Endogenous ligand for the apelin receptor (aplnra and/or aplnrb) (PubMed:17336905). Drives internalization of the apelin receptor (By similarity). Hormone involved in the regulation of cardiac precursor cell movements during gastrulation and heart morphogenesis (PubMed:17336905). Plays a role in early coronary blood vessels formation. Mediates myocardial contractility in an ERK1/2-dependent manner. May also have a role in the central control of body fluid homeostasis (By similarity). Expressed at the end of gastrulation (at protein level) (PubMed:24407481). Expressed in the embryo from 7 to 48 hours post-fertilization (hpf) (PubMed:17336905). Expressed in the axial mesoderm and its later derivative, the notochord at 11.5 hpf (PubMed:17336905). Expressed in the forming head and heart and in the posterior notochord at 18 hpf (PubMed:17336905). Morpholino knockdown of the protein show impaired gastrulation and heart formation (PubMed:17336905). Embryos display reduced migration of anterior lateral plate mesoderm (LPM) cells and cardiac progenitor cells (angioblasts) toward the midline during late gastrulation (PubMed:17336905, PubMed:26017639). Embryos show altered cardiac gene expression (PubMed:17336905). Embryos with CRISPR-induced apelin and apela null mutations show a strong decrease in the angioblast migration to the embryonic midline during late gastrulation (PubMed:26017639). Belongs to the apelin family.