In Greek mythology, Nereus (/, /; Greek: Νηρεύς) was the eldest son of Pontus (the Sea) and Gaia (the Earth), who with Doris fathered the Nereids and Nerites, with whom Nereus lived in the Aegean Sea.
In the Iliad the Old Man of the Sea is the father of Nereids, though Nereus is not directly named. He was never more manifestly the Old Man of the Sea than when he was described, like Proteus, as a shapeshifter with the power of prophecy, who would aid heroes such as Heracles who managed to catch him even as he changed shapes. Nereus and Proteus (the "first") seem to be two manifestations of the god of the sea who was supplanted by Poseidon when Zeus overthrew Cronus.
During the course of the 5th century BC, Nereus was gradually replaced by Triton, who does not appear in Homer, in the imagery of the struggle between Heracles and the sea-god who had to be restrained in order to deliver his information that was employed by the vase-painters, independent of any literary testimony.
In a late appearance, according to a fragmentary papyrus, Alexander the Great paused at the Syrian seashore before the climacteric battle of Issus (333 BC), and resorted to prayers, "calling on Thetis, Nereus and the Nereids, nymphs of the sea, and invoking Poseidon the sea-god, for whom he ordered a four-horse chariot to be cast into the waves."
Nereus was known for his truthfulness and virtue:
But Pontos, the great sea, was father of truthful Nereus who tells no lies, eldest of his sons. They call him the Old Gentleman because he is trustworthy, and gentle, and never forgetful of what is right, but the thoughts of his mind are mild and righteous.
The Attic vase-painters showed the draped torso of Nereus issuing from a long coiling scaly fishlike tail. Bearded Nereus generally wields a staff of authority. He was also shown in scenes depicting the flight of the Nereides as Peleus wrestled their sister Thetis.
In Aelian's natural history, written in the early third century CE, Nereus was also the father of a watery consort of Aphrodite named Nerites who was transformed into "a shellfish with a spiral shell, small in size but of surpassing beauty."
Also, the deepest-diving underwater ROV, which recently set a record for exploring the Challenger Deep of the Mariana Trench, is named after this figure. It was reported lost May 9, 2014, during a six-mile dive to the Kermadec Trench. 
In The Secrets of the Immortal Nicholas Flamel, Nereus appears as an Elder in Book 3 of the series, The Sorceress. In it he confronts and battles Perenelle Flamel in an effort to capture and subdue her, thereby making her his wife. He is described as being a man holding a trident from above the waist and below being an octopus. Perenelle ends up defeating him and his daughters, the Nereids, who appear as sharp toothed monsters.
In Percy Jackson and the Titan's Curse, Nereus appears as a homeless old man in some port in San Francisco who is described by Percy as 'Santa's evil twin'. He has that smelly ocean stench that differs him from the other homeless. He changes his shape as Percy clings on him in order to get answers. Despite of being a sea-related Ancient Greek creature, he apparently doesn't know the differences between a demigod of Poseidon and other gods.
He later appears briefly in The Son of Neptune, in which he recognizes Jackson in San Francisco, four years after The Titan's Curse.
In Disney's Hercules cartoon series, Nereus (voiced by Jim Cummings) makes occasional appearances. As in the myth, he is a grouchy old man who can transform into various monsters and creatures, and Hercules at one point has to fight him in some of his forms to gain information.
In the T.V. series Stargate SG-1, a minor Goa'uld was named "Nerus". He was a scientist and inventor of technologies. He was the one who while working for Ba'al (Leader of the Goa'uld), made the stargates all dial up simultaneously during the battle with the Replicators. Nereus later tricked General Landry into aiding the Ori by claiming that they should throw all their firepower at an Ori beachhead in the Milky Way.
In EVE Online, Nereus is an industrial transport ship, known to be fast and reliable. Referring to the mythological shape-shifting ability of Nereus, the ship provides a high level of customization, allowing the pilot to adjust the ship to meet his or her needs.
- Hesiod, Theogony 233-36, is unequivocal that Nereus is the Old Man of the Sea (ἅλιος γέρων), whereas the Odyssey refers the sobriquet to Nereus (xxiv.58) to Proteus (iv.365, 387), and to Phorkys (xiii.96, 345).
- R. S. P. Beekes, Etymological Dictionary of Greek, Brill, 2009, p. 1017.
- Iliad i.358, 538, 556; xviii.141; xx.107; xxiv.562.
- Or, as Proteus, Menelaus.
- On Argonautica iv.1396f, noted by Ruth Glynn, "Herakles, Nereus and Triton: A Study of Iconography in Sixth Century Athens" American Journal of Archaeology 85.2 (April 1981, pp. 121–132) p 121f.
- Glynn 1981:121–132.
- Papyrus Oxyrrhincus FGH 148, 44, col. 2; quoted by Robin Lane Fox, Alexander the Great (1973) 1986:168 and note. Thetis was the mother of Alexander's hero Achilles.
- Hesiod, Theogony 233
- Theoi.com; Glynn 1981.
- Aelian, On Animals 14.28
- "The Abyss: Deepest Part of the Oceans No Longer Hidden"
- "Remote-Controlled Research Sub Lost in Pacific"
- Favre, Liliana (2003). El Lenguaje NEREUS. Reporte interno. Grupo de Tecnologías de Software, INTIA. UNCPBA, Buenos Aires, Argentina.
- Nereus - EVElopedia
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