Cnidaria: Hydrozoans
By-the-wind Sailors (Velella velella)
Velella velella is a pelagic colonial hydroid, similar to the Portuguese Man O'War, Physalia physalis, and not a true jellyfish. Each colony has several different kinds of polyps, some of which are both feeding and reproductive (gonozooids) and others which are protective (dactylozooids). Each colony is either male or female, and capable of reproducing asexually by budding.
The main "body" of the colony is formed by the float, which is an oval disc, is deep blue in colour and can be up to 10cm in length. It is filled with air chambers that provide buoyancy to the organism. Short tentacles hang down into the water from the float. A thin semi-circular fin is set diagonally along the float acting as a sail. The direction of the sail along the float determines their direction of travel. If the sail runs north-west to south-east along the float it will drift left of the wind direction, if the sail sits in the opposite direction, it will drift to the right.
By-the-wind sailors feed on small plankton floating on or near the ocean’s surface. As they float, their trailing tentacles, which are covered in stinging cells, stun and trap prey before moving it toward the digestive opening at the centre of their bodies. The polyps are connected by an internal digestive canal which enables the colony to share the food ingested by individual polyps. Within the colony there are symbiotic single-celled algae, zooxanthellae, with provide sugars to the colony through photosynthesis. The presence of the zooxanthellae is revealed by the golden-brown colour which can be observed within the body.
Drifting on ocean currents, they are vulnerableto storms and mass strandings are not uncommon. Each year, mainly during the summer months, particularly in August, they are found stranded on the beaches on the west coast of the Outer Hebrides. The numbers are difficult to assess, but from time to time, very large number are observed. The animals quickly decompose, leaving just the translucent sails, which resemble plastic disks caught in the seaweed on the strandline.
Experienced beachcombers and nature watchers will also be looking for Janthina janthina, the violet sea snail, which preys on the by the wind sailors and can also be found washed-up on the beach during a stranding.