Mytilus reefs are composed of layers of living and dead mussels at high densities, bound together by the byssus threads secreted by the mussels and sometimes overlaying a great deal of accumulated sediment. Subtidal beds have been reported up to 120 cm thick (Simpson, 1977 cited in Seed & Suchanek, 1992). Well developed reefs in most UK sites rarely exceed 30-50 cm, however. Not surprisingly mussel bed thickness and structural complexity increase with age of the bed. Based on observations on beds of M. californianus, Suchanek (1979) described Mytilus beds in terms of three structural components;

i) a physical matrix of living and dead shells;

ii) a bottom layer of accumulated sediments, mussel faeces and pseudofaeces, organic detritus and shell debris.;

iii) an assemblage of associated flora and fauna.

M. edulis beds and reefs also have these components. The accumulated sediment is called ‘mussel mud’, and the faeces and pseudofaeces element is often the most important. Nixon et al. (1971) found 14.4 kg m^-2 dry weight of trapped sediment within M. edulis beds in Rhode Island, with an organic content of 3.86%. Davies et al. (1980) reported the weight of mussel mud to be 17-19 times the seed mussel production in crab-proofed cages in Morecambe Bay.

Accumulation of sufficient faecal and pseudo-faecal deposits together with dead shell to produce obvious mounds is largely restricted to those places, in estuaries or similar channels and flats, where there is a degree of shelter from wave action, but sufficient flow carrying seston for there to be good growth. Persistent and semi-permanent beds in sheltered locations can in these situations build up an obviously biomediated relief of mounds rising a metre or more above the surrounding seabed. These features can clearly be classified as "Biogenic Reefs".

Large, very raised reef areas often take a hummocked or sometimes ribbon-like form, usually up to a few metres across, often with patches of sediments or cobbles / boulders in between. Sometimes the whole of a mussel bed will have a shape that is obviously aligned with the dominant tidal stream axes in the channel, but superimposed on this there may be a cross tide wave form akin to mega-ripples superimposed on a sand bank. More often, however, the complex of mounds, formed by the patchy and variable growth of the mussels competing for seston and the way they attach byssally to each other, has a more irregular relief. Less projecting areas (especially where only 1-2 mussels deep) may be very extensive and continuous, particularly where they form on more solid substrata. The latter may usually be regarded as beds rather than reefs, and can be over 50 ha in extent (Dare, pers. comm.).

In some cases, for example some of the mussel reefs in Morecambe Bay and the Dornoch Firth, a reef may consist of a single year class of mussels which have settled in very high densities on only moderately sheltered gravel and cobble skears. At first the spat may be almost hidden within the deposit, but as they grow they build up sufficient mud that they emerge as a thin carpet overlaying a layer of mussel mud which may accumulate to a depth of 0.75m in four to five months (Dare, 1976). These areas are much more transient than reefs formed of a large number of year classes, and are often washed away by water movement, particularly during autumn storms (McKay & Fowler, 1997; Dare, pers. comm.).

In many situations, gradations can be seen from those where the mussels merely carpet the substratum to a patchy extent without building substantial mounds, to those where the beds obviously form reefs.

There are situations with stronger water movements where the mussels are on gravel and they so gather and bind together coarse material, rather than their own faecal mud. In beach gravel deposits, particularly where small rivers enter the sea, beds of Mytilus have been found on both sides of the Atlantic where the mussels live infaunally in a coarse gravel / small cobble deposit that they themselves stabilise (Stephens & Bertness, 1991).

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