Abundance
Musculus discors usually occurs as distinct clumps but occasionally forms dense, extensive beds (Könnecker & Keegan, 1983; Cartlidge & Hiscock, 1980; Hiscock, 1984b; Baldock et al., 1998).

Habit
Adults attach to their substratum using byssus threads. They then weave a 'nest' of several thousand of fine byssus threads around their shell, so that the shell is suspended in a network of byssus threads, similar to a 'ball of twine' . The byssus threads are not attached to the shell but only emanate from the byssal aperture. The nest completely encloses the adult so that the crenella is only visible when its valves are open and it is feeding with siphons extended (MacGinitie, 1955; Merrill & Turner, 1963). An adult may produce between 200 -500 threads/ week (Merrill & Turner, 1963). Nest construction is detailed by Merrill & Turner (1963).

The nest may incorporate a variety of pieces of seaweeds or detritus or may be fouled by epifauna, the exact composition depending on the location and habitat, which provide camouflage. For example, the nest may incorporate; the stolons of hydroids, bryozoans, small bivalves and annelids (MacGinitie, 1955; Merrill & Turner, 1963); fragments of Flustra foliacea (Forbes & Hanley, 1853), or fragments or blades of fucoids and laminarians (Thorson, 1935).

MacGinitie (1955) noted that specimens from East Greenland >20mm were nearly always covered by a byssal nest. However, in British Columbia Merrill & Turner (1963) noted that the smallest specimens with nests were 8.1mm in length and most specimens over 15mm had nests, although some specimens up to 18mm in length were without a nest.

Growth
Thorson (1935) reported that Musculus discors was 5-7mm in length by the first growth ring (presumably Ist year), 10 -11mm by the second and 13 -16mm by the third (presumably 3rd year) in east Greenland, however, growth rates will probably depend on environmental conditions.

Musculus discors forms gregarious clumps on the holdfasts of seaweeds, especially Corallina officinalis, Fucus spp. Laminaria spp. and Desmarestia sp. On laminarians, Musculus discors may cover the holdfast and bottom part of the stipe (Jeffreys, 1863; Ockelmann, 1958; Tebble, 1976). Small specimens (<20mm) were reported nestled in interstices between barnacles and the old holdfasts of tunicates (MacGinitie, 1955). Merrill & Turner (1963) found Musculus discors fouling the upper surface of the sea scallop Placopecten magellanicus.

Most records reported Musculus discors in the shallow subtidal to depths of up to 50m in the British Isles. However, it has been reported to be abundant above 30-40m in east Greenland, to form a well developed community at 60-100m in the Fosse de la Hague in the English Channel (Cabioch, 1968), to occur from 0 -374m in the Barents Sea (Ockelmann, 1958), and to be common in most trawls from 130-741 ft (ca 39-225m) at Point Barrow, Alaska (MacGinitie, 1955).

Musculus discors occasionally forms dense aggregations, especially in strong tidal streams, covering rock surfaces, forming the biotope MCR.Mus.