Mussels, mainly Mytilus edulis, are generally considered to be herbivorous
filter feeders with phytoplankton and bacteria the main part of their diet,
although zooplankton may be consumed as well (Davenport et al. 2000;
Rouillon & Navarro, 2003). Field (1911, see Newell et al. 1989) observed a
wide range of items in the gut of M. edulis, particularly diatoms and
dinoflagellates. These could be 200 μ m or more in length, though were
typically elongate so usually no more than 40 μ m in diameter. A similar
maximum diameter for typical ingested particles was reported in Oysters
(Crassostrea gigas) by Dupuy et al. (1999). There would seem, however, to
be a tendency for larger inorganic (SiO2) particles to be rejected with
Defossez & Hawkins (1997) reporting particles of 18.7μ m and above were
increased in relative concentration in the surrounding water – i.e. they were
not taken up. Lehane & Davenport (2002) reported a wide range of animal
parts inside the guts of mussels and oysters, indicating a degree of carnivory
in species commonly assumed to be herbivorous. Widdows et al. (1979)
reported that the size of food items ingested by M. edulis was typically less
than 40μ m, with the largest sized, around 30μ m, being taken up in spring
and summer.