Fig. 1. Hypothetical representation of cellular iron uptake pathways in fish combining data from gill and intestine. See text for more details. Briefly, ferric iron (Fe³⁺) is reduced via an apical membrane bound ferric reductase (FR). Ferrous iron (Fe²⁺) enters the cell via a Fe²⁺/H⁺ symporter (DMT1). Basolateral Fe²⁺ export occurs via an iron regulated transporter (IREG1), also known as ferroportin. IREG1 is linked to a membrane-bound copper-containing oxidase, termed hephaestin (HP), that oxidizes Fe²⁺ to Fe³⁺. Fe³⁺ binds to transferrin (TF) in the blood. Li, aquatic ligand.