Organisms that live freely at the ocean surface, termed neuston, include keystone organisms like the golden seaweed Sargassum that makes up the Sargasso Sea, floating barnacles, marine snails, nudibranchs, and cnidarians. Many ecologically and economically important fish species live as or rely upon neuston. Species at the surface are not distributed uniformly; the ocean's surface provides habitat for unique neustonic communities and ecoregions found at only certain latitudes and only in specific ocean basins. But the surface is also on the front line of climate change and pollution. Life on the ocean's surface connects worlds. From shallow waters to the deep sea, the open ocean to rivers and lakes, numerous terrestrial and marine species depend on the surface ecosystem and the organisms found there. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
The ocean's surface acts like a skin between the atmosphere above and the water below, and hosts an ecosystem unique to this environment. This sun-drenched habitat can be defined as roughly one metre in depth, as nearly half of UV-B is attenuated within this first meter. Organisms here must contend with wave action and unique chemical and physical properties. The surface is utilised by a wide range of species, from various fish and cetaceans, to species that ride on ocean debris (termed rafters).
Most prominently, the surface is home to a unique community of free-living organisms, termed neuston (from the Greek word νέω, which means both to swim and to float). Floating organisms are also sometimes referred to as pleuston, though neuston is more commonly used. Despite the diversity and importance of the ocean's surface in connecting disparate habitats, and the risks it faces, not a lot is known about neustonic life.
Overview Neuston are key ecological links connecting ecosystems as far ranging as coral reefs, islands, the deep sea, and even freshwater habitats. In the North Pacific, 80% of the loggerhead turtle diet consists of neuston prey, and nearly 30% of the Laysan albatross's diet is neuston."Hawaiian Seabird Feeding Ecology", Wildlife Monographs, 85: 3-71. Wiley. Diverse pelagic and reef fish species live at the surface when young, including commercially important fish species like the Atlantic cod, salmon, and billfish. Neuston can be concentrated as living islands that completely obscure the sea surface, or scattered into sparse meadows over thousands of miles. Yet the role of the neuston, and in many cases their mere existence, is often overlooked.
One of the most well-known surface ecoregions is the Sargasso Sea, an ecologically distinct region packed with thick, neustonic brown seaweed in the North Atlantic. Multiple ecologically and commercially important species depend on the Sargasso Sea, but neustonic life exists in every ocean basin and may serve a similar, if unrecognised, role in regions across the planet. For example, over 50 years ago, USSR scientist A. I. Savilov characterised 7 neustonic ecoregions in the Pacific Ocean.Savilov, A.I. (1969) "Pleuston of the Pacific Ocean". In Zenkewich, LA (Ed.) Biology of the Pacific Ocean: Part 2 The deep sea bottom fauna. Each ecoregion possesses a unique combination of biotic and abiotic conditions and hosts a unique community of neustonic organisms. Yet these ecoregions have been largely forgotten.
But there is another reason to study neuston: The ocean's surface is on the front line of human impacts, from climate change to pollution, oil spills to plastic. The ocean's surface is hit hard by anthropogenic change, and the surface ecosystem is likely already dramatically different from even a few hundred years ago. For example, prior to widespread damming, logging, and industrialisation, more wood may have entered the open ocean, while plastic had not yet been invented. And because floating life provides food and shelter for diverse species, changes in the surface habitat will cause changes in other ecosystems and have implications that are not currently fully understand or be able to be predicted.
Ocean surface life (neuston)
Invoking images of the open ocean's surface, the imagination can conjure up an endless empty space. A flat line parting the blue below from the blue above. But in reality a diverse array of species occupy this unique boundary layer. A tangle of terms exist for different organisms occupying different niches of the ocean's surface. The most inclusive term, neuston, is used here to refer to all of them.
Neustonic animals and plants live hanging from the surface of the ocean as if suspended from the roof of a massive cave, and are incapable of controlling their direction of movement. They are considered permanent residents of the surface layer. Many genera are globally distributed. Many organisms have morphological features that enable them to remain at the ocean's surface, with the most noticeable adaptations being floats.
