The fouling hydroid Ectopleura larynx: a lack of effect of next generation antifouling technologies.

The hydroid Ectopleura larynx is one of the main fouling organisms on salmon aquaculture cages in Norway; this study investigated novel surface materials and microtopographies to deter its settlement. The settlement preferences of hydroid larvae for 12 materials with wettabilities ranging from hydrophobic (54°) to hydrophilic (112°) were tested in a no-choice bioassay. Although settlement differed between materials, with the highest average settlement on polytetrafluoroethylene (95%) and the lowest on untreated polyurethane (53%), no trend regarding the tested wettabilities could be found and none of the tested materials was able to reduce average settlement below 50%. Furthermore, nine high-density polyethylene (HDPE, 100-600 μm microtopographies) and seven polydimethylsiloxane (PDMS; 40-400 μm microtopographies) microtextured surfaces were tested. There was no systematic effect of microtopography on the settlement of E. larynx larvae. However, there was a preference for settlement in channels on PDMS microtopographies between 80 and 300 μm. Similarly, there were no preferences for any of the examined microtopographies in a 12-day field test using PDMS surfaces at a commercial fish farm. The study indicated that neither surface wettability (hydrophilicity-phobicity) nor microtopographies were effective at deterring the settlement of the hydroid E. larynx. The high plasticity of the aboral pole and the hydrorhiza of the hydroids may explain settlement even under unfavourable conditions, highlighting the successful colonisation traits of this dominant biofouling species.