In the summer months, north-easterly winds frequently herald the arrival of bluebottles on beaches along Australia’s east coast. But while bluebottles – or to give them their more formal name, the Pacific man-of-war – are a common sight on Australian shores, they are not native to coastal waters. Instead, they spend most of their lives on the open ocean, drifting with the winds and the currents.
Bluebottles are just one of a collection of organisms that have made their home at the ocean’s surface. Some of these animals are hydrozoans like the bluebottle.
There is the by-the-wind sailor, Velella velella, which has a stiff, transparent, oval sail about five centimetres long attached to its bright blue float, and Porpita porpita, sometimes known as the blue button, which is shaped like a disc about three centimetres in diameter surrounded by stinging polyps. But there is also the strikingly beautiful sea dragon; crustaceans such as shrimp, buoy barnacles and tiny swimming copepods; and even molluscs such as the violet snail and Recluzia.
Known collectively as the neuston, these creatures are not tied to any one place. Instead, they move with the wind and the water. Sometimes they gather into huge drifts, living islands of velella and bluebottles like those that wash occasionally ashore on beaches in Australia or the western coast of the Canada and the United States. At other times they clump together around drifting debris or spread out sparsely over hundreds or even thousands of square kilometres.
Despite its ubiquity, the neuston remains comparatively poorly understood and critically understudied. A mere handful of papers concerning the ecosystem are published each year, and only three of the 400 proposals received for the International Zooplankton Production Symposium earlier this year concerned the neuston.
Marine ecologist Associate Prof Kerrie Swadling,from the University of Tasmania, puts it bluntly. “We know more about deep sea vents than we know about the neuston.”
The reasons for this ignorance are partly historical. Although several important studies of the neuston were published during the 20th century, they were written in Russian by scientists from the Soviet Union and were largely ignored outside the Eastern Bloc. But for the most part, the lack of research into the neuston is a consequence of the practical challenges involved in observing organisms that are scattered unevenly across the immensity of the open ocean.
Griffith University’s Prof Kylie Pitt specialises in jellyfish ecology. She says, “The neuston’s transient nature makes it difficult to study. You’ll see large numbers of jellyfish or bluebottles and then you won’t be able to find them again.”
In recent years, however, there has been an uptick in interest in the neuston. New research is revealing not just its importance to the health of ocean ecosystems as disparate as coral reefs and the deep ocean, but also important gaps in our understanding of how it will be affected by changes in the ocean environment.
The person most responsible for the increased visibility of the neuston is Dr Rebecca Helm. Now an assistant professor at Georgetown University in the United States, Helm was scrolling Twitter in 2018 when she came across a tweet about The Ocean Cleanup’s plans to remove plastic from the oceans by sweeping a floating net across the surface.
Helm says she immediately wondered about the potential impact of this technology on the neuston, and so began to investigate.
“Initially I was just doing a little digging in my free time. But once I did, I realised how little information there was available and how little had actually been done on this group of animals.”
Helm might have left it at that if the pandemic hadn’t meant she was locked out of her lab for several months. “I suddenly had all of this nebulous time to start looking into this more deeply, and became really fascinated.”
‘An inverted sea floor’
Helm’s response is easy to understand. The ocean surface is an extremely challenging environment: food is often scarce and survival requires an ability to withstand not just waves and storms, but also the heat of the sun and high levels of ultraviolet radiation. This last part may help explain why so many neuston species are blue: as well as acting as camouflage, the colour acts as an inbuilt sunscreen that reflects UV radiation.
However, survival in the neuston also requires animals to find some way to remain at the surface. For free-swimming species such as copepods and zooplankton, this is easy. But for other organisms it requires special adaptations.
Hydrozoans like the bluebottle and velella employ gas-filled floats, while the buoy barnacle extrudes air into the cement that it would otherwise use to attach itself to ships and rocks, creating a substance a bit like pumice that it uses as a float. Similarly, violet snails suspend themselves beneath rafts constructed out of hardened bubbles of mucus. There is even a form of free-floating sea anemone that hangs upside down from the surface with the aid of a float in their pedal disc.
Fascinatingly, this need for a float helps explain one of the more surprising discoveries to have come out of Helm’s research, which is that many of the animals that inhabit the neuston are not particularly closely related to other free-swimming species. Instead, they are descended from species that usually exist attached to the bottom of the sea that have migrated upwards, meaning that the neuston is, in a very real sense, what Helm dubs “an inverted sea floor” clinging to the ocean’s surface.
