Lab Experiments: Using laboratory and field-based experiments, we tested whether Gracilaria increased the survivorship and growth of Diopatra through direct or indirect provisioning of food, or through refuge from predation. Diopatra tubes extend up to 1 m below the benthos and it is difficult to collect Diopatra bodies without incidentally severing the worm. Severing, however, does not cause mortality because these animals are capable of both anterior and posterior regeneration (Berke et al. 2009). To standardize the initial size of the worm and create a point of posterior regeneration, we cut the field-collected worms to ~3 cm in length. We buried the worm in field-collected sediment contained in a 15 cm long × 3 cm diameter plastic tube in the laboratory. Tubes were held in racks in a recirculating seawater table at 22°C and a salinity of 30 ppt to allow the worms to regenerate their sediment-based tubes before experimentation, which usually happened within 24 h (Berke et al. 2009, N. M. Kollars pers. obs.).

In the laboratory, we offered Gracilaria-associated diet items to Diopatra and measured survivorship and growth after 6 wk. Diopatra were collected from the Fort Johnson mudflat in January 2013, and placed into plastic tubes that were encircled with window screen to create feeding chambers that still allowed water flow. Diopatra were randomly offered one of 4 diets: sediment-only control, Gracilaria, amphipods, or Gracilaria and amphipods (n = 24 per treatment). Gracilaria was offered ad libitum and replaced weekly. Three frozen, field-collected amphipods were offered daily (primarily Gammarus mucronatus, the most abundant amphipod species on Gracilaria; Wright et al. 2014). Though we offered Diopatra dead amphipods due to logistical constraints, we do have video evidence that shows that, in the laboratory, Diopatra are capable of pursuing and catching amphipods (see video Supplement 2 at www. int-res. com/ articles/ suppl/ m545 p135_ supp/). After 6 wk, we removed Diopatra from their sediment tubes and dried them at 60°C until no change in mass occurred and measured the final body mass. To examine the effects of diet treatment on Diopatra final dry weight, we used a 1-way ANOVA followed by a post-hoc Tukey’s test.

To assess the incorporation of the supplied food source into the new worm tissue, we quantified the carbon and nitrogen stable isotopic signatures in the posteriorly regenerated tissue of 5 worms from each diet treatment. We randomly selected 5 individuals from each diet treatment and sampled the dried posteriorly regenerated tissue (targeting muscle tissue and avoiding the digestive tract or fecal pellets). For comparison, we also analyzed 3 samples of each resource (collected from the field in April 2013): Gracilaria and amphipods. We generated the isotopic signature data using a Delta V plus spectrometer (ThermoFinnigan), with a Thermo Flash EA as the interface at the Skidaway Institute Scientific Stable Isotope Laboratory (Savannah, GA, USA).

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.4B - stable isotopes
MEPS_2016: Fig.5A - field expt 2012
MEPS_2016: Fig.5B - field expt 2013