A constant stream of new discoveries
Nothing is outlandish enough
Life in the sea has no standards: any size, any shape, any colour is allowed – no matter how bizarre. Anything and everything is included, from the tasselled wobbegong, a shark that actually looks like a bedside rug, to sea spiders with legs as long as a grown human, to jellyfish that glitter like disco balls or worms shaped like miniature Christmas trees.
The list of oddities and superlatives goes on and on, including creatures of all sizes and from all corners of the global ocean, but especially from extreme habitats such as the deep sea or polar oceans. These harsh environments require their inhabitants to develop highly sophisticated adaptation mechanisms, so that over the course of evolution, the shape, colour and functions of organisms have served one sole purpose: to survive as best as possible and reproduce.
- 1.1 > The body appendages of the firework jellyfish (Halitrephes maasi) appear bright red in the remotely operated underwater vehicle’s spotlight. This deep-sea species has been sighted in the Pacific and Atlantic oceans and the Mediterranean Sea. It also appears to be able to survive in low-oxygen waters. But not much more is known about the species.
- None of the beauties and presumed absurdities of marine life are just nature’s whimsy. Rather, every feature and trait is meaningful and has evolved through the process of evolutionary selection. But we humans have only just begun to fathom their meaning in individual cases, and to understand the role individual species play in the marine food web and network of relationships. And sometimes we don’t even properly put on our thinking cap – like when we voted for the world’s ugliest animal in 2013.
The blobfish (Psychrolutes phrictus) won the title because, in the photos shown, this deep-sea dweller resembled a grumpy-faced large boulder. However, in their natural environment, the deep waters of the North Pacific Ocean, the greyish to anthracite-coloured blobfish look just like ordinary fish. Only their head is slightly broader because it is armoured with bone plates. When brought to the surface, however, the loss of pressure distorts the appearance of the up to 70 centimetres long fish into a depressed-looking droopy mess. The image of the odd-looking fish impressed the voters more than the fact that blobfish can live to be over 100 years old and are threatened with extinction by deep-sea fishing.
The largest habitat on planet Earth
The blobfish, the tasselled wobbegong, the Japanese giant spider crab and the Christmas tree worm are among the approximately 248,000 marine species (as of August 2025) officially listed in the World Register of Marine Species (WoRMS). They are all considered “known”. On average, 2332 new species are discovered each year, and the list keeps getting longer with no end in sight. This is probably due to the sheer size of the global ocean.
It is by far the largest habitat on planet Earth. While its dimensions can be described in numbers and units, they really are beyond human imagination. The global ocean contains 1.3 billion cubic kilometres of water, covering a seabed area of 361 million square kilometres. This represents 71 per cent of the Earth’s surface, or an area more than eight times the size of the largest continent, Asia. The average water depth is around 3700 metres. However, in deep ocean trenches, such as the Challenger Deep in the western Pacific Mariana Trench, the distance between the sea floor and the water surface can be as much as eleven kilometres. Compared to such depths, even the world’s highest mountain, Mount Everest (8848 metres), appears merely medium-sized.
- 1.2 > Spirobranchus giganteus is commonly known as the Christmas tree worm. It owes its trivial name to its striking tentacle “crowns”. The calcareous tube it builds around its body serves as its home. It uses its tentacles to filter micro-algae and micro-organisms from the water.
- This space harbours life literally everywhere, from the deepest depths of the ocean trenches to the intertidal zones of the Wadden Sea and beneath the floating glacier tongues in Antarctica. Marine organisms brave the cold of the polar oceans as well as the extreme temperatures at hydrothermal vents, the so-called “black smokers”. Animal life is thriving even where humans have long thought it impossible – for example, on the sea floor of hypersaline deep-sea basins such as can be found in the Mediterranean. In the pore water between the sediment grains there is no oxygen, and salt concentrations are high: living conditions in the sea could hardly be more extreme. Nevertheless, scientists were able to identify three different species of tiny sediment-dwellers called Loricifera (from Latin, lorica, corselet [armour] + ferre, to bear) that colonize the upper layer of the sea floor.
Experts attribute the enormous resilience and specialization of marine organisms to the long evolutionary history of life in the sea. Some four billion years have passed since the once hot planet Earth cooled to below 100 degrees Celsius, liquid water first appeared and the first oceans formed. Not much later, the first life appeared on Earth. How and where is not known, but it is likely that the earliest life forms evolved near terrestrial hot springs and were single-celled organisms.
The primordial ocean then played an important role in their survival, as the Earth was repeatedly exposed to powerful meteorite strikes. The surroundings of the black smokers in the deep sea would have provided a safe haven for bacteria and archaea. Experts therefore believe that deep-sea regions were crucial to the development of life on Earth at that time.
- 1.3 > The sea anemone Edwardsiella andrillae could hardly have chosen a colder place to live than the underside of the Ross Ice Shelf in Antarctica. This is where researchers discovered it, burrowed deep into the ice. Only their tentacle tips peek out of the ice.
No more than a quarter of all marine species are known
It is estimated that the oceans host between one and two million different species today – from microscopic archaea and bacteria, to single-celled algae and fungi, to 30-metre-long whales and seaweeds. If we compare these figures with the WoRMS species list, we currently know only about ten to 25 per cent of all marine species. For the remaining 75 to 90 per cent, there is no written information, partly because humans have not yet explored many marine regions, ecosystems and groups of organisms in any detail.
By the end of 2023, scientists had mapped only a quarter of the world’s sea floor. Moreover, some of the least studied groups of marine organisms are thought to contain thousands, if not hundreds of thousands, of undescribed species. These include, for example, the marine isopods (Isopoda), the marine snails and slugs (Gastropoda) and the tanaids (Tanaidacea). The enormous diversity of protozoa, probably the largest and most diverse group of organisms in terms of taxonomy and evolution, is also poorly understood.
Moreover, surprising observations and research findings continue to show that we still have only a rudimentary understanding of how life is distributed in the oceans, what metabolic processes take place, and how the many different marine biocoenoses succeed in continuously driving the wheel of life – both in the sea and on Earth as a whole.
- 1.4 > Experts working on the World Register of Marine Species (WoRMS) keep a record of all newly discovered species. While the number of known marine species has now risen to almost 248,000, we still only know a small proportion of marine biodiversity.
Species
The “species” is considered to be the basic unit of biological systematics, but is not clearly defined. The biological concept of species is often used. According to this concept, a “species” is defined as a group of populations whose members have the potential to interbreed with one another and produce fertile offspring. It is important to note that the group referred to as a “species” should not be able to reproduce with other groups (criterion of intrinsic reproductive isolation).The ocean in crisis
Experts are of the opinion that the problem with our lack of understanding of marine life is that we have accepted this lack of knowledge as the status quo, and as a result we have lost sight of and forgotten about much of the ocean. Yet some 3.2 billion people, or 40 per cent of the world’s population, get a significant proportion of their animal-based food from the sea and are therefore directly dependent on functioning marine ecosystems.
But instead of conserving the marine habitat, we are systematically exploiting its resources. We are destroying marine ecosystems in many different ways and are weakening the ocean to such a degree that this threatens our well-being as a society.
There is increasing evidence of marine species extinctions. In recent decades, humans have fished sharks and rays to such an extent that more than three out of four of these predatory fish species are now threatened with extinction. Their disappearance has devastating consequences for marine biocoenoses. Many tropical coral reefs are in a similarly precarious situation. Together they are home to around a quarter of all known marine life. However, as a result of climate change, marine pollution, coastal defences and overfishing, warm-water reef-building corals in almost all regions are already dying at such a rate that by 2019 they covered only half to a quarter of the area they did in the 1980s.
If we are to halt these species extinctions and thus also safeguard a cornerstone of human existence, we need to understand marine life. We need to know which creatures live where in the sea, how they respond to man-made impacts, and how we can minimize the negative effects of marine exploitation. Experienced marine advocates sum up the current situation with the words, “People don’t protect what they don’t know”. This World Ocean Review therefore sets out to summarize the current state of knowledge about marine life and to identify solutions that will enable us to conserve the ocean’s biodiversity and restore it where it has already been destroyed.
- 1.5 > These marine creatures are among the most popular new discoveries of 2023: the worm-like mollusc Dorymenia boucheti, the bristle worm Alaysia solwarawarriors, the deep-sea sponge Abyssocladia falkor (top row, left to right), the sea daisy Xyloplax princealberti, the deep-sea jellyfish Santjordia pagesi and the Caribbean ribbon worm Tetranemertes bifrost (bottom row, left to right).
Population
“Population” refers to the individuals of a species that occur together in a given geographic area, interact with each other and form a reproductive community.Biodiversity, species richness and other key terms
The ocean is considered healthy and resilient when its biological communities support a large number of species that differ in their genetic make-up, appearance, characteristics and functions. If this is the case, experts speak of “high biodiversity” – a term derived from the words biological and diversity. Over the past 30 years, biodiversity has become one of the most important concepts in environmental protection and environmental management, and it’s now on everyone’s lips in politics, society, the media and the scientific community. However, people understand “biodiversity” in different ways.
The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) defines “biodiversity” as the variability among living organisms from all sources including terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are a part. This includes variation in genetic, phenotypic, phylogenetic and functional attributes, as well as changes in abundance and distribution over time and space within and among species, biological communities and ecosystems.
Terms such as “species diversity”, “species richness” or “biological diversity” are often used synonymously with biodiversity. However, as the definition shows, the concept of biodiversity is much broader. Species diversity and species richness are both simply measures of the number of species, be that in a particular ecosystem, habitat or even a scientific sample. Strictly speaking, therefore, they represent only one aspect of biodiversity.
Biodiversity
“Biodiversity” refers to diversity within species (genetic diversity), between species and between habitats and ecosystems.Diversity at all levels
When experts speak of marine biodiversity, they mean the diversity of life in all components of marine ecosystems and at all levels of biological organization – from genes and populations to individual species, biological communities and ecosystems.
Genetic diversity and individual trait differences
Genetic diversity and individual trait differences within populations allow marine species to adapt their behaviour and lifestyles to local environmental conditions. The North Sea three-spined stickleback provides an interesting example of how marine organisms indirectly pass on traits or characteristics across generations. When water temperatures rise, females of this species pass on trained mitochondria to their offspring that are adapted to the environmental conditions the females have experienced. In this way, the juvenile fish receive information about their mother’s living conditions and are to some extent better able to cope with the warmer waters. This gives them a clear adaptive advantage.
Differences between individual species
The differences between individual species are evident as organisms differ in appearance, have different evolutionary histories, colonize different habitats and take on different roles and functions within them. Particularly successful in this respect are the widespread cephalopods (Cephalopoda), a class that includes squid, cuttlefish, octopus and nautilus (Nautilidae). Together this class contains approximately 1000 species that live in all areas of the global ocean, from coastal waters to the deep sea. Only nautilus are restricted to the tropical waters of the western Pacific and Indian Oceans.
- 1.6 > In the ocean, diversity can be found at all levels of biological organization – from genetic diversity to ecosystems. Importantly, all levels are interconnected and facilitate the range of services provided by marine life.
- The various cephalopods have different appearances, behaviours and abilities. While the vampire squid (Vampyroteuthis infernalis), for example, takes a leisurely approach to foraging in the deep sea, stretching out its arms like a large umbrella to catch microplankton, sinking algae and faecal particles, cuttlefish are all about speed. They have a torpedo-like shape and use their mantle as a kind of nozzle through which water can be forced out of the mantle by powerful muscle contractions. This jet propulsion allows the squid to accelerate to speeds of up to 80 kilometres per hour. They then use their ten tentacles to grab their slower prey.
Differences between communities and ecosystems
In turn, the diversity of communities and ecosystems is based on species living together or alongside each other in different assemblages as well as influencing or depending on each other in different ways (interaction). For example, blue whales and Antarctic krill have a predator-prey relationship, meaning that one species eats the other. But two or more species may also compete for the same food source or live together in a symbiotic relationship, in other words one of mutual benefit. Well-known examples of such interactions in the ocean include the coexistence of clownfish and sea anemones, the symbiosis between large marine organisms and cleaner fish, and the close coexistence of corals and their unicellular algae symbionts.
The interaction of species in an ecosystem can, in turn, change local environmental conditions. Where kelp forests and seagrass beds grow, they influence local ocean currents. Flounders, worms and other seabed dwellers rummage through the top sediment layer, stirring up particles and sometimes altering the chemical balance of the upper seabed. Parrotfish nibble on coral stalks and excrete their remains as fine sand, determining the shape of coral reefs and the living conditions for themselves and all other reef inhabitants.
These examples show that the concept of marine biodiversity is multidimensional and opens up different perspectives from which to view and analyze the structure of life. There is widespread agreement among experts that a high level of biodiversity helps to stabilize the processes and interactions within communities over long periods of time. Species composition, in terms of the diversity of species traits, also plays a crucial role.
For example, if a coral reef contains several species of fish fulfilling similar ecological roles, these reef dwellers can fill any functional gaps that may occur if individual fish species are lost due to disease, overfishing or ocean heat waves. Under these conditions, ecosystems such as coral reefs are better able to cope with short-term environmental disruption or damage. Conversely, a loss of biodiversity can disrupt an ecosystem’s internal interactions. The productivity of the affected biocoenoses then often declines, rendering them unable to deliver the usual ecosystem services.
Earth’s biodiversity in numbers
Scientists quantify life on Earth in very different ways. The three main approaches are:
- to estimate or measure species richness (total number of species) in a given region,
- to calculate the total biomass of a species or group of species (e.g. all species in a genus), and
- to consider the phylogeny and lineage of a group of organisms, or in other words, whether this group is a separate branch on the “tree of life” and how many branch points the tree has.
- 1.7 > The diversity of marine life at different levels of organization is the result of different structural and functional traits. This table summarizes the characteristics that are of particular interest to science./dt>
Estimates of species richness
To understand the data on Earth’s biodiversity, it is important to know that taxonomists classify every living organism into one of three major categories of life on Earth, called domains. Their Latin names are Archaea, Bacteria and Eukaryota (living organisms with differentiated cells and nuclei). Archaea and bacteria are also known as prokaryotes because their cells have a relatively simple structure. Eukaryotic cells contain more sophisticated components called organelles, including a cell nucleus.
The domain of eukaryotes is divided into several supergroups. By far the greatest diversity and evolutionary variety is found in unicellular organisms, which include protozoa (meaning “first animals”). There are only two supergroups of true multicellular organisms: Archaeplastida, which includes land plants, and Opisthokonts, which includes fungi and animals such as jellyfish, fish and whales.
Currently around two million described species constitute the domain of Eukaryota. Of these, roughly half are insects and an estimated fifth are vascular plants. The remaining species represent an eclectic mix of life forms. About seven per cent are fungi and only four per cent of all known species are vertebrates.
Only estimates can be made as to how many living organisms remain undiscovered and undescribed. Ex-perts generally use their knowledge of species richness and relationships within better understood groups to infer the number of species in less well-studied groups. According to this approach, our planet should be home to an estimated 8.7 million eukaryotic species. Of these, 8.1 million would be plants and animals, with insects constituting the largest group with an estimated 5.5 million species.