This unexpected evolutionary link between the ocean’s surface and the sea floor echoes the growing awareness of the neuston’s role in connecting ocean ecosystems more generally. Many animals from other parts of the ocean rely upon it for food: numerous species of fish and fish larvae feed in the neuston, as do turtles and oceangoing birds such as fulmars, shearwaters, storm petrels and some albatrosses. The neuston also provides vital nutrition for many of the species that ascend each night from deeper waters to feed as part of the diel migration.
The neuston also plays a critical role in the life cycles of many fish, whose larvae spend time near the surface before migrating to other parts of the ocean as they mature. “The ocean surface is an incredibly important nursery ground for diverse species of fish,” says Helm. “Deep sea viper fish can be found at the surface when they’re very young. Many seahorses and pipefish, mahi mahi and billfish also seek out the ocean surface when they’re young.”
It’s likely many of the fish that spend time at the surface as juveniles do so because it is safer than deeper waters. Some shelter among the stinging tentacles of bluebottles and porpita, while others hide under floating mats of sargassum. Others join the many species that congregate around driftwood and other floating debris in search of food, protection or simply a scratching posts with which to remove parasites.
Plastic and the neuston
But wood and sargassum are not the only kinds of debris in the sea. Although most of the more than 12m tonnes of plastic that ends up in the oceans every year sinks, a considerable amount of that which remains accumulates in the subtropical gyres, huge current systems that circulate in the centre of the Indian Ocean, the North and South Atlantic and the North and South Pacific.
The regions at the centres of the gyres are often called garbage patches, but Helm rejects that label, arguing they are in fact neuston environments that have been invaded by plastic. Nonetheless, samples taken when long-distance swimmer Ben Lecomte swam through the North Pacific garbage patch in 2019 showed plastic and neustonic life clustered together.
This intermingling of plastic and neustonic life has severe impacts on species that feed upon the neuston. Unable to distinguish fragments of plastic from food, fish, turtles and other animals consume it, resulting in malnutrition and passing toxins into the food chain.
The effects of this can be catastrophic: Laysan albatrosses feed almost five tonnes of plastic to their chicks every year, while on Lord Howe Island plastic appears to be connected to rising mortality among shearwater chicks.
The effect of plastics upon the neuston itself seem to be more complex, however. While animals such as fish and buoy barnacles are likely to suffer adverse effects from ingesting plastic, larger pieces of floating plastic have the potential to provide shelter to some fish and fish larvae and appear to benefit sea skaters and other species that require objects on which to lay their eggs.
The effects of technologies designed to remove plastics from the ocean on the neuston also remain unclear. Partly as a result of Helm’s advocacy, Ocean Cleanup have adjusted their technology to minimise its impact on neustonic life.
But Helm is unconvinced. “I think it’s difficult to assess whether this technology is harming neuston. We don’t understand these animals … So while they may have made efforts that are perhaps trending in the right direction, I’m sceptical that can be stated with any confidence.”
Others are less concerned, believing the neuston’s dispersed distribution is likely to protect it against significant harm. Although she says her views may change if the operation scales up in the future, Swadling points to the fact Ocean Cleanup’s operation has only cleared a tiny fraction of the North Pacific gyre and says “the effect to date will be negligible.”
Nor is plastic the only area where our understanding of the human impact on the neuston remains worryingly incomplete. Oil and chemical spills have the potential to adversely affect neustonic life, as do rising air and ocean temperatures. Yet in a reminder of how little we know about the neuston, Swadling says that not only is she unaware of a single experiment gauging the thermal tolerance of neustonic organisms, our knowledge of the ecosystem is so incomplete that we don’t even possess a useful baseline from which to measure change.
To overcome these gaps in our knowledge, scientists are increasingly utilising the power of citizen science. Helm helped establish Go Sea, a Nasa-funded community that allows both scientists and the public to report sightings of surface life, and in collaboration with SeaKeepers has been helping train yachters to take samples of the neuston. Meanwhile the University of NSW is developing Bluebottle Watch, a bluebottle forecasting system that will use public sightings, ocean surveys, laboratory experiments and computer modelling to track and anticipate bluebottle swarms.
Nonetheless, there is no question that this crucial ecosystem deserves more attention. “People think of the open ocean as an empty environment, but it’s absolutely not,” says Pitt. “Just because we can’t see what’s going on doesn’t mean it’s not important.”
