Most organisms have several types of parasites associated with them. However, when species are introduced, they may lose some of their natural parasite load through the invasion process. This is because only a few individuals out of a population are transported at any given time, thus the number of parasites associated with this group of invaders is smaller than the number of parasites associated with the host’s native population. In essence, the invasion process itself acts as a filter limiting the number of parasites that are transported and introduced. Furthermore, parasite survival in the introduced population may be lower because some parasites have complicated life cycles requiring more than one type of host. For example, Toxoplasma gondii is a common parasite of cats that causes the disease Toxoplasmosis in humans. The parasite has two phases, a sexual phase and an asexual phase. The sexual phase can only take place in the cat (primary host), but the asexual phase can occur in several mammal species (secondary host) including cats, mice, humans, and birds. Because the parasite must infect a cat to reproduce and survive, its preferred secondary host is a mouse. If a mouse infected with Toxoplasma gondii were introduced into an area with no cats, the parasite would not be able to survive and establish in the new location. The term used to describe the ways in which the introduction process may reduce the number of parasites in the introduced population is called the parasite escape hypothesis. 

Dr. April Blakeslee, a Research Associate and former Postdoctoral Fellow at the Smithsonian Environmental Research Center (SERC), led a study investigating how invasion pathways and parasite life cycles influence parasite introduction in marine invertebrates on both coasts of North America (published December 2011 in the Journal of Biogeography). In the study, she and her co-authors explored trematode parasite species of two snails, the Eastern Mudsnail (Ilyanassa obsoleta) and the Rough Periwinkle (Littorina saxatilis). Both snails are native to the east coast of the United States but were introduced to San Francisco Bay during the 20th century. The mudsnail was also introduced to two other west coast bays, Willapa Bay, WA and Boundary Bay, on the border of Washington and British Columbia, Canada. Mudsnails were introduced through the importation of oysters during the early 1900s and the periwinkle was introduced in the 1990s with algal packing material used to package seafood and baitworms. 

Trematodes are parasitic flatworms commonly called flukes. These parasites have complex life histories and need to parasitize several different hosts in order to complete their life cycles. The snail is the parasite’s first host and is essential to the trematode life cycle because it is typically an obligate relationship. From the snail, the trematode infects a second host - this could be a worm, crab, mussel, fish, or another snail - then it moves to a third host where it reproduces and completes its life-cycle, called the definitive host. This definitive host is usually a fish or a bird. The larva produced in the snail host is a free-living stage called cercariae. The cercariae have a profound effect on the snail because at this stage the trematode asexually reproduces in the snail’s gonads preventing it from reproducing, in effect castrating it.

Dr. Blakeslee and her team collected thousands of snails from 49 east coast sites from Maine to Georgia and from sites in Willapa and Boundary Bays in Washington and San Francisco Bay, CA. The periwinkles were collected from 29 sites from Newfoundland, Canada to Long Island, NY and from San Francisco Bay, CA. Following collection, the snails were dissected in the lab to determine if any trematode parasites were present and if so, what species. They used this information to calculate an index of parasite escape, in other words, they determined whether there were more parasites in the native region than in the introduced region. 

What they found was that both introduced periwinkles and mudsnails on the west coast had fewer parasites than the native ones on the east coast. However, the periwinkles showed a greater decline in trematode infection in introduced versus native populations than the mudsnails. In fact, only 7 of the 1893 periwinkle snails sampled in the introduced San Francisco Bay region had trematode parasites. The difference in parasite prevalence between the two species in the introduced range may be due to three important factors: the prevalence of parasites in the source or native region, the transport mechanism, and the time since introduction. Mudsnails were transported nearly a century earlier than the periwinkles (1900s vs 1990s). In addition, mudsnails were transported through the oyster trade as hitchhikers in the large shipments of oysters that were brought over to establish a west coast fishery, which occurred on a massive scale over several years. In contrast, the periwinkles were transported in algae used to pack fish and baitworms, and this method of introduction occurred on a much smaller scale and many fewer snails were released at any given time compared to the oyster trade. Finally, this study found that the prevalence of parasites in the native region was three times higher for the mudsnails vs. the periwinkles. All of these factors together mean that there was a higher likelihood that a parasitized mudsnail was transported to the west coast than a parasitized periwinkle. But even with these differences both species probably benefit from having fewer parasites in the introduced range. 

Trematode parasites castrate the snails preventing them from reproducing; being free of parasites means an increased opportunity to reproduce giving both species an advantage over those in the native range. While the mudsnail’s means of introduction is no longer operating, the periwinkle’s invasion pathway remains active, providing opportunities for further invasion of trematode parasites to the west coast. Because these parasites have numerous detrimental impacts on their invertebrate and vertebrate hosts, it is important to further understand the invasion pathways and find management solutions to prevent further introductions.

Blakeslee, AMH, I. Altman, AW Miller, JE Byers, CE Hamer, and GM Ruiz. 2011. Parasites and invasions: a biogeographic examination of parasites and hosts in native and introduced ranges. Journal of Biogeography. doi: 10.1111/j.1365-2699.2011.02631.x