Enables monocarboxylate:sodium symporter activity. Involved in lactate transport; nicotinate transport; and pyruvate transport. Predicted to be located in apical plasma membrane. Is expressed in several structures, including immature eye; male organism; nervous system; neurocranial trabecula; and pleuroperitoneal region. Orthologous to human SLC5A8 (solute carrier family 5 member 8).