Belongs within: Pteriomorphia.
Contains: Brachidontes, Musculus, Modiolaria, Modiolus, Perna, Lithophaga, Mytilus, Crenella.
The Mytilidae are the mussels, a family of originally epifaunal, byssally-attached bivalves. Identifiable examples of this family are known in the fossil record from the Devonian (Skelton & Benton 1993). Free-living species are commonly nestling, forming distinct colonies. The beak of the shell is at the anterior end in Mytilinae but slightly behind the anterior in Modiolinae. Members of the subfamily Lithophaginae have become borers into their rock substrate. The Pacific genus Adula is characterised by an elongate, cylindrical shell with the beak behind the anterior margin and a thickened anterodorsal margin. The Crenellinae include byssally attached, infaunal and boring species in which the lobes of the mantle are anteroventrally fused (Beesley et al. 1998).
Plenty of mussels
Published 6 October 2025
Among the more distinctive families of bivalves in the world’s oceans are the Mytilidae, the mussels. These shellfish are characterised by more or less elongate, narrow shells that are equivalve and inequilateral—that is, the two valves are the same size as each other but each valve has the anterior and posterior margins differing in length. The beak of the shell is curved forwards, and the inside is typically pearly. The outside of the shell is usually covered by a thick periostracum that may appear hairy. The margins of the shell hinge are either smooth or bear only a few weak teeth, with a tough ligament posterior to the beak holding the valves together.
Some sources will refer to mytilids as ‘marine mussels’ to distinguish them from the freshwater Unionidae, though a small number of mytilid species are also found in fresh water. Remarkably, though mytilids and unionids are not closely related within bivalves, they share a unique mode of mitochondrial transmission known as doubly uniparental inheritance (Zouros 2000). Whereas most animals inherit mitochondria only from the female line, both males and females may pass on their mitochondria in mussels. Female mussels only retain mitochondria from their mothers, as in other animals, but males retain mitochondria from both parents. Furthermore, male gonads contain only mitochondria inherited from the father, so only the paternal mitochondria are passed on by males to their own offspring. The mechanism by which this happens is not well understood, but indications are that all larval mussels initially contain both mitochondrial lines before eliminating those mitochondria inappropriate to their sex. Why such an unusual mode of inheritance should have arisen in two seemingly independent bivalve lineages (it has not been found in other bivalves more closely related to either family) remains a mystery.
Mussels do not actively burrow themselves into sediment in the manner of clams but instead attach themselves to their substrate via the byssus, a cluster of threads secreted and placed by the long, finger-like foot. Depending on species, mussels may live epifaunally, attached to the surface of rocks, or more or less buried. Infaunal species may construct a nest or cocoon of byssus threads that traps sediment. Mussels may also bore into hard substrata such as wood, rock or even live corals; boring is achieved via secretion of corrosive compounds (Beesley et al. 1998). Many mussels cluster in dense colonies in intertidal habitats, and several larger species are economically significant food sources for humans.
Historically, mussels have been divided between several subfamilies on the basis of shell shape and life habit. However, recent studies of mytilid phylogeny have failed to support the monophyly of most subfamilies, with infaunal and rock-boring habits evolving on multiple occasions (Audino et al. 2020). Some authors have suggested that infaunal habits are ancestral for the family, as mussels of this type extend further back in the fossil record (at least as far as the Devonian), but phylogenetic relationships indicate the opposite. The earlier record of infaunal mussels is probably an artefact of their better placement for fossilisation. One group of mussels that is still resolved as monophyletic is the Bathymodiolinae, a deeper-living subfamily that is associated with hydrothermal vents as well as organically rich niches such as sunken wood and fallen animal carcasses. Bathymodiolines have reduced guts and obtain most or all of their nutrition from symbiotic bacteria living in the gills. Both sulphur- and methane-oxidising bacteria have been identified from bathymodiolines; some species may contain both (Duperron et al. 2009). Bathymodiolines dominate the hydrothermal vent niche, one of the most remarkable discoveries of the last century.
