By-the-wind sailors, also known as Velella velella, have been washing up on UK shores in significant numbers this summer. These marine hydrozoans, which are collections of polyps rather than single animals, are characterised by their bright blue disc and a translucent "sail" that catches the wind.
While typically arriving in numbers during stormy winter weather, their increased presence further inshore this summer may be due to calmer waters and favourable winds. Mass strandings have been reported along the Welsh coast and at St Bees in Cumbria.
The distinctive blue colour of the by-the-wind sailor comes from the carotenoid pigment astaxanthin, which provides camouflage and helps absorb harmful ultraviolet rays. Although largely harmless to humans, touching them can cause irritation.