Field experiments
To quantify the effect of Gracilaria on Diopatra growth and survivorship in the field, we manipulated Gracilaria density and predator access to Diopatra in a fully factorial and blocked field experiment over 4 wk in June 2012. We prepared worms in plastic tubes as above and out-planted them along a transect parallel to the shore at ~+0.15 m MLLW in one of 4 treatments: ambient control, Gracilaria addition, Gracilaria addition and predator exclusion cage, and cage only. Two worms were added per plot and 10 plots were generated per treatment. We report the final body masses of surviving worms (if both survived we used the average of the 2 worms). Treatments of a given replicate block were within 1 m of each other and blocks were separated by at least 1 m. We added Gracilaria to a plot by weaving 50.00 g (± 0.05 g, acceptable range of variation) blotted wet mass of the seaweed through 30 cm of 0.76 cm 3- strand rope and placing the rope in the plot center. A pre-experimental survey indicated that the maximum seaweed biomass on a Diopatra individual in the field was ~25.0 g wet mass (data not shown). Cages were 0.5 m wide by 0.3 m tall, constructed with PVC-coated chicken wire (2.5 cm mesh size) and embedded ~10 cm into the sediment. Plots without cages were marked with 3 PVC posts extending <10 cm out of the sediment and separated by ~30 cm. After 4 wk, we removed the worms and severed the regenerated tissue from the rest of the body (see photo graph in Supplement 1B at http://www.int-res.com/articles/suppl/m545p135_supp/) using a razor blade. We dried and measured the original body mass and regenerated tissue mass at 60°C until no change in mass occurred (see Supplement 1B for details). We collected all Gracilaria biomass within the plot, defaunated it, measured its wet mass and dry mass, and counted the number of whole, epifaunal amphipods.

To assess spatial and temporal variation in the effect of Gracilaria on Diopatra, we manipulated Gracilaria density in replicated field experiments in June 2013 at 2 sites near Charleston, SC (Fort Johnson, Stono River) and 2 sites near Savannah, GA (Priest’s Landing and Bull’s River). The 2013 experiments did not include a cage treatment be cause we did not detect a predator-exclusion effect in 2012 (see ‘Results’). Relative to the 2012 experiment, the 2013 experiments used only one worm per plot, had 30 replicates per treatment, and we reduced the initial level of Gracilaria biomass in the addition treatment from 50 g down to 25 g wet mass. We randomly distributed plots 1 m apart along a transect parallel to the shore at approx. +0.09 m MLLW. In addition, we attached the seaweed-embedded ropes to PVC posts using zip-ties and pushed the posts into the mudflat to in crease seaweed stability (see Supplement 1A). Plots where we did not add Gracilaria still contained a PVC post and empty rope. After 4 wk, we processed the worms, seaweed, and epifauna as in the 2012 experiment. Because of logistical constraints, we counted epifauna for only 2 of the 4 sites (Fort Johnson and Priest’s Landing) used in 2013.

Related Reference:
Kollars, N.M., J.E. Byers and E.E. Sotka (2016) Invasive decor: an association between a native decorator worm and a non-native seaweed can be mutualistic. Marine Ecology Progress Series (DOI: 10.3354/meps11602)

Related Datasets:
MEPS_2016: Fig.2A - survey
MEPS_2016: Fig.2B - Gracilaria growth rate
MEPS_2016: Fig.3 - growth rate and depth
MEPS_2016: Fig.4A - worm growth
MEPS_2016: Fig.4B - stable isotopes
MEPS_2016: Fig.5A - field expt 2012