Tackling tube worms

A new study aims to find an early warning for worm infestation, reports Nicki Holmyard.

I am always amused when people looking to come into the mussel industry think it will be easy. After all, don’t you just put a few ropes in the water and haul in a crop of black gold a year or two later? If only it was that simple!

The reality is that there are many technical and biological hurdles to overcome in order to get a successful crop to market, year after year, any of which can throw financial forecasts into disarray. And that is without taking into account “curve balls”, such as Covid-19 and Brexit, causing severe disruption to marketing plans.

In taking our own mussel farm (www.offshoreshellfish.com) offshore, we have faced many such hurdles, and we still have a lot to learn about the dynamics of farming on a large scale, six miles out in the open sea. However, it has meant that we can grow a crop from spat to harvest, in just over one year, which is a big advantage when compared to our previous sites in Scotland, where it took between two to three years to achieve the same thing.

Another advantage is that we no longer have an issue with biofouling organisms growing on the shells, such as barnacles (Balanus sp.) and tube worm (Pomatoceros triqueter). However, we do have occasional plagues of starfish, which settle around the same time as the mussels, and quickly outgrow them by feeding voraciously, leaving us staring at a devastated crop.

In Scotland and Shetland, tube worm can be a major problem in some years, with an estimated 500 tonnes of mussels sporting the calcareous white worm casts on their shells, causing financial losses of around £500,000 per year. As production increases, the losses will also stack up.

“On some of our sites, we have found as much as 20% of the shells affected. It’s seasonal, it’s not every year, and sometimes it is worse than others, but it can be a big problem and results in considerable wastage,” said Michael Tait, MD of Shetland Mussels.

Tube worm do not harm the mussels, but they are virtually impossible to remove, make the product less desirable to customers in terms of aesthetics and smell when cooked, and they can also damage vacuum packaging.

A new project, led by the University of Stirling’s Institute of Aquaculture, with support from Shetland Mussels and the Sustainable Aquaculture Innovation Centre (SAIC), aims to develop a rapid diagnostic tool for the presence of P. triqueter DNA in plankton and shell swab samples. Such a tool would enable producers to make informed decisions about dealing with tube worm, including environmental management and cleaning regimes.

Similar molecular diagnostic techniques are already common in finfish farming, but not in the shellfish sector. The project could therefore herald a significant step change for shellfish farmers, enabling them to improve stock management and product quality.

Currently, the only way to detect the presence of larval tube worm is by looking at a water sample under a microscope, but as project lead researcher Dr Stefano Carboni explained, the larvae can be easily confused with other organisms, and sampling only covers a small volume of water.

“Tube worm and other biofouling are of long-standing concern for the shellfish industry, and there is not a clear understanding about what drives the settlement of larvae. A more practical and reliable method for identification, would be an invaluable development for the industry, and it could be applied on a global scale. Once we have a molecular diagnostic tool, it could easily be adapted to identify other organisms of interest, including the D larvae of mussels, which would take all the guess work away from shellfish farmers,” he said.

Current strategies for dealing with early-stage tube worm settlement include monitoring water temperatures and weather patterns to estimate when it will occur, exposing shells to the air, washing them, or cleaning using acetic acid, brine or hot water. None of these are cost-effective and can have a detrimental effect on growth and survival.

For Michael Tait, who maintains separate spat collection and ongrowing sites, a detection tool could allow him to delay retubing seed mussels onto new lines until tube worm larvae are no longer registering in an area.

“This project has exciting potential to change the way mussel farms are managed in future, and it will be interesting to see the initial results. The potential for adaptation to a tool that identifies mussel larvae would also be of interest to us,” said Tait.

Regular water samples will be collected from Shetland Mussels, along with surface swabs from mussel shells, which the Stirling University team will analyse for the presence of tube worm DNA. The data will allow them to monitor patterns and seasonal variations that could inform cleaning schedules and potential site selection, as well as preventing future losses.

Heather Jones, CEO at SAIC, was pleased to support a project to help the growing Scottish shellfish sector.

“This project represents just one example of pioneering research that will support further sustainable growth to meet the global demand for protein. New data-led techniques such as this DNA diagnostic tool can help to drive the entire industry forward, with benefits spanning the environment, businesses operating in the sector, and the end consumer,” she said.

A similar project is currently underway in Sweden, where mussel farming company Bohus Havsbruk and the Swedish environmental institute IVL, are looking at how mussels can be heat treated to remove fouling.

The research, which is part of the EU Horizon 2020 AquaVitae project, is trialling the exposure of mussels to seawater at a higher temperature than the surrounding sea, for a short period of time.

Trials are currently still in the laboratory phase, with researchers investigating the temperature range that will result in high worm mortality, while maintaining high mussel survival. Once this has been established, the next stage is to undertake field trials to ascertain the feasibility of working at scale.