The core of my thesis research tested whether the association between Gracilaria and Diopatra is a novel mutualism.
Novel mutualisms occur when two species interact that do not have a shared evolutionary history and that interaction is mutually beneficial. Species introductions provide great systems for investigating the formation of novel interactions and the impacts those interactions have on biological communities and ecosystems.
Diopatra is a decorator worm that attaches material from its environment – shells, macroalgae, and pieces of seagrass or salt marsh grass – to a tube that it builds out of sediment and mucus. The picture above shows a Diopatra tube cap, the part of the tube that extends above the ground, decorated with Gracilaria (photo credit: Edna Diaz-Negron). Results from my research showed that this decoration behavior facilitates Gracilaria by anchoring the seaweed at a tidal height favorable for growth.
Reciprocally, Gracilaria indirectly provides Diopatra with a food source. Results from a manipulative lab experiment suggested that Diopatra benefits more from consuming the invertebrates that live on Gracilaria than from consuming Gracilaria directly. When we tested for a benefit of Gracilaria to Diopatra in the field, we found that the growth of Diopatra positively correlated with the density of Gracilaria and that there was a positive relationship between the density of Gracilaria and the invertebrates present on the seaweed. However, we also found that this benefit varied temporally and spatially – speaking to the conditionality that is often found in mutualistic interactions.
This association between Diopatra and Gracilaria is one of the first described mutualisms between a native and non-native species in a marine system. The association also reinforces existing ecological theory that facilitative interactions can have major impacts on biological communities and ecosystems. By facilitating Gracilaria, Diopatra is changing soft-sediment tidal flats in the southeastern USA to systems dominated by a macroalga that isn’t supposed to be there! This transformation is affecting trophic food webs and detrital processes in these estuaries. To learn more about the impacts of Gracilaria in South Carolina and Georgia, follow research from both the Sotka Lab and the lab of Dr. Jeb Byers (University of Georgia).
Kollars, N.M., Byers, J. E., and Sotka, E.E. (2016) Invasive decor: An association between a native decorator worm and a non-native seaweed can be mutualistic. Marine Ecology Progress Series 545:135-145; DOI 10.3354/meps11602.