Acantho-what?

A species of Centrorhynchus - note thorny head!

Let’s delve into my world of research for a moment.

I’ve been spending the last few days writing some manuscripts describing two new species of Acanthocephala, or thorny-headed worms as they’re commonly known. I’m one of very few parasitologists who study these worms in Australia, and shortly I’ll be travelling overseas to learn new techniques for working with them and hopefully also make some collaborative links for future research.

At the moment, I’m looking at species that occur in marine fish (although the species of Centrorhynchus featured in the pictures is from a bird).

The defining feature of the Acanthocephala is the proboscis. This is the ‘thorny head’ bit. It’s a weird structure protruding from the anterior end of the worm, and is covered in sclerotised hooks. The size and shape of the proboscis and the hooks varies greatly between different groups of worms. The hooks are arranged, conveniently, in longitudinal rows. The proboscis can be retracted into the proboscis receptacle within body cavity via a combination of hydraulics and muscles. As it retracts, the hooks fold in on each other from the top end down, and end up looking like a closed zipper. Well, to me anyway. A colleague was most confused the first time I described a partially-inverted proboscis as “zipped up”.

A proboscis, the body of the worm is at top left.

Acanthocephalans have a weird mix of characteristics that, to me, makes them look like they are a combination of all other parasite groups. They are widespread, occurring in all classes of vertebrates, yet can be quite rare within populations (i.e., not many individuals will be infected with them). They have intermediate hosts and sometimes paratentic hosts, much like many other parasitic helminths, but are not closely related to other helminth groups. They are nematode-like in appearance, yet get their nutrition through their tegument like tapeworms. Acanthocephalans typically live in the small intestine of their hosts.

Schematic life-cycle of an acanthocephalan from a fish.

Close-up of hooks from Centrorhynchus

Their phylogeny is just plain weird. In the early 2000s, it was established via analysis of 18S ribosomal RNA that their closest relatives were rotifers (Garcia-Varela et al., 2000, Garcia-Varela et al. 2002), not other kinds of parasitic worms as might be expected. This was supported in newer research this year, using cytochrome c oxidase  (Fontaneto and Jondelius, 2011).

These parasites have a variable effect on host health, dependent on many factors including size of parasite and host, intensity of infection, and host health and immune status. The thorny-head of the worm serves as an anchor or holdfast, and can cause localised pathology and damage to intestinal walls. Risk to human health is low, although there are some species occurring in terrestrial habitats that can infect humans (Taraschewski 2000).

So there you have it. All you ever wanted to know about Acanthocephala, in a conveniently sized chunk.

References

Fontaneto and Jondelius (2011) Broad taxonomic sampling of mitochondrial cytochrome c oxidase subunit I does not solve the relationships between Rotifera and Acanthocephala. Zoologischer Anzeiger 250, 80-85.

Garcia-Varela et al. (2000) Phylogenetic relationships of Acanthocephala based on analysis of 18S ribosomal RNA gene sequences. Journal of Molecular Evolution 50, 532-540.

Garcia-Varela et al. (2002) Phylogenetic analysis based on 18S ribosomal RNA gene sequences supports the existence of class Polyacanthocephala (Acanthocephala). Molecular Phylogenetics and Evolution 23, 288-292.

Taraschewski (2000) Host-parasite interactions in Acanthocephala: a morphological approach. Advances in Parasitology 46, 1-179.

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