and the presence and extent of a layer of spongy tissue that may partially cover the outside of the body. Particular species of arrow worms may be associated with particular bodies of water (such as particular currents) and changes in their distribution may indicate changes in the greater environment.


Kasatkina, A. P., & E. N. Selivanova. 2003. Composition of the genus Aidanosagitta (Chaetognatha), with descriptions of new species from shallow bays of the northwestern Sea of Japan. Russian Journal of Marine Biology 29 (5): 296–304.

Tokioka, T. 1965. The taxonomical outline of Chaetognatha. Publications of the Seto Marine Biological Laboratory 12 (5): 335–357.


  1. Looking at the WP page, Chaetognatha has apparently been reassigned from Deuterostomia to Protostomia since I last paid them any appreciable attention.

    1. Which implies that you last paid them any appreciable attention some time ago, because they've been established as protostomes for some twenty years or so. Not that that's a surprising lapse in attention by any means when it comes to arrow worms. I didn't even realise that not all chaetognaths were planktonic before researching this post.

    2. I don't think it can be quite twenty years, but maybe what I read was not up-to-date when I read it.

      I must've seen them as protostomes in phylogenetic trees of Bilateria etc. but it's evidently failed to register.

  2. Chaetognaths have remained hard to place though. This year there have been a couple papers including them in Gnathifera, along with other jawed groups like rotifers and gnathostomulids (Marlétaz et al. from transcriptomes, Vinther & Parry from fossils).

    1. I'm interested to see a strengthening of the argument for recognising Amiskwia as a chaetognath, an identification I've tended to be skeptical of in the past (I'd have been inclined to suspect it might be a mollusc). I'd be interested to know if there's anything to be made of a comparison between Amiskwia and Nectocaris.


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