Pelagia noctiluca may grow to 10 cm in diameter. The medusa colour varies from pale red to mauve-brown or purple and the exumbrella surface is covered in pink or mauve nematocyst-bearing warts. The mushroom shaped, deep bell has 16 marginal lobes, eight marginal sense organs and eight, hair-like marginal tentacles. These thin tentacles may extend as far as 3 m. All tentacles on Pelagia noctiluca are covered in nematocysts. The manubrium bears four thick, frilled oral arms. This species has no sessile stage and the adults release juvenile medusae in the autumn.