Receptor for apelin receptor early endogenous ligand (apela) and apelin (apln) hormones coupled to G proteins that inhibit adenylate cyclase activity (PubMed:17336905, 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:17336905, 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). 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 agtrl1a 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. Mesendodermal expression at the blastoderm margin appears by 4.5 hpf. At early gastrulation, expression is maintained ventrolaterally while expression in dorsal cells and random deep cells declines. During gastrulation and segmentation, expression is maintained in adaxial, intermediate, and lateral plate mesoderm. During late segmentation, expressed in several regions including the forming heart. By 24 hpf, expressed in the dorsal aorta, caudal vein, and intersomitic blood vessels. 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.