Blue Fleet: Beyond the Bluebottle

Introducing the bluebottle and the interconnected relationships and similarities it has with the other members of Australia’s Blue Fleet: blue dragons, the by-the-wind sailor, the violet sea snail, and the blue button.

Members of this specialised ocean community are all perpetual ocean castaways. They are all part of the pleuston food web - organisms living in the thin surface layer existing at the air-water interface of the ocean.

Blue dragons use their colouration for camouflage. They are a perfect example of ‘countershading’ used also by many other creatures including penguins and sharks, although the blue dragons countershade in reverse.

By floating upside down, its brightly coloured underbelly camouflages it against the ocean’s blue, concealing it from predators above. Its silvery back faces downwards to blend with the water’s bright surface protecting it from predators below.

Blue dragons are not naturally venomous. They incorporate toxic chemicals or stinging cells from their prey into their own skin in concentrated doses creating a potent defence mechanism against predators.

Blue dragons are hermaphroditic — each individual produces both eggs and sperm. An individual cannot fertilize its own eggs, however, so pairs must still mate.

Like blue dragons, this species is located in the tropical and subtropical Atlantic, Indian and Pacific oceans. Growing up to 4 cm, this ocean-roaming creature is one of the biggest organisms living in the pleustonic thin but expansive habitat.

The violet sea snail spends its whole life, often in large groups, drifting on the ocean surface at the mercy of the winds. Large groups are also found washed up on beaches all around Australia’s coastline.

The violet sea snail feeds on pelagic hydrozoans, especially bluebottles and by-the-wind-sailors. It has no means of propulsion, so must drift until it finds its prey.

It drifts by producing its own raft made from mucus it excretes which binds to a string of air bubbles it also produces. This all hardens to form its floating raft. If the raft ever breaks the violet sea snail loses its bubble float, sinks and dies.

Meet the by-the-wind sailor, another member of this specialised Blue Fleet ocean surface community. Often found in vast swarms of electric-blue sails in warm to warm-temperate waters throughout the world’s ocean.

Like others in this community, this ocean sailor moves across the surface of the water at the mercy of the winds and tides and is often found washed up in the thousands on beaches across the world after strong winds.

Once thought to be a colony of specialised polyps like the bluebottle, it is now thought to be one single highly modified hydroid polyp. Its body comprises a flat oval transparent float 6-7 cm in diameter containing a series of concentric air-filled chambers that provide buoyancy.

It also has an erect triangular sail projecting vertically at an angle to the axis of its body. Similar to bluebottles, some have a right-sided sail, while others have a left-sided sail — which means each type travels and comes to shore on different winds.

Velella feed on zooplankton, small fish and crustaceans immediately below the water surface. They catch their prey by means of short tentacles with stinging cells that are effective against their prey but generally harmless to humans.

They also have symbiotic algae in their tissues, allowing them to photosynthesise, providing them an additional source of nutrition. As with the bluebottle, by-the-wind sailors are preyed on by blue dragons and violet sea snails.

Just like other organisms in the pleuston food web, the blue button is a passive drifter, relying on currents and wind to transport it throughout the ocean. Often found in large aggregations, it is not uncommon for them to be washed up on the beach in large numbers.

Growing up to 3 cm in diameter, the blue button is found in tropical and subtropical Australian, Indo-Pacific, and Atlantic waters. Although it resembles a jellyfish, it is infact a colony of hydroidoan polyps that each function differently: some eat, some reproduce, some defend.

It also contains pores that are able to communicate with other blue buttons as well as its surroundings. The blue button has a single mouth located beneath the float, which is used for both the intake of prey, as a stomach and the expulsion of waste.

The hydroid colony resembles tentacles like those of the jellyfish. Each hydroid branches out in multiple strands that end in knobs of nematocysts containing venom to help capture its prey. Unlike the in-the-wind sailor which prefers a passive diet, the blue button will actively hunt its prey - feasting on small fish, eggs, crustacean larvae and other zooplankton.