Feature Story                                                                  April 2011 

Cryptogenic Species: Using History and Genetics to Solve Invasion Mysteries 

Trematode parasites,Cryptocotyle lingua

The Marine Invasions Research Laboratory at the Smithsonian Environmental Research Center (SERC) is working to develop a national database of marine and estuarine invasions of the continental U.S. and Alaska. This database provides detailed information on approximately 500 different introduced species in estuaries, bays, and coastal waters. An introduced species is one that lives outside its native range. But how do we know if a species is introduced or where its native range is? As Dr. Paul Fofonoff works to populate the Chesapeake Bay Database of introduced marine and estuarine species, and the National Exotic Marine and Estuarine Species Information System (NEMESIS), this question of status is always on his mind. Answering this question requires an understanding of the species’ natural history. Some clues about a species native and introduced range can be found by digging through records (reports, scientific journals, etc). In other cases there is not enough information to make this determination without alternative sources of evidence, such as molecular genetics. Dr. Fofonoff considers a species to be introduced only if the introduction is historically documented, or else is strongly supported by a wide range of biogeographic, ecological, and/or genetic criteria. Some species have a long and well documented history, for example, the importation of Brown Trout (Salmo trutta) from Europe as a sport-fish in 1883 by the United States Fish Commission. But for species that were not imported intentionally, it can be more difficult to determine when and how they arrived. For example, Barentsia benedeni is a Kamptozoan, a tiny filter feeder that looks like a cup with a rim of feeding tentacles. It was first described in Belgium in 1887 and has since been found throughout Europe, Japan, Australia, and the Pacific coast of North America. Because it is small and few people can identify it, its native range is not known. Researchers at the SERC first discovered this species in Chesapeake Bay during a survey of the fouling community in 1994. Because B. benedeni wasn’t found in earlier surveys in the Chesapeake Bay, and is known from only 2 other locations on the East Coast, we are confident that the species is introduced. But in spite of our efforts, there are some species whose introduction status remain a mystery, either due to lack of evidence or disagreement among multiple sources of evidence. These species are called “cryptogenic”, a term coined by Jim Carlton in 1996, which refers to those species that cannot be demonstrably classified as native or introduced in a particular region.

Dr. Blakeslee collects a tissue sample from a snail for genetic analysis.
 

One such species, whose native or introduced status in the western Atlantic has been debated for over 100 years, is the Common Periwinkle, Littorina littorea. The Common Periwinkle is one of the most common snails on rocky shores in northeast North America. For decades, researchers have been examining historical, ecological, archaeological, and molecular evidence as to its native or introduced status in North America and have found support for both. Dr. April Blakeslee (now a SERC post-doctoral fellow) and others at the University of New Hampshire used molecular genetics and parasites to resolve the species’ cryptogenic status (Blakeslee et al, 2008). They collected DNA from periwinkles and their most common trematode (flatworm, fluke) parasite, Cryptocotyle lingua, in North American and native European populations. What they found was genetically diverse populations of the Common Periwinkle and its trematode parasite in Europe but significantly lower genetic diversity for both in North America. Further, the populations in North America were not genetically distinct from those in Europe. This similarity between the periwinkle and its trematode suggests a recent human-mediated introduction. Dr. Blakeslee found further evidence of introduced status when comparing the diversity of trematode parasites in Common Periwinkles to those in two North Atlantic snails of the same genus, the rough periwinkle, L. saxatilis, and the smooth periwinkle, L. obtusata. Dr. Blakeslee found significantly fewer trematode species in Common Periwinkles from North America, compared to Europe, than in the two native North Atlantic periwinkles (Blakeslee & Byers 2008). Such a reduction in parasites is a common signature of recent introductions, lending additional support for the exotic status of the Common Periwinkle.

In a recent study at SERC, Drs. Blakeslee, Whitman Miller and Gregory Ruiz, as well as colleagues in Sweden, compared the genetic diversity in North Atlantic populations of the Common Periwinkle (L. littorea) and the native rough periwinkle (L. saxatilis). The aim of this study was to determine how genetically different these species are from each other and how different North American populations are from European populations. They found that the native rough periwinkle shows strong population divergence between Europe and North America with genetic connections only across North Atlantic islands, which supports a stepping stone migration of the species across the North Atlantic in the distant past. The Common Periwinkle, on the other hand, shows strong connections between numerous European and North American populations and recent population divergence. Altogether, these multiple lines of evidence are highly supportive of a recent human-mediated introduction of the snail from Europe.

Determining if a species is native or introduced to a region will always present a challenge. Species have been moved around the world for hundreds of years and new species, both native and introduced, are still being discovered. But it is important to know a species’ invasion status, because introduced species that survive and thrive in the new environment will invariably have some effect on the invaded region, be it through competition for space, food, or other resources, as a novel predator, by modifying habitat, or other such effects. Because little can be done about a species if its native or non-native status is uncertain, it is an important endeavor to resolve cryptogenic status and monitor the effects the species may be having in its potentially novel environment.
      

      
 Common Periwinkle, Littorina littorea

 Intern Kristin Maglies surveys snails along the shore

Carlton, JT. 1996. Biological Invasions and Cryptogenic Species. Ecology 77(6):1653-1655 [http://www.jstor.org/stable/2265767]

Blakeslee, AMH, and JE Byers. 2008. Using parasites to inform ecological history: comparisons among three congeneric marine snails. Ecology 89:1068–1078. [doi:10.1890/07-0832.1]

Blakeslee, AMH, and JE Byers, MP Lesser. 2008. Solving cryptogenic histories using host and parasite molecular genetics: the resolution of Littorina littorea's North American origin. Molecular Ecology 17 (16): 3684–3696 [DOI: 10.1111/j.1365-294X.2008.03865.x]