The vast open ocean, far from land, seems like a barren and lifeless place to many people. But is it really devoid of all marine life? The answer may surprise you.
If you’re short on time, here’s a quick answer to your question: Yes, there are fish and other marine life even in the most remote parts of the open ocean, thousands of miles from land.
Pelagic Fish Thrive in Open Waters
Tuna, Swordfish, Marlins
Some of the most iconic pelagic fish found in the open ocean include tuna, swordfish, and marlins. These large, fast fish are perfectly adapted to cruise the endless blue waters in search of prey (1). Bluefin tuna are among the fastest and most powerful fish, reaching speeds over 70 km/h during high-speed predation (2).
Their torpedo-shaped bodies, retractable fins, and thunniform swimming mode all contribute to their speed and hydrodynamic efficiency. Swordfish also swim at high speeds, slashing through shoals of smaller fish and squid.
Their bills serve multiple purposes – from stunning prey to balancing at high speeds. Marlins are also large open ocean hunters, known for their elongated jaws filled with sharp teeth.
Lanternfish
In contrast to the large pelagic predators are the abundant lanternfish, which measure just a few centimeters long. Lanternfish are likely the most numerous fish genus on Earth, with over 250 species filling oceans from surface to seafloor (3).
Their name comes from the bioluminescent photophores that dot their bodies, allowing them to disguise their silhouette when viewed from below. Lanternfish avoid predators by staying deep during the day, then rising to the surface at night to feed on plankton.
Some migrate vertically up to 1 km every day (4)! Their huge abundance and daily migration make them an essential food source, supporting many open ocean food chains.
Jellyfish
Jellyfish are common drifters in warm, tropical waters around the world. The cannonball jellyfish (Stomolophus meleagris) and the moon jellyfish (Aurelia aurita) pulse through the open sea in huge blooms that consist of hundreds of individuals.
Jellyfish don’t have strong control over where they swim, simply drifting along with ocean currents. Fortunately, they don’t have to work hard to find food since their diet mainly consists of tiny zooplankton that bump against their tentacles.
While jellyfish blooms indicate healthy open ocean ecosystems, increases beyond normal levels can have detrimental impacts like disrupting fisheries by clogging nets (5).
The Deep Scattering Layer
The deep scattering layer (DSL) refers to a layer in the ocean consisting of marine organisms that migrate vertically on a daily basis. This layer was discovered in the early 1940’s when scientists using sonar noticed a thick layer in the ocean that scattered or reflected sound waves.
They were amazed to detect such a dense layer of organisms living at depths between 600 and 1,000+ feet, far below the penetration of sunlight and the food-producing photosynthetic zone.
The organisms that make up the DSL are astounding in their abundance and diversity. They include a myriad assemblage of jellyfish, crustaceans, zooplankton, fish and squid. Scientist Edith Widder describes the deep scattering layer as “almost like a city at that depth strung out.” At around dusk every day, a mass migration of organisms, numbering in the tens to hundreds of millions per mile, ascend from these dark depths up toward the ocean’s surface to feed under the cover of night.
What’s fascinating is we still don’t fully understand what’s driving this great migration. Theories suggest it could be following vertically migrating zooplankton, seeking to avoid predation in daylight, saving energy, feeding on material sinking from upper waters, or some combination of these factors.
Likely, the migration is a complex biological choreography to balance meeting life’s basic needs: finding food while trying not to become food! Dr. Klevjer says, “There are still so many unanswered questions surrounding the deep scattering layer—it remains one of the least understood communities in the oceans.”
Significance of the Deep Scattering Layer
The DSL represents a massive hidden biomass and food source that scientists are still working to quantify and study. By conservative estimates, if the global DSL were harvested completely, it could provide three times as much seafood as is currently fished from the surface.
The DSL forms an important energy and nutrition pipeline between the deep sea and waters closer to the surface. It’s a key food source for many larger predators living above, also playing an integral, but still mysterious role, in the ocean’s carbon cycle.
