Invasive décor: an association between a native decorator worm and a non-native seaweed can be mutualistic

The likelihood of invasion success increases when non-native species engage in mutualisms with a native or non-native species. Mutualisms formed between native and nonnative species have been termed ’novel mutualisms’, and research in terrestrial systems has advanced our understanding of the ecological processes involved in their formation and persistence. However, documentation of novel mutualisms in marine systems is rare. In Atlantic estuaries of the southeastern USA, the native polychaete worm Diopatra cuprea actively decorates its tube with the non-native red seaweed Gracilaria vermiculophylla. We used field and laboratory experiments to test whether the Diopatra−Gracilaria interaction is mutualistic. We found that Diopatra facilitates Gracilaria by securing the seaweed onto the soft-sediment benthos within a favorable tidal elevation for growth and where hard substrata for attachment are otherwise rare. A combination of laboratory and field experiments also suggests that Gracilaria can enhance the growth of Diopatra by increasing access to epifaunal crustacean prey. However, field removal experiments showed that the benefits of Gracilaria to Diopatra only occurred in some sites and years. We found no evidence that this invader has a significant negative effect on Diopatra, and it appears in some instances to even benefit the worms, which suggests that Gracilaria (and its associated impacts on ecosystems) are likely to remain an important component of southeastern USA estuaries for the foreseeable future.