Cytokine that acts as a physiological ligand for receptor tyrosine kinases LTK and ALK (PubMed:29078341, PubMed:29317532). Required for neural crest cell differentiation and iridophore development during embryonic iridophore development and adult stripe development by acting as a receptor for LTK (PubMed:29078341, PubMed:29317532). Also required for iridophore formation in the adult eye (PubMed:29078341). Homodimer. Highly expressed in the swim bladder and single cells of unknown identity in the head. Reduced number of iridophores in the eye (PubMed:29078341). Fishes lacking alkal2b do not exhibit any defects in trunk iridophores in adults (PubMed:29317532). Fishes lacking both alkal1 and alkal2b show a complete loss of eye iridophores (PubMed:29317532). Fishes lacking alkal1, alkal2a and alkal2b are embryonic lethal and display total loss of iridophores (PubMed:29317532). Belongs to the ALKAL family.