Photographic Advent Calendar, Day 21: By-the-wind Sailor

Today I am going to focus on a curious little organism which has a fascinating story to be told: the By-the-wind Sailor (or Velella velella, from velum: a sail). Large wrecks of these disc-shaped creatures have been appearing around much of the UK's western coastline in the last month or so, and we have also been seeing plenty washed up along Bardsey’s shoreline. Living in temperate and warm waters, Velella velella is found in all of the World’s oceans, often occurring in wrecks during inclement weather conditions in the autumn and early spring. These wrecks can involve tens of thousands of individuals, such as occurred in 1992 and 2004.

The By-the-wind Sailor is a member of the Cnidarian clade of invertebrates, which includes familiar species like true Jellyfish and Anemones. They belong to a class called Hydrozoa, which they share with their close relative the Portuguese Man-of-war. Both of these species are known as Neustons, which simply refers to organisms that float either on top of or just beneath the surface of the water. Like the Portugese Man-of-war, the By-the-wind Sailor is not a single organism but is in fact made up of a colony of hydrozoan polyps...

Velella velella leads a fascinating life: like nearly all Cnidarians, this species has a two-part life cycle, composed of a medusa (or jellyfish) stage, followed by its more familiar polyp stage. The medusa stage takes the form of a tiny 1mm jellyfish, which is released from the polyps via an asexual budding process. The second stage of its life cycle involves many individual polyps joining forces to create a colony that forms the curious spinning top-shaped organism that 'sails' the oceans. The tiny individual animals (the polyps) are specalised to perform specific tasks, just like bees in a hive colony! Some polyps form the gas-filled disc that is hardened with chitinous material, enabling the whole colony to stay afloat and range over vast tracts of ocean; others are involved with prey capture, forming the tentacles that dangle from the underside and possess nematocysts for paralyzing small planktonic organisms; others still are involved with reproduction and the digestive system.

The sail that stands erect atop the disc of polyps can either run in a north-east to south-west direction, or a north-west to south-east direction. The orientation of the sails affects whether the sailor will drift left or right of the wind direction, ultimately deciding where it will travel. The blue colour of the outer disc comes from astaxanthin proteins in the mantle tissues, although in many of the individuals encountered around Bardsey these proteins seem to be absent.