Catalysis of the formation of a double bond between C8 and C9 of the long chain base of a sphingolipid. For example, sphinganine (d18:0) = 8-sphingenine (d18:1delta8); phytosphinganine (t18:0) = 8-phytosphingenine (t18:1delta8); and 4-sphingenine (18:1delta4) = 4,8-sphingadienine (d18:2delta4,8).