Mussel beds Modiolus modiolus on Atlantic sublittoral sediment

Modiolus modiolus is an Arctic-Boreal species, with a distribution ranging from the seas around Scandinavia (including Skagerrak & Kattegat) and Iceland south to the Bay of Biscay. Beds of M. modiolus occur with a patchy distribution in the colder waters of the North-East Atlantic, with a range that extends from Russia in the southern parts of the Barents and White Seas, off Norway and down to Sweden (the Sound and Kattegat) and off the UK, in the southern North Sea and Irish Sea. Beds are also known to occur around Iceland and the Faeroes.

Modiolus beds form when horse mussels (Modiolus modiolus) aggregate together by attaching to each other and the substratum with byssal threads. The degree of aggregation at which individuals are considered to be sufficiently dense to constitute a “bed” has been determined to be >30% cover. The depth range over which M.modiolus forms beds is unknown, but they typically occur from the lower shore to about 70 m, with clumps found below 100 m in the Irish Sea. Off the Faeroes, they occur to about 200m depth, being most dense between 65-95 m. The majority of beds are located in current- swept fully-saline locations, although some can be found in sheltered bays, fjords or lochs, with some beds restricted to depths below haloclines.

Modiolus beds occur on a range of substrata, most often on cobbles through to muddy gravels but have also been found on bedrock. Beds are often persistent features which build up through accumulating faecal pellets, shell and trapped sand, so that they may become de-coupled from the substratum on which they were originally founded. They can be self-sustaining to the extent that spat survival is greatest in the crevices amongst the byssal threads of the mature clumps. A diverse range of epibiota and infauna often exists in these communities including hydroids, red seaweeds, solitary ascidians and bivalves such as Aequipecten opercularis and Chlamys varia. The clumping of the byssus threads of the M. modiolus creates a stable habitat that also attracts a very rich infaunal community including areas with a high density of polychaete species. 

Indicators of quality:

The condition of Modiolus beds may be judged in several different ways:

- Spatial integrity, such as whether fishing gear tracks cut across a bed.

- Topographic integrity, such as the continued presence of ridges, mounds and other biogenic relief.

- The size distributions of the mussels and whether the populations are being adequately renewed by successful spat settlement and juvenile survival through the first years when they are most vulnerable to predation.

- The abundance, composition, condition and diversity of the associated biota.

For physical disturbance impacts, changes in soft epifauna are more likely. Some of the vagile epifauna, such as brittlestars (e.g. Ophiothrix fragilis) are known to fluctuate markedly in abundance, so caution is needed when interpreting change. Damage also leads to increased abundance of scavengers, which are attracted to disturbed areas.

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats