By-the-wind Sailor (Velella velella)

Description: By-the-wind sailor, purple sailor, sail jellyfish, or Velella velella is a type of hydrozoa usually found in warm to warm-temperate waters. These hydozoa look quite unique; when washed up, they look like a flat blue disc with a clear or white half-disc that sticks up (which looks similar to a sail, hence the name “sail jellyfish”). Underneath their little disc-shaped base are tiny tentacles. By-the-wind sailors can be found washed up along coastlines in huge groups, but each individual is very small, usually only 4-6 centimeters wide and sometimes 8 centimeters wide.

Habitat: On the Pacific Coast of North America they’re found as far north as Washington state, and on the Atlantic Coast of North America they’re usually found as far north as North Carolina. These hydrozoa wash onto shore in huge groups because they are found floating on the surface of the open ocean, and their little sails are at mercy to the winds and waves. 

Diet: Even though their tentacles are small, by-the-wind sailors are still able to use them to catch prey like crustaceans, small fish, and invertebrate eggs floating in the water. They also contain symbiotic algae to help obtain nutrients and minimize damage from over-exposure to the sun.

Reproduction: Like most hydroids, the part we typically see is the polyp stage which will release the microscopic medusa to reproduce sexually. Unlike other hydroids, the polyp stage of the By-the-wind Sailor is not attached to a surface but instead floats on the surface of the water - upside down. This is thought to be an adaptation for life in the open ocean where hard surfaces are either few or very deep.

Tide Pool Tidbits:

• By-the-wind sailor is able to float on the ocean’s surface by filling its blue base with air.

• Their tiny “sails” are not placed horizontally in a straight line across the middle of the base. Instead, they are angled diagonally either to the left or the right. Individuals of one population can be found with either a left-angled sail or a right-angled sail, and although they are in the same group, they will be blown by the wind in different directions due to the different angles of each sail.

• Not many creatures eat by-the-wind sailors, but most of their predators are pelagic snails and nudibranchs. 

• By-the-wind sailors do not have a sting that is harmful to humans, but touching one of these washed-up hydrozoa (live or dead) may cause irritation to hands or eyes if one touches their eyes after handling a by-the-wind sailor.

Reference: Animal Diversity Web, Scripps Institution of Oceanography