London (CNN) A same-sex penguin couple will raise an adopted "genderless" chick for the first time, a London aquarium announced Tuesday.

The four-month-old Gentoo penguin chick will not be named or characterized as male or female , Sea Life London said.

The chick is being raised by two female penguins Rocky and Marama, who were given the egg to relieve the penguin's birth mother of the pressure of raising two chicks.

It will be "the world's first penguin to not have its gender assigned," Sea Life London said.

Experts at the aquarium usually give newborn penguins gender-based names, but in this case "decided it was more natural for the chick to grow and develop into a mature adult as genderless which is normal in the wild until they mature," the centre said in a press release. The chick has been fitted with a purple tag on its wing so it can be identified.

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