Endogenous ligand for the apelin receptor (aplnra and/or aplnrb) (PubMed:24316148, PubMed:24407481). Hormone required for mesendodermal differentiation, blood vessels formation and heart morphogenesis during early development and for adult cardiovascular homeostasis (PubMed:24316148, PubMed:24407481, PubMed:26017639, PubMed:29117894). Acts as a motogen by promoting mesendodermal cell migration during gastrulation by binding and activating the apelin receptor (PubMed:24316148, PubMed:24407481, PubMed:29117894). Acts as an early embryonic regulator of cellular movement with a role in migration and development of cardiac progenitor cells (PubMed:24316148, PubMed:24407481). 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 (PubMed:26017639). Positively regulates sinus venosus (SV)-derived endothelial cells migration into the developing heart to promote coronary blood vessel sprouting. Involved in cardioprotective functions during heart failure. Mediates myocardial contractility in an ERK1/2-dependent manner (By similarity). Interacts with aplnra and aplnrb (PubMed:24316148). Expressed ubiquitously during late blastula and gastrula stages and becomes restricted to the lateral mesoderm, endoderm, and anterior and posterior notochord after gastrulation (PubMed:24407481). Expressed from the mid-blastula transition (MBT) to 3 days post-fertilization (dpf) (PubMed:24316148). Ubiquitously expressed in dividing cells of the blastoderm before becoming restricted after gastrulation to axial structures, with most prominent expression in the neural tube (PubMed:24316148). Zinc-finger-nuclease (ZFN)- or TALEN-mediated apela gene inactivation induces embryonic lethality due to heart dysgenesis around 6 days post-fertilization (dpf) (PubMed:24316148, PubMed:24407481). Mutant embryos show cardiac dysplasia ranging from a rudimentary heart to no heart (PubMed:24316148, PubMed:24407481). Embryos have minimal blood circulation, with an excess of erythrocytes accumulating at the intermediate cell mass (ICM) (PubMed:24316148, PubMed:24407481). Larvae display variable posterior truncations and, at times, tailbud duplications (PubMed:24316148). Posterior tissue defects range from loss of the ventral fin to complete tail and trunk truncations (PubMed:24316148). Embryos also display malformed pharyngeal endoderm, abnormal left-right positioning and formation of the liver (PubMed:24407481). By mid-gastrulation, embryos show reduced endodermal cell numbers and endodermal and mesodermal progenitor cell migration toward the animal pole (PubMed:24316148, PubMed:24407481). CRISPR-induced apela null mutations show also reduced endodermal cell numbers, increased rates of endodermal cells death and reduced endodermal and mesodermal cell migration at mid-gastrulation (PubMed:29117894). Show a strong decrease in the angioblast migration to the embryonic midline during late gastrulation (PubMed:26017639). Was named Toddler based on the phenotype. Belongs to the Elabela/Toddler family.