- 1.8 > Guardians of biodiversity: The droppings of Adélie penguins fertilize the Antarctic coastal areas where they raise their chicks. In this way, the birds provide the nutrients nitrogen and phosphorus that microorganisms in the soil, as well as mosses and other plants, need to survive.
- If all undiscovered species were to be identified and described using conventional methods (manual identification of differences in shape and body structure), it would take more than 300,000 taxonomists about 1200 years. But that’s not all: initial analyses using new molecular biology methods suggest that the domain of eukaryotes might, at an extreme, contain up to a billion species.
According to current knowledge, terrestrial ecosystems are much richer in species than marine communities. About 77 per cent of the approximately two million described and thus known species live on land, about twelve per cent in the ocean and eleven per cent in lakes, rivers, streams and ponds. This difference is due to the fact that, in the course of their evolutionary and dispersal history, terrestrial organisms have been forced to specialize much more frequently. Where barriers such as mountains, valleys, rivers or an island location prevent the dispersal of living organisms, new species or communities that only occur in a particular area of the world (endemic species) develop more quickly. Moreover, flowering plants and pollinators have co-evolved on land and many insect species only pollinate very specific flowering plants, which is why there are so many different terrestrial insect species. In the ocean, however, flowering plants are rare.
Experts also point out that marine biological communities have been affected by mass extinctions throughout Earth’s history. They may have been slow to recover, which could be one reason why the biodiversity of the oceans is significantly lower than that of terrestrial ecosystems.
- 1.9 > The total number of species on Earth is still a matter of estimation. This is mainly because the diversity of terrestrial and marine bacteria, prokaryotes and archaea is difficult to quantify.
- 1.10 > New molecular methods of analysis have contributed to the fact that the phylogenetic tree of the eukaryote domain now has many more major branches (supergroups) than it did 25 years ago. Our diagram is a simplified representation of the state of knowledge in 2020.
- 1.11 > Results from molecular biology studies allow scientists to reconstruct the evolutionary history of related groups of animals – whales in this case – and to pinpoint more precisely at what point in time new species arose.
Living biomass quantities
A different picture emerges when experts analyze which living organisms have the most biomass. At 80 per cent, plants make up the vast majority of the Earth’s biomass. They account for a total weight of around 450 gigatonnes of carbon, with land plants contributing the largest proportion. However, when researchers only consider animal biomass, they find that marine arthropods top the list of animal groups with a total weight of around one gigatonne of carbon. Marine arthropods, for example, include more than 30,000 different species of crustaceans, with some marine “species of fame” such as the European lobster (Homarus gammarus) and the Antarctic krill (Euphausia superba), approximately 500 different species of sea spiders and five different species of horseshoe crab. The huge shoals of Antarctic krill alone are estimated to weigh in at 500 million tonnes.
- 1.12 > The family of scarab beetles contains 30,000 to 40,000 different species, including many well-known beetles such as the dung beetle, rose chafer, cockchafer, stag beetle and the Hercules beetle (centre), the world’s largest beetle with a body length of more than 15 centimetres.
- Fish make up the second largest animal group in terms of total biomass. They weigh in at around 700 million tonnes of carbon. Those inhabiting the mesopelagic zone, i.e. fish living at depths of 200 to 1000 metres, account for the largest proportion by weight. Molluscs (Mollusca) and annelids (Annelida) share third place with 200 million tonnes of carbon each.
Phylogenetic history and phylogenetic diversity
A third way of measuring diversity is to look at the evolutionary or phylogenetic history of species or groups of species. Essentially, this involves asking how many branches and branch points (forks) there are in the family tree of a group of organisms, and how long each branch is. Each branch point represents a split in the species. The length of the branches and the number of branch points reflect how far back in time the split occurred and how many different traits have developed. Although this method provides different information than the consideration of species diversity, phylogenetic diversity largely correlates with the number of species within particular groups. This is because for each additional species, the group’s own branch in the phylogenetic tree receives an additional branch.
A look at the tree of life shows that the phylogenetic diversity of our planet is dominated by bacteria. This is hardly surprising, as bacteria and archaea are among the oldest and most widespread forms of life on Earth. Moreover, their enormous diversity of species and functions provides essential metabolic processes and cycles, such as the nitrogen and sulphur cycles, without which life on Earth would not be possible.
Depending on which approach to quantification the scientists employ and from which perspective they analyze the diversity of life on Earth, different players come to the fore: animals dominate in terms of known species diversity, plants in terms of biomass and metabolic activity, and bacteria in terms of phylogenetic diversity and metabolic innovation.
From microscopes to genes: New biodiversity research methods
Up until now, researchers have had to go to great lengths to understand which organisms live in a particular area of the ocean. They cast nets of different mesh sizes to catch, count and identify fish, microalgae and microfauna. They scraped mussels, sponges and other sessile creatures from the ocean floor, dived into deep waters with ROVs or research submarines to take samples, or used photo and video cameras to document any creature at loose in the water column or on the seabed.
Particularly time-consuming was the counting of organisms and the identification of individual species based on their body shape, internal structure, behaviour and reproduction and other special characteristics. Moreover, the analyses were usually limited to those organisms that could be seen with the naked eye or at least displayed clear characteristics under the microscope.
However, it is difficult to distinguish bacteria and other single-celled organisms by their shape. They often have no distinctive morphological features. The same is true of the larval stages of many marine invertebrates. Even experts find it difficult to distinguish between the tiny juveniles of bivalves, echinoderms and crustaceans, such as oysters, sea urchins and lobsters.
- 1.13 > Biodiversity can be quantified in a number of ways. Here you can see the distribution of global biodiversity across the major kingdoms of life using the metrics of species diversity, phylogenetic diversity and biomass.
- We also now know that the different shapes of living organisms do not necessarily mean that they belong to different species. Sometimes the organisms simply go through different life-history phases. Common jellyfish, for example, spend most of their lives as a small cup-shaped polyp on the ocean floor. This polyp grows over the winter and then releases small, free-swimming juvenile jellyfish in the spring. This free-swimming form of jellyfish is called a medusa and usually dies a few months after sexual reproduction. Despite their completely different appearances, the jellyfish polyp and the medusa are one and the same creature. They are genetically identical.
The opposite is true of orcas. Once thought to be a single species, researchers have now identified three, if not four, distinct populations, or ecotypes, that differ in their behaviour and appearance and may represent more than one species, even if they look very similar. In a February 2024 publication, US scientists first reported on the fourth population. According to the study, the 49 animals live far off the west coast of North America. They hunt sperm whales, sea turtles, elephant seals and other marine mammals and have both pointed and rounded dorsal fins. In the other populations, however, the whales have uniform dorsal fin shapes. In two populations they are rounded, in the third they are pointed.
- 1.14 > Umbrella jellyfish belong to the cnidarians and appear in two different forms after the larval stage – as a medusa and as a polyp. Their transformation from polyp to medusa is known as metagenesis or alternation of generations.
- Therefore, marine organisms that look similar may or may not belong to the same species. It is quite common for unrelated beings living in the same environment to have developed similar or identical body shapes, features and traits over the course of evolution. This principle is known as convergent evolution. An example of convergent evolution is the development of shovel-shaped forelimbs which both the European mole (Talpa europaea) and the Australian marsupial mole (Notoryctes) use to burrow through the ground. But the two species are not related. Body morphology can therefore no longer be the sole criterion for identifying different species.
One barcode for every species
To be on the safe side, researchers are increasingly using molecular methods to describe and differentiate between species and to identify their functions in the ecosystem. This involves analyzing the organisms’ genetic information, or more precisely, sections of their DNA molecules. These DNA segments, known as genes, form a species-specific blueprint of life and allow scientists to precisely differentiate species and their functions in the ecosystem.
