Parasites and their (in)tolerant hosts
The unwanted houseguest–it is a host’s worst nightmare. Even if the unwelcome visitor is not terribly unpleasant, the host always pays in some way and in extreme cases, the relationship can become destructive. Either way, nobody likes a parasite.
Biologist Mark Forbes, the Tier II Canada Research Chair in Ecological Parasitology/Wildlife Conservation at Carleton, investigates how parasitic relationships evolve.
“Disease organisms pose a threat because parasites can be ecosystem engineers,” he states. “If we understand how they find and infect their hosts, why some organisms get really nasty, and how hosts fight back, we can learn more about ecological problems such as why infectious organisms appear and re-appear and what we can do to protect species and environments that are at risk.”
All ecosystems are made up of living organisms and non-organic matter. In a healthy system, there is not too much of one element or too little of another. When there is imbalance, however, things can go wrong. In the case of the great Irish potato famine of 1845, an airborne parasite infected millions of potatoes–Ireland’s staple crop–turning them to black, inedible lumps. For the more than three million Irish peasants who survived on a diet solely made up of potatoes, the loss was devastating. The crop failures led to a severe food shortage and over a million people starved to death. Many of those who did not die left Ireland for British colonies such as Canada.
Forbes investigates parasites in animals rather than plants but the principle is the same. “Parasites exist in all healthy systems but when they become virulent and the host species are strongly affected, an ecosystem could be in serious trouble.”
The mud shrimp Corophium volutator is one of the species he studies. Just as the potato was essential to the Irish economy of the mid 1800s, this little crustacean is vital to the ecology and fishing industry of the Bay of Fundy. Each year, billions of shrimp dig into the Fundy intertidal mudfl ats and are the main prey for many species including local groundfi sh.
The Corophium population has shrunk signifi cantly from time to time at some mudfl at sites and Forbes recently looked at the relationship between Corophium and a newly-discovered single-celled parasite to see if this organism was a contributing factor. What he suspects is that the behaviour of this microorganism leads to possible population control: they “feminize” the males (i.e., change them into females).
To affect the sex change, it appears that the parasite lodges itself in the mother shrimp’s ovaries. The organism is passed along with her eggs and those destined to become a male shrimp may become female if parasitized. Their ovaries then carry the parasite’s offspring and so the cycle continues. If males become too rare, the shrimp population can fall, sending a ripple through the whole system: fewer shrimp means fewer fish, and ultimately fewer dollars flowing into the local economy.
The jury is out on whether this “feminization” of male shrimps actually occurs but Forbes says it is a good example of how one tiny organism might affect an entire ecosystem. “The parasitized shrimp are not diseased but what we learn from parasitic relations might teach us more about how some disease organisms, such as HIV or avian flu, are, or become, deadly.”
Mark Forbes is the 2007-08 Davidson Dunton Research Lecturer.