Hello Andreas,
There are very few Caribbean Pectinidae with so few ribs.  Based on your
description:
 
<The size of the greatest specimen is about 6cm in breadth. It is as
broad as wide and bears 10 ribs, all the shell bears many radiating
thread-like riblets.>
 
the only possibility I can think of is immature Lyropecten nodosus.
This species generally has 8 or 9 main ribs, so 10 would not be out of the
question, and it has the riblets you describe.  Such small ones (6 cm and
under) often lack the rounded knobs seen on larger ones.
 
Paul M.