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Caution: Feeding the ducks may be harmful!
Have you ever seen this sign posted at the edge of a pond and wondered exactly why feeding ducks may be harmful? After all, ducks need to eat, and feeding them makes people feel good - that they are doing something to help the wildlife. So where is the harm?
There are a number of reasons that feeding the ducks may be harmful. Here are a just a few of them.
So clearly, feeding the ducks may be harmful to them. But humans aren’t the only ones that can harm ducks by feeding them. One creature that can cause considerable harm is a small crustacean called an amphipod. Amphipods don’t mean to do any harm to ducks, and they certainly don’t feed the ducks on purpose. It is just that they can’t help themselves. You see, what the amphipod offers to the duck is itself!
Amphipods are small aquatic crustaceans that somewhat resemble shrimp. Their body is flattened side to side and contains 13 segments. There are 8 pairs of legs on the thorax, with the first pair ending in large grasping claws. They use these claws to hold on to things, like a female during mating. Most people never notice amphipods, because to be noticed is usually to be eaten. So whenever an amphipod is disturbed, it heads for the bottom of the pond and finds a nice dark place to hide.
Amphipods have many enemies, but the one we are interested in for this story is a duck - the mallard. As mentioned above, mallards will eat a wide varied of foods, including amphipods. If a mallard sees an amphipod swimming at the surface, it will eat it. A mallard may also inadvertently swallow an amphipod that is holding on to a bit of vegetation that the mallard eats. So, when disturbed, amphipods head for the bottom to hide. Except when they head for the surface and swim around in plain sight or grab onto vegetation, where they are likely to become duck food. So what, you might ask, would cause an amphipod to behave this way?
There are many species of spiny-headed worms. All of them are parasites, living in and feeding off other animals. Typically, they need two hosts to complete their life cycle - an intermediate host and a definitive host. The intermediate host is usually an arthropod such as an insect or crustacean. The definitive host is a vertebrate - a fish, mammal or bird. In our story, the intermediate host is the amphipod, Gammarus lacustris and the definitive host is the mallard duck, Anas platyrhynchos. The spiny-headed worm is Polymorphus paradoxus.
Our spiny-headed worm starts life as an egg. After the egg hatches, the larva may get eaten by an amphipod. So far so good. Here, inside the amphipod, the worm will grow into its next stage, a cystacanth. It may need to grow for two weeks in the amphipod before it is ready to move on to the mallard. If the amphipod gets eaten before this time, the cystacanth will not survive in the duck. If, however, the duck eats the amphipod after this time, then the cystacanth will mature into an adult spiny-headed worm. So the trick for the worm is to make sure the amphipod offers itself to the duck only after the two week waiting period, only after it is able to survive in the duck.
Once swallowed by the duck, the worm will then imbed its head in the lining of the duck’s intestine, holding on with the spines. There, the worm, which has no mouth, will absorb nutrients directly through its skin, nutrients meant for the duck.
When our amphipod encounters a mallard, or any disturbance, it does what any amphipod intent on survival does - it dives to the bottom of the pond and looks for a dark place to hide. This is true of an uninfected amphipod or a newly infected amphipod. However, once an amphipod has been infected for a couple of weeks, once the worm’s cystacanth is now able to survive in the mallard, the amphipod begins to behave in a different manner. It seemingly offers itself up to the mallard for dinner! Instead of diving to the bottom, the amphipod now heads for the surface, where it swims around until it bumps into something and immediately grabs that object and holds on for dear life. If a mallard spies an amphipod swimming at the surface, a most unamphipod-like action, the mallard wastes no time in making a meal of it. If the amphipod instead happens to grasp on to a bit of floating plant, it may again become food for the mallard when the mallard eats the plant. Either way, by enticing the amphipod to essentially offer itself to the duck, the worm has now ended up where it needs to be to complete its life cycle. While one spiny-headed worm in the intestine of a duck may not do it much harm, a heavy infestation of these parasites can make the bird quite ill, and may eventually kill it. So clearly, enough infected amphipods feeding the duck may be quite harmful.
All spiny-headed worms are thought to alter the behavior of their intermediate host in some way. Of course, to be successful, they must alter the behavior in a way that helps them complete their life cycle. For instance, one spiny-headed worm has an intermediate host of an amphipod and a definitive host of a fish. It would do no good for the worm to induce the fish to grab and hold on to a piece of plant as in our example above. The fish does not eat plants and therefore would not eat the amphipod. Instead, this amphipod will swim towards the light, where it may be seen by the fish. The cystacanth in this instance is a bright orange in color, and can be clearly seen inside the amphipod. An amphipod with a bright orange spot swimming around in the light - what self-respecting fish could pass that up? And so, by inducing the amphipod to offer itself up to the fish, it is insuring that it will end up in an appropriate host and can complete its lifecycle.