Mytilinae
Based on a morphological/anatomical characters; b molecular data; c paleontological data. †The World Register of Marine Species (WoRMS; www.marinespecies.org) used this source in their classification of the Mytiloidea. §Based on the fossil record, shell morphology, life history traits and spat behavior, Ockelmann & Dinesen (2009) concluded that the Adulinae, a subfamily of mechanical borers into soft rock, should be included in the Mytilinae rather than the Lithophaginae, another subfamily of borers. ‡A freshwater subfamily.
Figure 1.4 Different external shell forms in the family Mytilidae. (A) Mytiliform Mytilus galloprovincialis.
Source: From MgA. Jiří Novák (biolib.cz).
(B) Modioliform Modiolus lulat.
Source: From MgA. Jiří Novák (biolib.cz).
(C) Elongate coral boring form Lithophaga lithophaga.
Source: From MgA. Jiří Novák (biolib.cz).
(D) Shell with pronounced anterior and posterior radiating ribs, as in Musculus niger.
Source: From Guido Poppe © G. & Ph. Poppe (http://www.poppe‐images.com/).
(E) Radiating sculpture over entire shell surface, as in Brachidontes erosus.
Source: From MgA. Jiří Novák (biolib.cz).
(F) Perna viridis.
Source: From Guido Poppe © G. & Ph. Poppe (http://www.poppe‐images.com/) (See colour plate section for colour representation of this figure).
Significant Evolutionary Developments in the Family Mytilidae
As already mentioned, one of the most significant developments in bivalve evolution was the emergence of the heteromyarian form, possibly from a putative isomyarian infaunal ancestor (Figures 1.3 and 1.5A). The form – most pronounced in the Mytilidae – when coupled with secure byssal attachment allowed mussels to anchor themselves and live in high population densities in wave‐exposed habitats (Morton 1996). Species of Mytilidae that exhibit varying degrees of heteromyarianism are found in the subfamilies Arcuatulinae, Brachidontinae, Mytilinae, Modiolinae, Musculinae, Septiferinae and Limnoperninae. Infaunal (endobyssate) mytilids inhabit both hard and soft substrates and are less heteromyarian, but more diverse, than epifaunal (epibyssate) rocky shore mytilids. The most significant adaptation in infaunal mytilids is a shell that is long and narrow (modioliform), allowing large numbers of individuals to form dense beds. The following text is heavily reliant on Morton (1992).
The posteriorly elongated shell keeps the inhalant and exhalant orifices in these fixed bivalves above the sediment water interface (Figure 1.5B). Enlargement of the posterior face of the shell and reduction of the anterior has resulted in the development of larger posterior adductor and posterior byssal retractor muscles and reduced anterior adductor and anterior byssal retractor muscles. In Perna, the anterior adductor muscle has been lost entirely, making the genus the only monomyarian one that has retained the heteromyarian form (Morton 1987). In addition, there has been an enlargement of the ligament, which probably acts to keep the two shell valves correctly aligned, particularly since another consequence of heteromyarianism is either a large reduction in hinge plate and teeth or their loss. Another adaptive feature, exemplified by species of Musculista, Modiolus and Arcuatula,
