Derceto was not a Greek goddess but a Syrian deity who entered the Greek world through the conquests of Alexander and the cultural exchange of the Hellenistic age. The Greeks called her Derceto, a corruption of the Aramaic name Atargatis. She was the consort of Hadad, the storm god whom the Greeks identified with Zeus, and together they ruled the heavens and the earth. Her primary sanctuary stood at Hierapolis Bambyce in northern Syria, a vast temple complex with sacred lakes, golden statues, and towering pillars. She was depicted enthroned between lions, holding a scepter in one hand and a spindle in the other, wearing a mural crown and rays upon her head, the very image of sovereign power.

But Derceto had another form, one that made her famous across the Greek world. At Ashkelon on the coast of Philistia, she was worshipped as a mermaid, a woman with the head and arms of a human and the body of a fish. The historian Diodorus Siculus, writing his Bibliotheca Historica in the 1st century BCE, tells the story of how this came to be. Derceto fell in love with a beautiful youth named Simios. She bore a daughter from this union, but overcome with shame at her illicit passion, she killed her lover, exposed the infant in the wilderness, and threw herself into a lake near Ashkelon. The goddess Aphrodite, whom the Greeks identified with Derceto, transformed her into a fish, though her head remained human.

The child that Derceto abandoned did not die. A flock of doves found her in the wilderness, sheltered her with their wings, and fed her milk from their beaks. The royal herdsmen of the Assyrian king discovered the child and her unusual nurses, and one of them, a man named Simmas who had no children of his own, adopted her. He named her Semiramis, after the Syrian word for doves, which thereafter became sacred birds. Semiramis grew into a woman of extraordinary beauty and intellect. She married Onnes, the governor of Syria, and accompanied him on military campaigns where her strategic brilliance earned the admiration of King Ninus. After Onnes took his own life amid royal intrigue, Semiramis married Ninus and became queen of Assyria, ruling with unmatched prowess and founding great cities.

Lucian of Samosata, who visited Hierapolis in the 2nd century CE, described the cult of Derceto. He saw her image in Phoenicia as half-woman and half-fish, but at Hierapolis she was depicted entirely as a woman, enthroned and majestic. Lucian noted that the people of Hierapolis abstained from eating fish, honoring the sacred animal that had saved their goddess. He also described a ceremony in which water was poured into a chasm in the temple, because tradition held that the waters of the Great Flood had drained away through that opening. The temple was said to have been founded by Semiramis herself, dedicated to her mother Derceto rather than to Hera.

The image of Derceto as a fish-tailed goddess has echoed through millennia. In Ovid's Metamorphoses, she appears as the Babylonian Dercetis who transformed into a fish. In medieval bestiaries, her story became part of mermaid lore, the cautionary tale of a goddess who loved a mortal. Modern scholars have traced her origins back to the Bronze Age Canaanite goddess Asherah, the Lady of the Sea, and have found her name on cuneiform tablets from Ugarit. She is the ancestor of every mermaid story ever told, the fish-tailed goddess who guards the boundary between the human world and the depths, the mother of Semiramis and the divine protector of Ashkelon.