a slender, horny, flexible, unsegmented fibril which strengthens the embryonic fin fold and which may persist in the outer edge of the adult fin membrane or in the adipose fin. It develops intercellularly rather than cellularly. Persists in fins of Elasmobranchii, Holocephali, and sometimes in Teleostomi distal to the lepidotrichia that replace them. Actinotrichia are translucent, exhibit birefringence (double refraction) and are composed of a scleroprotein called elastoidine. They may be homologous with ceratotrichs found in cartilaginous fishes because of their horny or keratinous nature
Dictionary of ichthyology. 2009.