Receptor for apelin receptor early endogenous ligand (apela) and apelin (apln) hormones coupled to G proteins that inhibit adenylate cyclase activity (PubMed:17336906, PubMed:24316148, PubMed:24407481). Plays a key role in early development such as gastrulation, blood vessels formation and heart morphogenesis by acting as a receptor for apela hormone, promoting endoderm and mesendoderm cell migration and regulating the migration of cells fated to become myocardial progenitors, respectively (PubMed:17336906, PubMed:24316148, PubMed:24407481, PubMed:26017639). Positively regulates angioblast migration toward the embryonic midline, i.e. the position of the future vessel formation, during vasculogenesis (PubMed:26017639). May promote sinus venosus (SV)-derived endothelial cells migration into the developing heart to promote coronary blood vessel development (By similarity). Required for cardiovascular development, particularly for intersomitic vein angiogenesis by acting as a receptor for apln hormone (By similarity). Also plays a role in various processes in adults such as regulation of blood vessel formation, blood pressure, heart contractility, and heart failure (By similarity). Acts redundantly with agtrl1b in heart development (PubMed:17336906). Internalized to the cytoplasm after exposure to apelin (apln). After exposure to apelin receptor early endogenous ligand (apela), internalized from the cell surface into an endosomal recycling compartment, from where it is recycled to the cell membrane. First expressed before epiboly in dorsal precursors. During epiboly, expressed in the enveloping layer, yolk syncytial layer and migrating mesendoderm. During segmentation stages, expressed in epithelial structures such as adaxial cells, border cells of the newly formed somites, developing lens, otic vesicles and venous vasculature. Morpholino knockdown of the protein induces embryonic lethality due to cardiac dysplasia with little to no blood circulation around 6 days post-fertilization (dpf) (PubMed:24316148). Mutant embryos show pericardial edema and accumulation of erythrocytes in the intermediate cell mass (ICM) at 30 hours post-fertilization (hpf) (PubMed:24316148). Display decreased angioblast migration to the embryonic midline during late gastrulation (PubMed:24316148, PubMed:24407481, PubMed:26017639). Belongs to the G-protein coupled receptor 1 family.