IS INTERSEXUALITY A COST OF ENVIRONMENTAL SEX DETERMINATION IN GAMMARUS-DUEBENI

Three possible causes of intersexuality in Gammarus duebeni, a crustac ean with environmental sex determination, were investigated. Intersexu ality appears to be a cost of the flexible sex determining mechanism o f this species: the occurrence of intersexes is influenced by photoper iod, which also cues sex determination. Intersexuality is heritable: i ntersex mothers produce more intersex offspring than do true females. Parasitism by a feminizing microsporidian is ruled out as a significan t cause of intersexuality.