ABSTRACT Invasive marine parasites can be established long before their introduction mechanisms are resolved, and factors contributing to their successes are often unknown. Understanding the ecology of these invasive parasites is urgently needed for economic and resource conservation efforts. In the eastern Pacific, the introduced Asian bopyid parasite, Orthione griffenis , extends at least from Sitka, Alaska, USA to San Quintín, Baja California, Mexico. In the new range, it infests at least two native hosts and one introduced host. We examined the genetic structure of O. griffenis from Morro Bay, California, to Ketchikan, Alaska, based on Double digest restriction‐site associated DNA (ddRAD) sequencing, and estimated its larval dispersal range from laboratory‐based survival tests. There was a lack of genetic diversity, structure, and isolation by distance across O. griffenis populations. There was also a lower‐than‐expected genetic polymorphism, consistent with previous hypotheses of its dispersal away from a single colonization event by a small number of initial propagules. Orthione griffenis larval survival appears sufficient for dispersal in coastal ocean currents over the observed northern invasion range and for transpacific dispersal via ballast water. The natural history and interaction of O. griffenis with its new and original hosts provide a unique system for understanding species adaptation in invaded ecosystems. This work demonstrates how genetically homogeneous invasive parasite populations can rapidly expand and potentially alter marine communities. Expanded efforts to understand the interactions of parasites and their vectors in their native and non‐indigenous habitats are critically needed for detecting, limiting, and mitigating their effects on endemic marine communities.
