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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?
Widespread |
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
Widespread |
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.
Widespread. |
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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References
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