Genetic material can be found in any cell. Skin flakes, mucus, hair, individual microalgae, bacteria, faecal particles or even urine are often sufficient to extract the DNA of their originators. Selected areas of the DNA molecule are then amplified and read (sequenced) using high-throughput techniques. Researchers focus primarily on selected molecular markers. These are short gene fragments found in the DNA of the highest possible number of organisms.
The special feature of these molecular markers is that on the selected gene segment, the four nucleic acids – adenine, guanine, thymine and cytosine – are uniquely arranged for each species. At the same time, their sequence is so similar that the gene sequences of different species can be compared.
For example, DNA regions for the production of ribosomes, which are required for protein synthesis, can be used as molecular markers. Ribosomal gene fragments are mainly used to identify unicellular organisms such as microalgae, archaea and bacteria. In animals, however, experts usually sequence a gene segment called “cytochrome c oxidase subunit 1”. It encodes information for energy generation in the mitochondria, the powerhouses of the cell.
- 1.15 > This shows the rich diversity of shapes in the plankton community of Australia’s Great Barrier Reef. It includes fish eggs, hydromedusae, copepods and larvae of sea cucumbers, mantis shrimp, snails, prawns and decapods.
- For each gene sequence read, the researchers create a barcode of nucleotides, similar to the barcodes on supermarket products. This process is called bar-coding when the analysis is performed on a few specific DNA molecules. If the environmental sample analyzed contains an assortment of genetic material (environmental DNA, eDNA), so that thousands of different DNA fragments from different organisms need to be amplified and sequenced, experts speak of metabarcoding.
The barcodes are then compared to the millions and millions of gene sequences that scientists have already collected and catalogued in digital reference databases.
If a code is found in the database, the DNA owner can be identified immediately. However, if the gene sequence is unknown, individual analyses must be carried out. This requires that the unknown organisms are available as individuals, so that taxonomists can identify them based on their appearance and at least, as a first step, assign them to a taxonomic group or genus. Their genetic material is then sequenced, a reference barcode is created and all available information is transferred to the appropriate database for cataloguing.
Environmental DNA
Environmental DNA (eDNA) is the total set of biological remains found in a water sample, the genetic analysis of which provides insight into the biodiversity of a specific marine area. Experts also refer to this as “free-floating DNA”- Barcoding methods can be used to detect all living organisms that have left genetic traces in the sample being analyzed – from bacteria to whales or seabirds – provided their gene sequences are known. This is increasingly but not invariably the case. On German marine biology expeditions to the Arctic Ocean, for example, 30 to 70 per cent of the microalgae species sampled cannot immediately be identified using molecular methods. For this reason, there are always taxonomists on board who pick out unknown algae species cell by cell and then analyze these using both conventional and novel methods.
Using barcoding, scientists are increasingly able to trace the evolutionary history of organisms or determine the dates of evolutionary milestones. For example, researchers have only just recently been able to use molecular methods to pinpoint more accurately the origin of vertebrates. The corresponding gene changes took place between 530 and 505 million years ago. For this study, the team of scientists sequenced the genome of the eel-like brown hagfish (Eptatretus atami) and the jawless sea lamprey (Petromyzon marinus), among others.
In addition to purely scientific species identification, barcoding can also be used for other, more extensive purposes. For example, it can be used to identify illegally caught sharks. An international team of researchers recently developed a compact test kit that allows even lay people to determine within an hour whether a sample of fish meat originated from selected endangered shark species. The test is sensitive to gene sequences from three species: the bigeye thresher (Alopias superciliosus), the pelagic thresher (Alopias pelagicus) and the shortfin mako shark (Isurus oxyrinchus).
Metabarcoding can tell us which species are present in a given marine area at what time of year, including organisms that cannot be seen or distinguished with the naked eye or caught in nets. However, it does not provide information on the number of individuals, nor does it reveal the functions of individual species within an ecosystem or how these functions are changing. Shifts in community composition only become apparent through metabarcoding if the analyses are repeated at regular intervals and certain species are detected at higher or lower frequencies.
