It is clear that the Pinna byssus has unusual properties in comparison to its equivalent in the bivalve mussel, and is conspicuously different in terms of crystallinity.

Common mussel attachment threads

Bivalves such as oysters attach themselves by cementing one valve to the substrate, and such cementation has evolved many times in different groups. Others, notably the mussels, employ protein threads that are secreted by a gland adjacent to the foot and provide a remarkably resilient attachment. In point of fact the threads have effectively a three-fold structure, the adhesive plaque that connects thread to rock and then to two units that act effectively as a shock absorber. Interestingly the adhesive plaque is histidine-rich and has an association with copper: just such a toughening is found in the jaws of the glycerid polychaetes. The distal end of the thread structure is stiff and resilient, while the proximal position is more elastic and not surprisingly each has a different molecular structure. Thus although the overall structure depends on the protein collagen, in the proximal region there are structural domains that resemble elastin, one of a number of convergent elastic proteins. Correspondingly the stiffer unit, that accounts for about two-thirds of the thread, has a crystalline structure and intriguingly has fibroin-like units that are remarkably similar to those found in silk.

Pen shell byssus threads

The similarity of mussel thread fibroin strcture to silk finds another resonance with sea-silk, that is the byssus threads derived from the ‘pen shell’ or ‘fan mussel’ Pinna. Although there is some controversy as to the archaeological antiquity of sea-silk, with dismissal that the Argonauts’ Golden Fleece might have been so woven, there was a minor industry in the Mediterranean that made artefacts, including cups, cravats and ties out of it. The oldest known piece of this fabric comes from the 4th century, AD, from Roman Budapest (Aquinum), and a later cap of 12th century age is known from France (St. Denis). It is very likely that the byssus of Pinna has additional fibroin units that confer its silk-like nature, but there has been little detailed investigation. It is, however, clear that the byssus has unusual properties in comparison to its equivalent in the mussel, and is conspicuously different in terms of crystallinity (it has almost no x-ray diffraction properties, and if it is made of collagen then it has an unusual composition). The byssus of Pinna would be well worth further investigation to clarify its composition and any convergence with mussel attachment threads or even silk.