Originally titled "The Deep", Freeform announced the series title change to "Siren" in April 2017.
Mermaids and Sirens are both mythical, beautiful, female creatures; however, until the 7th century, they differed in appearance. Mermaids are half-woman and half fish while Sirens, before the 7th century, were half-woman and half-bird much like the Harpies. After the 7th century, Sirens were women from their heads to their navels and fish from below their waists; in other words they were mermaids. The show uses the post-7th century interpretation of Sirens although mermaid is said on the show.
Eline Powell performed the theme song ("Call of the Siren") during San Diego Comic Con 2018, which led to many fans thinking she really sang it in the series. The theme and all mermaid sound effects are actually performed by Mariana Berreto whose voice is also electronically altered to double for Donna's voice.
Eline Powell incorporates the traits of both sea and land mammals in her role as Ryn. She is also inspired by singer Björk.