Field and laboratory observation of Mediterranean mussel Mytilus galloprovincialis predation by flatworm Stylochus mediterraneus

Abstract

Recently, large numbers of Polyclad flatworms (Stylochus mediterraneus) have been observed in Istrian County shellfish farms connected to intensive ascidian fouling. To assess a possible threat and determine whether the flatworm feeds only on weakened dead mussels or can also infest healthy mussels, we have conducted a field research and laboratory mussel predation experiments. Performed field observations (August 2022–May 2023) indicated increased mussel mortality and, at the same time, increased abundance of flatworms from 70 % time filtrating with the valve open > 50 %, with 1–5 resting periods), the behavioural changes of exposed mussels compared to control mussels, the timing of flatworm attack, the response of mussels during predation, and the timing of mussel death. We conclude that the flatworm S. mediterraneus is most likely a very opportunistic species that prefers weaker mussels but can also feed on healthy mussels. This investigation brings new knowledge and assumptions on the behavioural patterns of investigated flatworms. Further studies should consider that ''experienced'' flatworms attack and kill the mussels, while the rest simply eat a free meal using the strategy of the predator pack.

Keywords

• Mytilus galloprovincialis
• Behavioural response
• Stylochus mediterraneus
• Predator-prey interaction
• Valve Gaping Mussel Monitoring System