Floaters (pleuston) Floaters, sometimes called pleuston, are the organisms that live floating at the ocean surface. Cnidarians(jellyfish) Velella, Porpita, Physalia, and Actinecta Numerous floating cnidarians (jellyfish) live at the ocean's surface, some famous (or infamous) and others rarely seen. Species like Velella sp. (by-the-wind sailor) and Porpita sp. (blue button) are central to the surface food web. They possess symbiotic dinoflagellates in their tissue, and like their benthic coral cousins, these symbionts may allow them to survive in oligotrophic waters. Velella and Porpita are the only two genera of the chondrophore clade within Hydrozoa, and likely evolved convergently with another neustonic Hydrozoan genera: Physalia (Portuguese man-o-war). Both Physalia and Velella poses "sails", which allow them to travel based on wind direction. These by-the-wind sailors float near the surface of the ocean with their tentacles hanging below in the water. Velella has a raised transparent "sail" on a blue oval disk. Short fringing tentacles hang below from the disc. Movement is powered by wind hitting the sail. Some Velella have a right-hand sail and some a left-hand sail, ensuring they don't all get blown in the one direction at the same time.Browne, J. (2019) Velella velella: By-the-wind Sailor in Museums Victoria Collections, Australia. Accessed 4 December 2021. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License. Physalia also utilises trailing tentacles that serve as a sea anchor on in the open ocean, and pack a powerful sting. Sea anemones in the genus Actinecta are rarely seen, but also float submerged on the ocean's surface, similar to Porpita, but using a bubble float on the pedal disc. Mollusks(marine snails) The mollusks Janthina, Recluzia, and Glaucus Bubble rafting snails Recluzia and Janthina construct floating rafts by dipping their anterior foot into the water's surface and wrapping trapped air in a layer of mucus to form a bubble, which they then adhere to a raft. The enigmatic Recluzia feeds upon the sea anemone Actinecta, and both are brown-yellow in colour. In contrast, the violet snails Janthina prey on Velella, Porpita, and sometimes Physalia , though they cannot move or hunt. Instead, Janthina rely on passive contact with their prey. Other species include the nudibranch Glaucus (blue sea dragon), which also feeds on floating hydrozoans and swallows air to stay afloat. There are multiple cryptic species of Glaucus, and species in this genus may show a high degree of regional isolation. Crustacean
The only truly neustonic barnacle, Dosima fascicularis (Buoy barnacle) lives at the ocean's surface by first attaching to floating objects as larvae (such as feathers), and secreting an airy pillow-like float rather than the normal hard cement used by other barnacles. This float allows Dosima barnacles to eventually outgrow their larval home and drift independently. Macroalgae(seaweed)
Neustonic seaweeds like Sargassum fluitans and Sargassum natans have numerous gas-filled floats to remain at the ocean's surface. These algae create habitat for a variety of Sargassum-associated species, particularly at the western edge of the North Atlantic Subtropical Gyre, known as the Sargasso Sea.Coston-Clements, L., Settle, L.R., Hoss, D.E. and Cross, F.A. (1991) Utilization of the Sargassum habitat by marine invertebrates and vertebrates, a review. NOAA Technical Memorandum, volume 296, NMFS-SEFSC-296. In the Pacific the algal genus Turbinaria reproduces with floating fronts. In addition, over 20 species of algal have been found floating at the surface, and eight species of sea grass. Microorganisms(bacteria, protists etc.)
Diverse microorganisms occupy the ocean's surface ecosystem, and may play a significant role in gas exchange between the ocean and atmosphere. Many of these organisms have been identified from the sea surface microlayer, which, depending on definition, extends from 100 to 1000 μm below the ocean's surface. The ocean's surface has unique chemical and physical properties that may concentrate species specifically adapted to these conditions. For example, bacterioneuston living in the sea surface microlayer are often brightly coloured, possibly as protection against solar radiation. The surface microlayer may be largely dominated by heterotrophic organisms, including both bacteria and microeukaryotes, which take advantage of surface associated compounds. Other species may extend beyond the sea-surface microlayer but still associated with the surface, including the ecologically important cyanobacterium Trichodesmium. Still, as with larger organisms, surface microorganisms are generally poorly known.
Epineuston Epineuston are organisms that use water surface tension to keep them on the ocean surface. Insects
There are very few marine insects.Why are there so few insects at sea? Deutsche Welle, 9 July 2018. The only true open-ocean insects are Halobates. Epineustonic organisms live on the water's surface, and in the open ocean all epineustonic species belong to the insect genus Halobates. Known as "sea skaters", Halobates sp. prey on other neustonic species and zooplankton trapped at the surface. Halobates lay eggs on a variety of objects, including floating feathers, wood, plastic etc., and unusually on pelagic molluscs like Atlanta turriculata.
Hyponeuston Hyponeuston are the mobile organisms that live immediately below the surface. Copepods
A wide variety of copepods can be found at the ocean's surface. Some neustonic copepods possess remarkable adaptations, especially within the pontellid copepods. Pontellid adaptations include specialised surface attachment structures, blue pigmentation. and even flying behavior to escape predators. Sapphirinidae copepods are often also associated with the surface, and some species have incredible structural colouration. As in many marine ecosystems, copepods represent a major food source for a variety of neustonic and surface-associated species.
The sea surface microlayer (SML) at the air-sea interface is a distinct, under-studied habitat compared to the subsurface and copepods, important components of ocean food webs, have developed key adaptations to exploit this niche. The ocean-spanning SML forms the boundary between the atmosphere and the hydrosphere. Despite having a thickness of less than one millimetre, the SML has profoundly different physicochemical and biological characteristics from the underlying water (ULW). The SML provides a biogenic gelatinous framework and is typically enriched with organic matter, heterotrophic microorganisms as well as higher trophic level organisms. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.