You searched for: “actinotrichia
1. Threadlike (hairlike) fibers in the finfolds of fish larvae that eventually develop into fin rays.
2. Slender, horny, flexible, unsegmented fibrils which strengthen the embryonic fin fold or the unjointed horny rays at the edge of fins in many fish.

Actinotrichia are translucent, exhibit birefringence (double refraction) and are composed of a scleroprotein called elastoidine.