Erik Sotka and Stacy Krueger-Hadfield describe an unusually successful invasion—and a novel partnership.

In his classic 1958 book The ecology of invasions by animals and plants, an English zoologist named Charles Elton warned of the growing scourge of invasive species, or species which humans accidentally or intentionally introduced outside their native range. The book arrived when the threat of nuclear Armageddon was constant, but Elton argued boldly and persuasively that the impacts of rodent, plant and insect pests on native ecosystems were tantamount to the effects of a nuclear bomb. The “ecological explosions” of invasive species are “not making such a loud noise and taking a longer time, but just as impressive.” Unfortunately, in the intervening 65 years since Elton’s book raised awareness of the issue, the rates of invasion have not just continued unabated, but accelerated.

Today, non-native species occur virtually everywhere, including within our oceans and estuaries. As an example, over 400 species of marine macroalgae, or seaweeds, have been introduced into coastal habitats worldwide. Some of these introductions were intentional, such as seaweeds grown for food, fertilizer or extractable compounds. Wakame, the brown seaweed Undaria pinnatifida, is consumed widely and was intentionally introduced to Europe during the 20th century. However, many seaweeds are introduced accidentally, either by hitching a ride with commercially-produced invertebrates (e.g. oysters), as insulation during 19th century shipping (e.g. fucoids) or released into local waters after being part of the aquarium trade (e.g. Caulerpa taxifolia, nicknamed killer algae).

Critical questions persist, the answers to which are important to management strategies as they provide information on where and how to target our resources toward prevention, mitigation or both. Are invasive populations increasing or stabilizing in number? What are their impacts? And how did a particular species succeed when others failed?

We have been attempting to answer these questions with an non-native red seaweed called Gracilaria vermiculophylla (or Gverm). Gverm is originally from the north-west Pacific, but invaded most temperate estuaries of eastern and western North America and Europe within the last 30–40 years. Therefore, it is arguably one of the most geographically widespread and successful invasions in the ocean that has ever been recorded. In Atlantic estuaries of the south-eastern United States, Gverm arrived around 2003 and within 10 years, the biomass has dramatically increased to cover upwards of 100% of some high-salinity mudflats (inset image). We are currently using DNAbased tools to understand the history of this incredibly rapid expansion. Our preliminary evidence suggests that the seaweed spread to Europe and the west coast of the United States via exportation of Japanese oysters sometime after World War II. Within coastlines, the spread of Gverm was likely facilitated by commercial and recreational gear that harvests native shrimp and crabs.

Gverm has the potential to transform ecosystems into which it is introduced by outcompeting native seaweeds, adding structural complexity to mudflats and altering detrital and consumptive food webs. It is important to remember, however, that in common with many invasive species, some of the ecological impacts of Gverm are positive. In Atlantic estuaries of the southeastern United States, the invasive Gverm has formed a novel mutualism with a native decorator worm called Diopatra cuprea (Figure 1). Much as a spider weaves a web, the decorator worm produces mucus-based tubes embedded in mudflats. The worm then decorates its home with Gverm, a behaviour benefiting both worm and Gverm. The non-native Gverm gains a foothold in shallow water where photosynthesis is possible. The native worm ‘farms’ small prey (e.g. amphipods, isopods and decapod larvae) and simultaneously seeks refuge in the 3D structure created by the seaweed.

We believe Gverm may have succeeded where other seaweeds have failed due to rapid evolutionary changes enabling particular strains of Gverm to spread. Seaweeds in the genus Gracilaria (ogonori) have been used as a source of agar in the Japanese mariculture industry for at least 300 years. Gverm itself was intensively cultivated for its high quality agar. Cultivation practices impose artificial selection, not unlike what occurs in terrestrial crops and ornamental plants. Algal individuals, or genotypes, are chosen based on agar yield, recovery and growth rates, and these same hardy genotypes are likely able to withstand large fluctuations in temperature, salinity and light. To confirm these suspicions, over the next three years, we will use population genetic data that reconstruct invasive history and common-garden experiments that compare phenotypes among native and non-native populations.

Over the last sixty years, we have witnessed an enormous homogenization of the Earth’s biota. At face value, this is dispiriting, given the clarion call to action that Dr Elton provided. At the same time, biologists have learned an enormous amount about the ecology and demography of invasion. This ecological knowledge has been used to successfully prevent other invasions and mitigate their impacts. Ultimately, we hope that using Gverm as a case study, we can help understand to what extent microevolution should be incorporated into management decisions.

Fig. 1. A tube cap formed by Diopatra cuprea, to which Gverm has been added as decoration. Image: Stacy A. Krueger-Hadfield.

Dr Erik E. Sotka (, Associate Professor.

Dr Stacy A. Krueger-Hadfield (, Post-doctoral Fellow. Grice Marine Laboratory, College of Charleston, Charleston, SC 29412, USA.


Erik Sotka and Stacy Krueger-Hadfield