There's quite a clever play on words in the aetiology (origin account) of the cockle, shellfish or sea-snail, which, according to Aaron Atsma of The Theoi Project website, was called a nȇritȇs. The Greco-Roman writer Aelian, c. late 100s—early 200s AD, reports that while the epic poets, like Homer and Hesiod, fail to mention a character called Nerites in their works nearly one thousand years earlier, sailors in Aelian's time celebrated such an individual in their folklore. Nerites, they said, was the lastborn child of the sea deities Nereus and Doris, born an only son after his fifty sisters who were called the Nereides [Nereids]. The word-play here would have it that Nerites is equivalent to a masculine form of Nereis [Nereid], "Offspring of Nereus."
The goddess Aphrodite grew up in the sea together with its divine inhabitants. Among these was the young sea-god Nerites, who was more beautiful than any other god or man, and who became her childhood sweetheart. When Aphrodite was elevated to the rank of one of the highest Olympian deities, she invited Nerites to join her on Mt Olympus, but he preferred to live in the sea with his parents and sisters, so he declined the offer. Aphrodite attempted to sweeten the deal by granting to her first love the power to grow wings, which Nerites, in Aelian's words, "counted as nothing" (perhaps because wings cannot be much use underwater).
Enraged at this, Aphrodite plucked off Nerites' wings and transformed him into a shellfish. Now there was another young and beautiful god named Eros1, who was either a son of Aphrodite (said to have been engendered in different ways, in one version parthenogenetically [without the assistance of male seed]) or who ascended Mt Olympus at the same time as this goddess did. Aphrodite made Eros into her companion and attendant in order to replace Nerites and she gave this newer god the wings of which she had just deprived her lover.
Thus the story explains why Eros has wings, in addition to narrating the origin of a certain pretty type of saltwater mollusk, which happens to have a name that sounds like it belongs to the son of the Old Man of the Sea.2
Aelian does not explicitly say it, but my assumption is that Nerites was not transformed into an immortal snail or shellfish but that the animal which he became eventually died after having produced the second generation of these mollusks, from which the cockles, shellfish or sea-snails of the present day are descended.
The Wikipedia article on this Nerites character adds that Nerites's sister Nerea begged their brother-in-law Poseidon (who was married to the Nereid Amphitrite) to restore Nerites to his original form, which Poseidon did. The only ancient source cited by the article is Aelian's book On Animals, which mentions neither this request nor anything like the idea that Nerites was restored to his former glory by Poseidon or anyone else.
Aelian reports a different version of this mariner's tale, which does not feature Aphrodite at all but rather has Nerites becoming the charioteer of Poseidon. Going by this account, the sun-god Helios, for some unexplained reason, changed Nerites into the sea animal which came to bear his name. Aelian suggests that Helios and Poseidon had beef with each other and that the Sun's action was intended as a swipe against the Sea-King. Aaron Atsma speculates that there might have been jealousy on Helios's part, as he, being a charioteer himself, envied Nerites's swift driving of his master's vehicle through the sea, perhaps having been challenged to a race by the young water-deity.
Moreover the Nerites Wikipedia article says that the bit about Nerea's intervention in order to retrieve her brother from his shell-locked fate is an undifferentiated constant in this second version. She approaches Poseidon for assistance and he acquiesces by changing his charioteer back to his old self. Again, the ancient sources do not appear to have anything about this.
In his book The Poetic Achievement of Ezra Pound, Michael Alexander says that the Imagist poet Ezra Weston Loomis Pound invented the character Nerea in the 1st stanza of his unfinished poem The Cantos (first published 1925). I have found no references to Nerites in Pound; the Wikipedia article may be the earliest piece of literature which contains the idea that a sister of Nerites was involved in the misadventure of his metamorphosis.
1. Known more famously by his Roman name Cupid
2. Halios Geron, "Old Man of the Sea," was a nickname of Nereus