The small creatures have been identified as Velella velella, not strictly a jellyfish but floating colonies of microscopic hydrozoansthat typically live far offshore in open ocean waters, where their little ‘sails’ help distribute them using the force of the wind.
Hundreds have washed up on the Costa Blanca coast between Altea and Denia. Sightings have been reported on beaches including L'Olla in Altea, the sandy stretch of Arenal-Bol in Calpe and Cap de la Nau in Javea.
The species, known in English as ‘By-the-Wind Sailor’, are fairly common in open waters but are rarely seen washed up in Mediterranean waters. Such mass strandings are common on the West coast of the USA.
Recent high southerly winds and strong sea currents are blamed for washing carpets of them on shore at beaches along Spain's eastern Mediterranean coast.
Although the species is related to the deadly Portuguese Man O’War, Velella are completely harmless to humans.
During the last week of April, some beaches in the region were closed to swimmers after Portuguese Man O'War were spotted close to shore, but there were no reported stingings.
Here's one that washed up in Altea:
Cierran temporalmente la playa de Altea (Alicante) debido a la presencia de medusas azules (Carabela portuguesa) horas después de que Cruz Roja de Águilas hallara en el litoral murciano varios ejemplares. pic.twitter.com/oblcFUWiSL
— EFE Murcia (@EFE_Murcia) April 25, 2018