No interactions present.
The threads must be byssus threads (mussel, cockles etc also have them).But most Donax v. shells you find don't have them & I was unable to find any mention in texts or on the net; & no photos either in texts/ internet sites, but (about a yr back) in google images but many pages down) I found one at conchology.be, (belgian), twice as bushy & it has a whole page of Donax v. for sale (2 euros), several even more bushy. Now (at least on my google images) the bushy donax is on 1st page. Have since fd a 2nd D. v with slighter bush.
I am here bause I am about to post my own D.vittatus - http://www.ispotnature.org/node/871471
I hadn't thought much about byssal threads on clams, thinking maybe that only mussel-types had them for anchoring, It seems unlikely that clams buried in sediments would need to develop them,
However I notice that these are sometimes called Surf mussels and, in South Africa's cousin, the white mussel. So there might be an association beyond the threads.
You pic is remarkable because, as you write, there are no others on the web showing such features, makes me suspect they may indeed be seaweed strands.
It seems, in 2017, that http://www.conchology.be/ is a shell retailing site. But there is an interesting link or two on the home page; this one http://www.conchology.be/?t=166
Collecting Shells In Times of Internet
Guido T. Poppe
I can't read it because I am a 'PC & Android man'
THREE MARINE PROJECTS
Lat/Lng: 51.5805, -4.2954
OS grid ref: SS410893