When talk about invasive species includes mention of their parasites, stories usually focus on an absence of the later. In fact, when species invade new habitats, one of their biggest advantages is liberation from their usual “bugs.” There are several ways that invasive species escape their parasites, and studies show that relatively few are replaced by parasites native to invaded ecosystems . This means that invaders can produce more offspring, grow larger, and sometimes reach high densities than what would normally be possible.
These situations might be better than the alternative though. In some cases, introduced species do bring along their parasites and transmit them to native species, sometimes with devastating outcomes. I recently wrote about a Eurasian snail that transmits its natural parasites to North American waterfowl, causing massive die-offs in the northeastern Unites States. Some other examples include European elm bark beetles that have helped spread Dutch Elm Disease in North American, and more recently, managed bees that have been shown to transmit disease to native pollinators. And here’s one more…
When Europe’s crayfish populations began declining in the 1960’s, North American signal crayfish were imported to sustain local fisheries. To their dismay, the larger and more aggressive signal crayfish began outcompeting and displacing the native white-clawed species. It also drove out fish and amphibians, and degraded habitats by destroying vegetation and burrowing in stream banks. Later, biologists even found that the signal crayfish carried the very disease that was causing native species to decline. In fact, it was probably even responsible for introducing the pathogen in the first place.
The disease, called crayfish plague, is caused by a type of water mold, a group of organisms related to algae and the infamous potato blight. Although it seems to have little to no effect on North American crayfish, European natives are poorly adapted to the pathogen and die within a few weeks after infection. As a result, white-clawed crayfish populations have continued to plummet, and according to the IUCN, the species is now globally endangered.
Parasites can be a good thing
However, in an exciting twist that could benefit the white-clawed crayfish, several leech-like worm species have also been introduced to European waterways. The worms, called branchiobdellids or crayfish worms, are not parasitic in the strict sense. They live on the outer surface of crustaceans or in their gill chambers, where they feed on zooplankton, bacteria, and algae. Because the worms rid their hosts of debris and potentially harmful organisms, they usually provide an overall benefit. But the relationship between the worms and their hosts is complex, and positive outcomes are far from universal.
Scientists have found that in some situations the worms have no effect on their hosts or even cause harm. The exact outcome seems to depend a number of factors, but the parasites have another trick up their sleeves.
A recent study from Cardiff University showed that signal crayfish that were infested with crayfish worms behaved differently than those without. The infested crayfish were less likely to fight or exhibit threatening displays and were more likely to retreat or avoid competitors altogether. Researchers also found that the worms were transferred to uninfected crayfish during interactions and that infested crayfish captured fewer prey items.
It’s possible that the signal crayfish is affected so drastically because it isn’t adapted to the species of worm it encounters in Europe. Both are native to North America, but the signal crayfish is found in waterways on the eastern side of the continent, and the worm is found in the west.
Parasites and conservation
The ability of parasites to influence the behavior their hosts is nothing new. Toxoplasma gondii, for example, has long been known to affect the behavior of infected rats and humans. Even in crayfish, trematode infections have been shown to alter host behavior and increase vulnerability to predation. Pathogens have also been used to address invasive species problems, like the introduction of Myxoma virus to control Australia’s non-native rabbit populations. However, unintentionally introduced species (like the crayfish worms) are rarely found to benefit ecosystems by lessening the impacts of other invasives.
The crayfish worm studied by the group at Cardiff University has the potential to mitigate impacts of the invasive signal crayfish. Not only does the species influence the invader’s behavior, but it also seems to prefer foreign species, like the signal crayfish, over natives. More research is needed, but it’s possible that crayfish worms could influence the invasions of other non-native crayfish too.
Featured Image Credit: Terry Matthews