Acanthocephalus dirus

The word parasite has a lot of connotations associated with it, and "maternal" is certainly not one of them. To most people, the term "freeloader" comes to mind (hopefully, this blog will show you that parasitism is actually a very challenging way of life). They also have a reputation as being pretty lousy parents. In most textbooks, parasites are usually considered as "r-strategists" - which produce many, many offspring and don't take good care of them (as opposed to a K-strategist which produces fewer offspring, but invest a lot into parental care - like an elephant). But not all parasites are bad parents, and today, I am going to tell you about a study on a maternal parasite which sacrifices everything (literally) for her offspring.

Acanthocephalus dirus has a reproductive strategy that is unusual for its group - the acanthocephalans or the thorny-headed worms (Acantho = "thorns", Cephala = "head"). In fact it is unusual compared to most intestinal parasites. Unlike some tapeworms, which profligately cast off segments (each containing hundreds of eggs) into the wilderness with abandonment, A. dirus has rather different approach. The impetus that spurred on this piece of research were two separate observations: (1) fish that are infected with A. dirus do not have any worm eggs in their feces (unlike most animals infected with intestinal parasites) and (2) perfectly healthy and intact female worms were often expelled from the definitive host. What the researchers found was that instead of simply laying eggs that are expelled from the worm and from the host, a female A. dirus actually retains her eggs until she become completely bloated with them - at which point she exits gracefully from the host fish's digestive tract. Some readers might recall a nematode that has a similar reproductive strategy, and that both lineages have evolved such a reproductive strategy independently. So why has A. dirus evolved such an extreme strategy instead of just laying eggs normally like other thorny-head worms?

One reason could be that A. dirus infects creek chub - which, as its name indicates - lives in flowing creeks. The chub acquire the worm through eating infected isopods in the stream (the picture shows the light-coloured infected isopod on the right, and the darker uninfected individual on the left), which become infected when they ingest worm eggs resting on the creek bed. Acanthocephalan eggs tend to float - so if the eggs are simply expelled into the environment, they would get washed away downstream and deposited where the isopods do not occur. Whereas with A. dirus, the worm's own body can act like a weight belt which would carry the eggs down to the sediment layer, so by the time the worm herself decays, the eggs are already in the sediment where isopods can pick them up.

Furthermore, laboratory tests showed that isopods like to eat egg-filled female worms as much as their usual food - leaf litter - and the worm body itself actually enhances the infection success of the eggs. Researchers found that when exposed to fresh eggs alone, fewer than one in four isopods became infected, whereas when exposed to gravid females, over 80% became infected (natural infection comes somewhere in between those at about 60%). By making the ultimate maternal sacrifice, A. dirus gives her offspring the best possible start in life.

Image from figure in: Seidenberg (1973) Journal of Parasitology 59: 957-962

Reference:

Kopp, D.A., Elke, D.A., Caddigan, S.C., Raj, A., Rodriguez, L., Young, M.L. and Sparkes, T.C. (2011) Dispersal in the acanthocephalan Acanthocephalus dirus. Journal of Parasitology 97: 101-105