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.

Keywords:

• anatomy
• convergent evolution
• Crenellidae
• fossil record
• Mytilidae
• Modiolidae
• Musculidae
• pericardium
• Septiferinae
• systematics

Acknowledgements

This manuscript is based on a talk presented at a symposium to celebrate the 90th birthday of Kurt W. Ockelmann and which was convened within the Marine Laboratory of the University of Copenhagen, Helsingor, Denmark, 4–5 March 2014. I am grateful to K.R. Jensen (University of Copenhagen) and G. Dinesen (Technical University of Denmark) for organising the symposium and for inviting me to participate. I am also grateful to G. Dinesen for help with the figure labelling and for the benefit of useful discussions.

Disclosure statement

No potential conflict of interest was reported by the author.

Funding

The Carlsberg Foundation, Denmark, provided funds to facilitate attendance of the symposium at which the results of this research were presented.

Notes

¹The status of this taxon, Modiola laevigata Gray, 1824, M. substriata Gray, 1824 and Modiolaria corrugata Stimpson, 1851 are variously described as independent species or as subspecies or varieties (Backeljau 1986).