Acanthocephalus rhinensis

image from figure 1 of the paper

The study which forms the basis of today's post features an acanthocephalan - also known as a thorny-headed worm - which lives in the intestine of European eels in Lake Piediluco in central Italy. Acanthocephalans spend their adult lives like tapeworms, clinging to the wall of their host's intestine, and absorbing nutrients from the pre-digested gut content. But unlike tapeworms, which mostly use suckers and small hooks to cling to the intestinal wall, an acanthocephalan has a formidable bit of armament which puts the tapeworms to shame. As its name indicates, at the front of the acanthocephalan is a hook-laden proboscis (see the picture on the right) to stab into the intestinal wall and firmly anchor themselves in place.

In Lake Piediluco, some eels were found to be infected with up to 350 Acanthocephalus rhinensis, though most eels had fewer than 50 worms. The eels become infected through eating little shrimp-like crustaceans called amphipods. The amphipods live mostly amongst the aquatic vegetation at the edge of the lake, and they are parasitised by the larval stage of A. rhinensis. If you thought the idea of having dozens of prickly-headed worms clinging to your intestinal wall with their nightmarish probosces is bad, A. rhinensis is downright brutal to the amphipod host.

image from figure 3 of the paper

The larval worm (called a cystacanth) occupies a large part of the little crustacean's body (see picture on the left), displacing many of its internal organs. About one in ten amphipods at Lake Piediluco are infected with A. rhinensis, and each amphipod had one or two worms inside them (probably because there wouldn't be much room for more). Acanthocephalus rhinensis imposes a massive burden on the little crustaceans - infected females can only successfully produce half as many eggs as uninfected females.

Armed with that formidable anchor, you would think that A. rhinensis would be able to establish itself in the gut of just about any fish it finds itself in. But it appears to be remarkably faithful to eels, which are the only fish found to have A. rhinensis in their intestines. Perhaps there are other immunological or ecological reasons that prevent this species from successfully infecting other fish.

In addition to establishing the life-cycle of A. rhinesis, another discovery made by the researchers actually served to amend an existing error in the scientific literature. In the original description of A. rhinensis, which was made based on nine specimens, this species is supposed to have a distinctive band of orange-brown (think spray-on tan) pigment just behind their proboscis, a feature that apparently distinguishes it from all the other Acanthocephalus species. However, the researchers who wrote this paper examined a total of over a thousand worms and not a single one had the supposed distinguishing band. But what gave those worms that orange-brown collar? The researchers suggested that this was caused by discolouration from being jammed so deeply into the intestinal wall that the worms inadvertently absorbed pigment from host's intestinal vessel which gave them a distinctive tinge just behind their proboscis.

So in addition to working out the life-cycle of A. rhinensis, this study also served to clarify old mistakes, which will help out any future researchers who work on this species.

Reference:
Dezfuli, B.S., Lui, A., Squerzanti, S., Lorenzoni, M. and Shinn, A.P. (2012) Confirmation of the hosts involved in the life cycle of an acanthocephalan parasite of Anguilla anguilla (L.) from Lake Piediluco and its effect on the reproductive potential of its amphipod intermediate host. Parasitology Research 11: 2137-2143.