Endogenous ligand for the apelin receptor (APLNR) (PubMed:28137936). Hormone required for mesendodermal differentiation, blood vessels formation and heart morphogenesis during early development and for adult cardiovascular homeostasis (PubMed:26611206, PubMed:28137936). Drives internalization of APLNR. Acts as a motogen by promoting mesendodermal cell migration during gastrulation by binding and activating APLNR. Acts as an early embryonic regulator of cellular movement with a role in migration and development of cardiac progenitor cells. May act as a chemoattractant for the activation of angioblast migration toward the embryonic midline, i.e. the position of the future vessel formation, during vasculogenesis. Positively regulates sinus venosus (SV)-derived endothelial cells migration into the developing heart to promote coronary blood vessel sprouting. Plays a role in placental vascular development; promotes placental trophoblast invasion and spiral artery remodeling in the uterus (By similarity). Involved in the regulation of maternal cardiovascular homeostasis to prevent gestational hypertension and for potent cardioprotective functions during heart failure (PubMed:26611206). Mediates myocardial contractility in an ERK1/2-dependent manner (PubMed:26611206). Interacts with APLNR. Found in blood plasma. Found in serum of pregnant mice, peaking at midgestation; indicating a maternal and zygotic origin of circulating APELA during pregnancy. Belongs to the Elabela/Toddler family.