Evolution and adaptive radiation in the Mytiloidea (Bivalvia): clues from the pericardial–posterior byssal retractor musculature complex

Abstract

Mytiloidea can be divided into four anatomical categories based upon the relationship between the location of the pericardium and the positions and arrangements of the posterior byssal retractor muscle units. In Category 1, the pericardium is situated above multiple posterior byssal retractor muscle blocks and is typical of the Modiolinae only. This subfamily contains among the oldest mytiloid fossils and is considered to represent the earliest condition. In Category 2, the pericardium is situated anterior to a reduced number, between one (Lithophaga, Botula, Adula and Brachidontes) and six (Mytilus) posterior byssal retractor muscle units. This category is typified by the Mytilinae and related subfamilies, for example, the Botulinae, Adulinae, Lithophaginae and Septiferinae. In Category 3, the pericardium is situated between either two (Limnoperna, Bathymodiolus, Trichomusculus) or one and numerous (Modiolarca) posterior byssal retractor muscle blocks. This category is typical of the Musculinae, Arcuatulinae, Limnoperninae and Bathymodiolinae. Category 4 has a reduced musculature overall, only one pair of small posterior byssal retractor muscles, is seen only in the Dacrydiinae and Crenellinae, and results from the reduction/loss of the anterior component of the posterior byssal retractor muscles. These anatomical categories suggest the Mytiloidea comprises four families: the Modiolidae; Mytilidae; Musculidae; and Crenellidae. The Septiferinae, having a long accessory posterior adductor muscle, may also be a distinct family but affiliated with the Mytilidae.

Mass spawning by the date mussel Lithophaga lithophaga

“…Female L. lithophaga individuals released eggs in two parallel streams (Fig. 3c ) reflecting how the paired ovaries and gonadal apertures open into their supra-branchial chambers 14 (Fig. 4e ).…”

Functional morphology, biology and sexual strategy of the circumboreal, adventitious crypt-building,Crenella decussata(Bivalvia: Mytiloidea: Crenellidae)

“…The problem arises from the fact that few mytilid taxa have been described anatomically so that ascertained diagnostic differences between them mainly comprise shell features such that virtually all of the classification schemes described above are based on these characters. Only a few authors have examined mytiloid morphology and these have been identified by Morton (2015a). Notwithstanding, the most recent classifications of the Mytiloidea have arisen from the results of molecular studies.…”

“…(2000), Steiner & Hammer (2000) and Giribet & Wheeler (2002) all suggest that the Mytiloidea constitutes a monophyletic clade, which shell morphology evidence broadly supports. Distel (2000) and Steiner & Hammer (2000) suggested, however, that subfamilial relationships of the Mytiloidea need re-examination and to this end Morton (2015a) re-examined the pericardial-posterior byssal retractor muscle relationships of a large number of mytiloid taxa and concluded that the superfamily comprised four families – the Modiolidae, Mytilidae, Musculidae and Crenellidae – a view which supported the classification suggested by Bieler & Mikkelsen (2006) albeit with the latter author's addition of the Septiferidae. Finally, in a classification including fossil taxa, Carter et al (2011), recognized the extinct Modiolopsoidea P. Fischer, 1886 as separate from the Mytiloidea and suggested that the latter comprised three families and 10 subfamilies – the Mytilidae (with the Mytilinae, Arcuatulinae, Bathymodiolinae, Lithophaginae, Modiolinae and the extinct Xenomytilinae Squires and Saul, 2006), Crenellinae (with the Crenellinae and Musculinae) and Septiferidae (with the Septiferinae and Limnoperninae).…”

“…This is not true, however, as this study will show. Morton (2015a) has shown for Crenella ecuadoriana Pilsbry & Olsson, 1941 (Soot-Ryen, 1955) that there is a posterior adductor muscle, minute paired posterior byssal retractor muscles, but no anterior byssal retractor nor posterior pedal retractor muscles and a small anterior adductor muscle aligned thinly along the fused anterior mantle margins. Crenella ecuadoriana is currently synonymized with C. decussata s.s. due to indistinguishable adult shell features, but with the proviso that such taxa may comprise a species complex, that is C. decussata s.l.…”

Evolução da margem do manto em Pteriomorphia (Mollusca: Bivalvia): um modelo para compreensão da macroecologia do bentos marinho

“…Estudos anatômicos identificaram supostas associações entre diferentes modos de vida e possíveis adaptações morfológicas, relacionadas principalmente ao formato e estrutura das valvas, organização da musculatura adutora e presença de especializações do manto Morton, 2015). Contudo, a maioria dessas hipóteses não foi testada em um contexto filogenético ou por métodos comparativos.…”