Marine Mussels. Elizabeth Gosling

molecular data suggests that Mytiloidea is a monophyletic clade (Morton et al. 2016 and references therein). For more than five decades, there has been no consensus on the number of families contained within the Mytiloidea. For example, Boss (1982) recognised only a single family, the Mytilidae, divided into four subfamilies, Mytilinae, Crenellinae, Lithophaginae and Modiolinae. Coan et al. (2000), and subsequently Coan & Valentich‐Scott (2012), also recognised a single family, but added three additional subfamilies: Bathymodiolinae, Dacrydiinae and Septiferinae. Bieler et al. (2010) added one more, Limnoperninae, making a total of eight subfamilies within the Mytilidae. In contrast, others have recognised between three and four families in the Mytiloidea; Carter et al. (2011), for example, in a classification including fossil taxa, recognised three families, Mytilidae, Crenellidae and Septiferidae, while Morton (2015) proposed four, the Mytilidae, Crenellidae, Modiolidae and Musculidae – although he suggested that the Septiferinae might be eligible for family status subject to additional morphological and molecular analyses. Bieler & Mikkelsen (2006) initially recognised five families, the Mytilidae, Crenellidae, Musculidae, Modiolidae and Septiferidae, although as we have seen they later recognised only one, the Mytilidae (Bieler et al. 2010). Table 1.1 presents a summary of five different taxonomic classifications of extant Mytiloidea, which are only a fraction of the published classifications for this superfamily. The majority of classifications have been heavily reliant on shell features, such as shape and characteristics of the valves, hinges and dentition, with few authors to date using anatomical features (reviewed in Morton 2015) or molecular data.

The Mytilidae is the largest family within the Mytiloida, with subfamily numbers varying between four and eight (Table 1.1). The family consists of 52 genera and 411 species (www.marinespecies.org). Species are recognised as members of the family by the shell form, sculpture, hinge and muscle scars (see Soot‐Ryen 1955 for details). Mytiliform species, with a broadly triangular shape and terminal umbones, are placed in the genus Mytilus (Figure 1.4A), while those with subterminal umbones with a distinct anterior margin – the modioliform species – are consigned to the genus Modiolus (Figure 1.4B). Elongate forms that burrow in rock or mud and have parallel dorsal and ventral shell margins and subterminal umbones are placed in the genus Lithophaga (Figure 1.4C), species with pronounced anterior and posterior radiating ribs, except for a smooth median area, are placed in the genus Musculus (Figure 1.4D), and species with radiating sculpture over the whole outer shell surface are assigned to the genus Brachidontes (Figure 1.4E). Classification solely based on external shell features is problematic as these can be influenced by environmental conditions and similar forms may arise as a result of convergent adaptations to common environmental conditions. For example, a phylogenic study of four subfamilies of Mytilidae found that Crenellinae and Lithophaginae were monophyletic but Mytilinae and Modiolinae were polyphyletic⁴ (i.e. they do not share a common ancestor), suggesting that the mytiliform and/or the modioliform body plans have evolved independently in at least two mytilid lineages (Distel 2000). However, in a recent study using species in five subfamilies (Mytilinae, Modiolinae, Bathymodiolinae, Lithophaginae and Crenellinae), Liu et al. (2018) found that phylogenetic trees reconstructed with a combination of two mitochondrial (COI and 16S rRNA) and three nuclear (18S and 28S rRNA and histone H3) genes indicated monophyly of the family Mytilidae and the subfamily Bathymodiolinae, polyphyly of the subfamilies Modiolinae and Lithophaginae and paraphyly⁵ of the subfamily Mytilinae.

Table 1.1 Classification schemes in the superfamily Mytiloidea. Families within the Mytiloidea are in bold, while those in plain text are subfamilies recognized within the individual families. Source: From Morton (2015). Reprinted with permission from Taylor & Francis.

Boss 1982 a	Coan et al. 2000 a	Bieler et al. 2010 a,b,c	Carter et al. 2011 a,b,c	†Morton 2015 a
Mytilidae	Mytilidae	Mytilidae	Mytilidae	Mytilidae
Mytilinae	Mytilinae	Mytilinae	Mytilinae	Mytilinae
Crenellinae	Bathymodiolinae	Bathymodiolinae	Arcuatulinae	Brachidontinae
Lithophaginae	Crenellinae	Crenellinae	Bathymodiolinae	Botulinae
Modiolinae	Dacrydiinae	Dacrydiinae	Lithophaginae	Adulinae§
	Lithophaginae	Limnoperninae‡	Modiolinae	Lithophaginae
	Modiolinae	Lithophaginae	Crenellidae	Septiferinae
	Septiferinae	Modiolinae	Crenellinae	Crenellidae
		Septiferinae	Musculinae	Crenellinae
			Septiferidae	Dacrydiinae
			Septiferinae	Modiolidae
			Limnoperninae	Modiolinae