Cryptic invasions occur when introduced taxa go undetected because they appear similar to native or other non-native taxa already established in that range. Trapa natans, an annual aquatic plant, has been recognized as an invasive species in the northeastern United States since the 1920’s. However, in 2014 a new morphotype of Trapa was discovered in the Potomac River, Virginia. This morphotype is distinguished by having two-horned fruits, compared to the four-horned characteristic of established invasive Trapa (T. natans). As such, we hypothesize that the new two-horned Trapa represents a cryptic introduction of a Trapa taxon that is distinct from the four-horned Trapa previously identified in North America. We tested this hypothesis by sampling Trapa from across the northeastern US and collaborated with scientists from Asia and Africa for collections of Trapa from its native range. We genotyped 308 Trapa individuals at 129 amplified fragment length polymorphism loci in an effort to distinguish the two morphotypes and identify their origin.
Results/Conclusions
Indeed, the two-horned and four-horned varieties are genetically distinct. The two-horned Trapa taxon was most genetically and morphologically similar to samples from Taiwan identified as T. bispinosa Roxb. var. iinumai Nakano. The US four-horned Trapa, previously identified as T. natans L. is most genetically similar to T. natans collected from Japan. However, our data show that the US four-horned Trapa and T. natans from Japan are genetically distinct. Therefore, it is unclear whether the US four-horned Trapa was introduced from a source not captured in our sampling, or whether the genetic differences result from genetic divergence following introduction from Japan. Further, we found Trapa recognized under the same species name that are genetically distinct groups. Therefore, we recommend a more thorough investigation into Trapa taxonomy. A similar study with more thorough sampling of Trapa’s native range and more detailed genetic markers may be able to resolve the taxonomy and origins of the invasive US morphotypes.