Douglas Harper's Etymology Dictionary
actino-
before vowels actin-, word-forming element meaning "pertaining to rays," from Greek aktis (genitive aktinos) "ray, radiance;" perhaps cognate with Sanskrit aktuh "light, ray," Gothic uhtwo "dawn, daybreak," Lithuanian anksti "early."
Wiktionary
actino-
pre. 1 (context biology English) Used to form the terms relating to organisms etc. that have a radial form 2 (context physics English) Used to describe terms related to actinic radiation