Imagine an existence where every moment of your life is spent floating at the ocean surface; for some people this may sound like bliss, for others not so much. Yet for some marine creatures this is reality. The floating community of ocean organisms called pleuston inhabit the air-sea interface, a habitat covering 71% of the Earth’s surface.
The highly specialised members of this unique group have adopted a lifestyle where their entire adult life is spent floating at the ocean surface, sailing and drifting in winds and currents and feeding and breeding as they wander the World’s oceans. Among them are species of sea snails, sea slugs and sea jellies that live only at this boundary between air and sea. The more familiar pleuston may be encountered bobbing alongside our kayaks, when they are pushed into coastal waters, or more often when they end up stranded on beaches, occasionally in great numbers as a result of onshore winds. Most conspicuous and common are Portuguese man of war and the smaller By-the-wind sailor and both rouse a lot of interest from beach goers when stranded. Grouped with in the Hydrozoa of the larger Cnidaria group that includes all sea anemones, jellyfish, corals and the like, here we’ll take a closer look at these two brightly coloured floaters.
Portuguese man of war (PMOW), Physalia physalis is both striking and peculiar in appearance with a cornish pasty-shaped float and deep purple coloured stringy tentacles. It is aptly named after a warship at full sail: traveling by wind for thousands of miles, dragging behind long tentacles that deliver a deadly venomous sting to its prey and foe. Though it looks like a single animal it is actually an intimate collection of four different types of animals called zooids that are specialised for separate functions and dependent upon each other for survival. Like a well-oiled team each works together for the collective good of the colony. The float is one type of zooid that buoys up all the others; it is filled with gas (carbon monoxide) and contracts or inflates to adjust the surface area and buoyancy allowing a certain amount of control over where it goes. This has been likened to the trimming and reefing of a sail and may allow enough of a diversion to evade predators. The float is curved in either a right or left handed direction which is determined at a young age and also affects its direction of travel. Trailing beneath the float like fine streamers are the tentacles, a collection of zooids specialised for prey capture that may grow up to 30 metres long. Embedded in the surface of each tentacle are hundreds of stinging cells called nematocysts that characterise the Cnidaria group to which they belong. As the POMW sails along the tentacles bump into an unwitting, plankton or small fish in the water which triggers the stinging cells; barbs loaded with a potent neurotoxin are fired into the soft tissue of the victim paralysing them. The painful sting and a life cycle with seasonal blooms, results in periodic mass beach strandings making the PMOW one of the most infamous floating stingers. Though lesser creatures may suffer at their fearsome sting, some predators are undeterred. Both leatherback and loggerhead turtles are too tough to feel the sting and happily chomp on them, while two species of pleustonic sea slugs Glaucus atlanticus and Glaucus marginatus go a step further. Not only are they immune to the sting and able to feast on the PMOW but they are able to store the intact nematocysts ready to redeploy them for their own defenses.
Following the successful bump and envenomation by the stinging cells, the PMOW passes the fishy prey to the third group of zooids responsible for digestion. Located near the base of the float, the zooids either secretes digestive enzymes on to the prey or gobble small fragments into their tiny mouths. More than 50 digestive zooids have been observed to completely cover the surface of a 10 cm fish with their mouths, dissolving the flesh and slurping it up. The resulting soup is released into the main gastric cavity to share with the whole colony for their respective chores.
The fourth members of the colony have the sole purpose of reproduction releasing either eggs or sperm to the water depending on whether the colony is male or female (colonies have separate sexes). Once water-borne, sperm fertilise the eggs and swimming larvae develop from the eggs, spending an unknown amount of time below water, until they grow a sufficiently buoyant float to maintain a life at the surface. Here they join other juveniles and adults in a life of drifting and sailing the high seas and feasting on fish soup.
By-the-wind-sailor, Velella velella is smaller in comparison to the PMOW but no less striking. It has an oval disc with concentric rings that form chambers to the float and a triangular sail atop and purple tentacles trailing beneath. Like the PMOW the Velella is not one animal but a team of zooids with specific roles in buoyancy, prey capture, feeding and reproduction. The float is not a gas-filled bag but a series of concentric chambers flattened into a disc and topped with a small rigid sail. The Velella sails align along the direction of the wind acting as an aerofoil so that the animals tend to sail downwind at a small angle to the wind. In this way they are at the mercy of the wind and often end up stranded on beaches in their thousands. Reassuringly their sting is no concern for us, though effective at securing a meal for themselves. When all goes well they travel thousands of miles across the world's oceans dragging the stinging tentacles to capture their preferred diet of fish eggs and fish larvae. Once captured the prey is passed along to the digestion team for processing and shared among members. When nutrition needs a top-up or prey is scarce, they are also able to supplement their diet with food from the sun. Like reef-building corals and some sea anemones, Velella harbours single-celled algae in its tissues called zooxanthellae that are used as tiny solar panels for energy; not a surprising addition to a lifestyle spent bathed in bright sunlight at the sea surface. As with the PMOW procreation is left to a specialised few in the Velella club and involves release of eggs and sperm to the water, fertilisation of the egg and development of a swimming larva into the adult form.
Even in the life of a nomadic, ocean drifter, teamwork and compassion for ones comrade zooids prevails, we have much to learn from our fellow floaters ...