I bought this book, Echinoderms of Florida and the Caribbean-Sea Stars, Sea Urchins, and Allies, by Hendler, Miller, Pawson & Kier about 9 years ago when it was on sale at the bookstore in the Smithsonian's National Museum of Natural History in D.C. I hadn't had a chance to use it until last week when my son found this sea star at a beach in Florida.
There are no dichotomous species keys in this book, which is fine with me, because I don't care much for dichotomous keys anyway. Instead, one tries to identify the specimens from descriptions, habitats and the color pictures.
This sea star, apparently a juvenile with a diameter of 55 mm, comes closest to Echinaster sentus. The book, however, notes that the "variability of Echinaster species makes their identification decidely difficult." To make things worse, several other Echinaster species that also occur in Florida are listed, but no clues are given as to how to tell them apart.