The Pinnidae are a taxonomic family of large saltwater clams sometimes known as pen shells. They are marine bivalve molluscs in the order Pterioida. 

Examples of 4 of the 5 types of Pen shell found on the Arabian coastline…Pinna muricata (top left),Pinna bicolor (bottom left),streptopinnna saccata (bottom), Servatirina pectinata (bottom left, centre)

These bivalves are fragile, anchoring themselves in the sand by use of byssus threads (sea silk) with only the top, wide end of the shell above the sand.There is a layer of nacre inside the shells near the anchored end. Sea silk is an extremely fine, rare, and valuable fabric that is made from the long silky filaments or byssus secreted by a gland in the foot of pen shells

Pinna bicolour…This is the largest I have found…31.4 cms

Plentiful around the Arabian coastline, sometimes the paired shells are found intact on beaches with little tidal movement, but it is rare to find them completely un-damaged. I have found all sizes and for the first time early this year, found a live habitat, much to my discomfort…I was wading through a sandy, sea grass area and stepped on the upper end of one. Sharp, a few cuts inflicted, time to buy some scuba shoes!

Just poking up from the sea bed, waiting for my feet….

The nacreous inside…

the 5th type of pen shell found in Arabia …Atrina vexillum…

Of interest is the Pinna Nobilis, a native of the Mediterranean Sea bed, whose byssus threads were used in clothing manufacture for many centuries. The effect is a golden hue and items such as cloaks, stockings, shawls and gloves were much prized. Pinnidae are also used as a food source around the Pacific rim shores.

I found this article on Go Nomad about Chiara Vigo, the last person in Sardinia who weaves using byssus threads. It’s fascinating….