One of a group of threadlike (hairlike) fibers in the finfolds of fish larvae that eventually develop into fin rays.

The actinotrichia are slender, horny, flexible, unsegmented fibrils which strengthen the embryonic finfold, 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.