Systematics of Mytilidae
<==Mytilidae |--+--ModiolinaeH00 | | |--ModiolusPC11 | | |--Modiolula phaseolinaPC11 | | |--PernaH00 | | |--AdipicolaBRW98 | | |--Lioberus castaneus (Say 1822)BRW98, S-S90 | | `--ModiolusiaBRW98 | `--LithophaginaeCS04 | |--LithophagaPC11 | |--AmygdalumF26 | | |--A. beddomei Iredale 1924MG-H11 | | `--A. watsoni Smith 1895MG-H11 | |--Zelithophaga Finlay 1927F26 | | |--*Z. truncata (Gray 1843)P61 [=Lithodomus truncatusP61, Lithophaga truncataF26] | | `--Z. nelsoniana [=Lithophaga nelsoniana]F26 | |--AdulaH00 | | |--A. cinnamomea [=Lithodomus cinnamomeus, L. cinnamominus]C64 | | |--A. falcata [incl. A. gruneri]C64 | | `--A. stylina Carpenter 1864C64 | `--Inoperna Conrad in Kerr 1875 [incl. Pharomytilus Rollier 1914-1915]H00 | |--*I. carolinensis Conrad in Kerr 1875 [=Modiolus (Inoperna) carolinensis]H00 | |--I. lilliputensis Hodges 2000H00 | `--I. plicata Sowerby 1819 (see below for synonymy)H00 `--+--MytilasterPC11 | |--M. lineatusCS77 | `--M. monterosatoiP75 `--+--MytilinaeG88 | |--MytilusPC11 | |--Limnoperna fortuneiG88, M96 | |--AustromytilusBRW98 | | |--A. australisBRW98 | | `--A. rostratusBRW98 | `--XenostrobusBRW98 | |--X. atrata (Lischke 1871)XZ10 | |--X. pulexHS01 | `--X. securisHS01 `--CrenellinaeBRW98 |--CrenellaS98 |--LanistinaBRW98 |--TibialectusBRW98 |--MusculistaPC11 | |--M. glaberrima (Dunker 1857)S-S90 | `--M. senhousia (Benson in Cantor 1842)PC11, HJ08 `--Gregariella Monterosato 1884P61 [incl. Trichomusculus Iredale 1924P61, F26] |--*G. sulcata [=Modiolus sulcatus]P61 |--G. barbata (Reeve 1858)P61 [=Lithodomus barbatusH09, Modiolaria barbataF26, *Trichomusculus barbatusF26] `--G. petagnaeBBB-S95 Mytilidae incertae sedis: Geukensia demissaGW02 |--G. d. demissaDK89 `--G. d. granosissima Sowerby 1914DK89 BrachidontesF26 Aulacomya Moerch 1853P61 |--*A. magellanica (Lamarck 1819)P61, F26 [=Mytilus magellanicusP61] |--A. ater (Molina 1782)GO78 `--A. maoriana (Iredale 1915)P61 [=Mytilus (Aulacomya) maorianusF26, Brachyodontes maorianusF26] Trichomya Ihering 1924F26 |--T. hirsuta (Lamarck 1819)F26, H09 [=Mytilus hirsutusF26, Brachyodontes hirsutusH09] |--T. huttoni [=Mytilus huttoni]F26 |--T. rostrata [=Mytilus rostratus]F26 `--T. torquata (Marshall 1918) [=Mytilus torquatus]F26 MusculusM54 Crenomytilus grayanus (Dunker 1853)VKK10 Solamen Iredale 1924DK08 | i. s.: S. spectabilis (Adams 1862)XZ10 `--S. (Exosiperna Iredale 1929)DK08 `--S. globularis (Tate 1886) [=Crenella globularis]DK08 BotulaBW09 |--B. fusca (Gmelin 1791)BW09 |--B. silicula (Lamarck 1819)BW09 `--B. vagina (Lamarck 1819)K90 Septifer Recluz 1848P61 |--*S. bilocularis (Linnaeus 1758)P61 (see below for synonymy) |--S. bifurcatus [=Mytilus bifurcatus]C64 |--S. cumingianus [=Mytilus cumingianus]C64 |--S. curvirostrisG31 `--‘Mytilus’ mutabilis Gould 1861C64 Stavelia Gray 1858W60 `--S. horrida Récluz 1852WMJ09 [=Mytilus horridusH09] Falcimytilus Cox 1937W60 Ryenella Fleming 1959P61 |--*R. impacta (Hermann 1782) [=Mytilus impactus]P61 `--R. cumingiana (Dunker 1857) [=Musculus cumingianus]S-S90 Dacrydium [Dacrydiinae]P61 | i. s.: D. vitreumHG98 | D. viviparumBRW98 `--D. (Quendreda Iredale 1936)P61 |--D. (*Q.) fabaleP61 `--D. (Q.) pelseneeri Hedley 1906P61 SinomytilusG88 ArcuatulaC96 ChoromytilusJB12 PhthoniaJB12 PletomytilusJB12 PromytilusJB12 Mytilops precedens (Hall 1870)SB93 ModiolariaC64 Bathymodiolus Kenk & Wilson 1985AJH07 [BathymodiolinaeBRW98] `--B. breviorR01 IschadiumBRW98 ModiolatusK90 |--*M. plicatus (Gmelin 1791) [=Mytilus plicatus]S-S90 `--M. hanleyi (Dunker 1882)K90 Volsellina Newell 1942DX84 `--V. zhesiensis Liang in Ding, Xia et al. 1984DX84
Inoperna plicata Sowerby 1819 [=Mytilus plicatus non Gmelin 1791 (not preoc. if in dif. gen.), *Pharomytilus plicatus]H00
*Septifer bilocularis (Linnaeus 1758)P61 [=Mytilus bilocularisP61, Brachyodontes (Septifer) bilocularisS-S90; incl. M. efflugensBRW98]
*Type species of generic name indicated
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