Modiolus modiolus

By comparison with Mytilus, mussels of the genus Modiolus are best described as being adapted to live semi-infaunally with endobyssate attachment to the substratum (Meadows & Shand, 1989; Seed & Suchanek, 1992). However, the ecological niche they can occupy is quite broad, so that they can be found living attached epifaunally in the manner of Mytilus, or they may be found hidden within the sediment. With this broad habitat spectrum in mind and in relation to the dynamics and sensitivity of those situations where biogenic reefs form, biotopes where Modiolus is important can be divided into a number of primary and secondary sub-divisions, and these are presented, with descriptions, comments and examples, in the table below.

There are gradations to circumstances not qualifying as biogenic reefs but there are two more or less distinct types of end points where the mussels are deemed to truly form bioherms;

  • semi-infaunal reefs, which occur in various gradations of density and thickness, and to which it presently seems that the majority of Modiolus reef biotopes probably belong. Large accumulations of faecal mud and shell build up, probably over many years so that mounds of a scale visible on echosounders build up. The living mussels in this case form an irregularly clumped layer over the mound, with the largest individuals living with about two thirds of their length embedded in the deposit and small individuals find refuge amongst the byssal threads of the clumps of larger ones.
  • infaunal reefs, usually on coarser grounds and in strong currents, where the mussels bind together banks of gravel and live virtually as nested infauna within the coarse deposit. They can form wave-like mounds or bioherms which in the Bay of Fundy have been reported as up to 3 m high and hundreds of metres long. The best described examples of the latter within Britain are those off the north east of the Isle of Man, which contain numerous steep faces up to around 1 m in height, where the Modiolus are more concentrated and the associated fauna much richer. Similar areas have recently been found off the Codling Bank, Ireland.

Descriptions of Modiolus biotopes from personal observations of E I Rees and T J Holt

Description Biogenic reef? Comments and examples

1. Epifaunal Modiolus

1A Carpeting steep surfaces by Modiolus. More commonly in the infra-littoral in the cold water parts of the range where it may replace Mytilus at depth on offshore structures.

1B Isolated individuals or small relict clumps of large and probably very old animals. Sometimes these may be at LWS levels on boulder shores.


Not biogenic reef


Not biogenic reef


Parts of Lochs Duich, Long and Alsh?



2. Semi-infaunal Modiolus

2A Massive, nearly continuous beds often forming true bioherm mounds. A range of variation is found both in the way biogenic material is retained to build the mounds and in the mix of associated fauna living on the horse-mussels, in the crevices and in the accumulated soft sediment dominated by faecal material.

2A.1 Low turbidity open sea or sea loch mouth beds in strong tidal streams. Beds in these situations may not retain much of the faeces and pseudo-faeces so that though extensive they may hardly build up sufficient thickness to qualify as reefs.

2A.2 Higher turbidity areas with moderately tidal streams and where more faeces and pseudo-faeces builds up. Mixed with fine sand swept onto the beds the faecal mud and generations of shell all such beds to mound up several metres above the substratum on which they where founded.

2A.3 Sheltered sea loch beds. These are distinguished more by the mix of associated species, but the mounds may not be as massive as those in less quiescent conditions.

2B Discontinuous beds on gravel grounds with numerous scattered clumps that may or may not be large enough to be classified as biogenic reefs.


Can often be considered as biogenic reef

Not usually biogenic reefs


These beds undoubtedly qualify as biogenic reefs.

Can often be considered as biogenic reef

Some areas may qualify as biogenic reefs


South east Isle of Man where they do build up thickly in places

Off Lleyn Peninsula


Scottish Lochs and Voes, Strangford Lough



3. Infaunal Modiolus

3A Gravel wave or mound deposits in areas of v. strong currents in which abundant Modiolus are almost completely hidden. The mass of mussels "nesting" within the deposit put out byssal threads in all directions, and so bind the deposit, sometimes with much dead shell, into waved mounds. Steep faces up to one metre high and many metres long, with much live & dead shell, well bound & with rich epiphytic biota have been seen.

3B Abundant small Modiolus in offshore coarse sediments that apparently do not survive and grow to produce true mussel beds.

3C Isolated individual Modiolus of moderate size or small clumps living "nested" within coarse sediments.


Steep faces may often qualify as biogenic reef


Not biogenic reef

Not biogenic reef


Several km2 to the north of the Isle of Man found recently (Holt & Shalla, unpubl); also Codling Bank, Ireland.

Freq between S Wales & Ireland.


Densities of Modiolus themselves may not necessarily be particularly high in some of these more infaunal reefs, though true figures will only be gained if diver surveys are carried out. Holt et al. (unpubl) estimated maximum densities of large Modiolus from photographs as 20 - 40 / m2 (probably occasionally more) off the north of the Isle of Man, but it was difficult to pick out the Modiolus as only the last few mm of the shells were visible, and the field of view covered by the camera probably varied greatly due to the rough terrain. The combination of these mussels and their byssus threads binding the sediment together, plus often very large amounts of dead shell, nevertheless clearly creates a dramatically altered habitat deserving in many cases to be called a reef. The bioherms described in the Bay of Fundy had similarly low densities (4-78 / m2) but were frequently up to 3m high (Wildish & Fader, in press; Wildish et al., in press).

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