- 1.16 > Caribbean reef sharks (Carcharhinus perezi) are an endangered marine species. To better understand the distribution and population structure of these predatory fish, and to be able to make recommendations for their conservation, scientists take a small piece of the shark’s fin and use molecular techniques to analyze the genetic information it contains.
Inferring functions from genomic information
In the field of marine microorganisms, scientists are therefore going one step further with their molecular analyses. They no longer sequence only selected gene fragments, but the entire DNA contained in an environmental sample. All the gene sequences are then sorted into so-called metagenomes. In this way, the researchers can also identify functional genes, which, for example, reveal the metabolic processes in which certain types of bacteria are involved. This in turn allows them to deduce the microorganisms’ function in the ecosystem being studied.
A study of the Global Ocean Gene Catalogue 1.0 shows the insights that researchers are gaining using this method, termed metagenomics. The digital database contains gene sequences obtained by scientists who analyzed 2102 water samples from different regions and depths of the global ocean. They then used supercomputers to identify more than 308 million groups of genes. They were able to assign just over half of these to the three domains of bacteria, archaea or eukaryotes, with bacteria accounting for the largest proportion of the global ocean’s microbial genome, followed by eukaryotes and archaea. The researchers were also able to visualize the distribution of microorganisms across ocean regions and depth zones, and use functional genes to identify the metabolic tasks performed by specific groups of microorganisms in each ocean region. For the first time, the researchers were also able to show that more than 50 per cent of the genetic material analyzed came from fungi in the twilight zone, at depths of between 200 and 1000 metres.
- 1.17 > The soft coral Iridogorgia magnispiralis can produce light signals on its own – a property known as bioluminescence, which has been present in the animal kingdom for 540 million years.
Extra Info Artificial intelligence: A technology revolutionizes marine research
- Other researchers used functional genes to discover that soft corals (Octocorallia) were able to independently emit light signals as early as 540 million years ago – a trait now termed bioluminescence. To this end, they reconstructed the evolutionary history of soft corals using a large dataset of genetic sequences and fossil remains of relevant species. A computer model was then applied to analyze the large amount of data.
But molecular methods have a lot more to offer. A third method, known as metatranscriptomics, allows scientists to identify the species-specific traits that help marine microbes thrive in their habitat. In this context it is important to know that certain gene markers are switched on or off in an organism depending on environmental conditions, such as when an organism acclimaizes to rising water temperatures. Metatranscriptomics can be used to identify such adaptation strategies. For example, tropical coral experts are using this method to find out why some microalgae that live in symbiosis with corals and provide them with nutrients survive ocean heat waves better than others, with the result that their symbiont corals bleach more rarely.
These and other detailed insights from research on molecular biodiversity are urgently needed to understand the functioning of marine life. Knowing which marine organisms live where, what functions they perform, and what traits enable them to do so, will help us to understand how climate change and other stressors are likely to change marine communities and what services they will be able to provide in the future.
- 1.18 > When looking for environmental DNA (eDNA), the pore size of the seawater filter determines which organisms, cell components or even extracellular DNA fragments are filtered out and then amplified and read for metabarcoding
- However, it is also true that there are not enough taxonomists in the world to identify unknown marine organisms and carry out the many survey counts that have become necessary as part of fine-scale marine monitoring. Experts therefore believe that molecular methods will soon become the standard for marine monitoring. And sometimes the results of molecular analyses highlight problems that have yet to be solved, such as when genetic material from land animals such as chickens or cattle suddenly appears in water samples from the German North Sea. Rainwater probably washed their faeces off the fields. Rivers then carried them to the sea, thus contributing to the eutrophication of coastal waters. Their presence in the North Sea is further evidence that our lives on land have a major impact on the health of marine ecosystems.
- 1.21 > Using high-throughput molecular techniques such as metabarcoding, metagenomics and metatranscriptomics, researchers can identify species, infer their phylogeny and determine the function of specific genes.