Numbers alone can’t convey the wonder and magic of watching this great migration emerge out of the black depths every evening. As explorer Edith Widder, who was one of the first to film it describes, “It’s almost like a fairy tale magical city appearing out of the gloom.” To catch a glimpse and learn more, see her mesmerizing video here: Deep Sea Exploration Video
There is still so much to uncover about the deep scattering layer hidden in the ocean’s twilight zone! But with continued research, this important yet little-understood zone between 200-1000 meters that links the surface and depths, is slowly revealing its secrets.
Nutrient Sources in the Open Ocean
Upwellings
Upwellings are areas in the open ocean where cold, nutrient-rich water from the deep rises to the surface. This process brings essential nutrients like nitrogen and phosphorus up from the depths, fueling the growth of phytoplankton.
Phytoplankton are microscopic marine algae that form the base of the ocean’s food web. When upwelling occurs, it kickstarts enhanced productivity in the surface waters.
The major upwelling regions in the world’s oceans are along the eastern edges of ocean basins. For example, upwelling happens along the west coast of North and South America, off the coast of northwest and southwest Africa, and along the south coast of Australia.
In these areas, prevailing winds combined with the Coriolis effect drive surface waters offshore. This allows deep water to well up near the coast, bringing nutrients to the sunlit surface waters where phytoplankton can flourish.
Coastal upwelling regions are some of the most productive marine ecosystems in the world. The influx of nutrients in upwelled water can generate blooms of phytoplankton that support diverse food webs, from zooplankton to small forage fish, to seabirds, whales, and apex predators like tuna.
In fact, over 50% of the world’s fish catches come from upwelling areas, which only make up about 1% of the ocean’s surface.
Whale Falls
When a whale dies and sinks to the seafloor, its carcass becomes a concentrated source of organic matter and nutrients. This ephemeral habitat is known as a “whale fall” and it provides sustenance for unique communities of deep-sea organisms.
A 40-ton gray whale carcass contains around 2,000 kg of carbon, enough organic material to sustain ecological succession for 50-100 years. Scavengers like hagfish, sleeper sharks, octopuses, and crustaceans first consume the soft tissue.
Then, within months, lipid-rich bones harbor mats of chemoautotrophic bacteria that can synthesize organic compounds using energy from sulfur or methane. This process of chemosynthesis supports specialized species like bone-eating zombie worms, mussels, and snails.
After 1-2 years, the skeletal remains of the whale are colonized by deep-sea corals, sponges, and anemones. A whale skeleton can thus provide habitat and sustenance for over 100 species that are not found anywhere else in the deep ocean.
While the number of great whales has declined due to whaling, natural whale falls may still input over 190,000 kg of carbon per year into food-limited deep-sea ecosystems worldwide.
Unique Adaptations of Open Ocean Fish
Bioluminescence
One of the most amazing adaptations of open ocean fish is their ability to produce bioluminescence. Bioluminescence is light produced by a chemical reaction within an organism’s body. Many mesopelagic fish that live 200-1000 meters below the ocean’s surface utilize bioluminescence for essential tasks like attracting prey, camouflage, mating, and communication.
The light is produced by special photophore organ cells that contain luciferins and luciferase enzymes. When luciferins combine with oxygen, the luciferase speeds up the reaction, causing the luciferin molecules to become electronically excited and release photons.
Some species like lanternfish have photophores scattered over their entire body, while viperfish and anglerfish have a conspicuous ‘fishing rod’ protruding from their heads with a photophore at the end. The viperfish uses its lure to attract smaller fish within striking distance of its sharp fangs.
Other bioluminescent fish like the flashlight fish have luminescent underbellies to disguise their silhouette from predators swimming below. The range of bioluminescent colors and blinking patterns allows fish to communicate and identify each other in the perpetual darkness of the deep sea.
Truly amazing!
Large Eyes and Excellent Vision
Another essential adaptation of open ocean fish is their oversized eyes and excellent vision. With little to no sunlight hundreds of meters below the surface, fish need to make the most of any photon available. Many midwater fish have tubular eyes positioned on the top of their heads.
This gives them the best view of any potential prey swimming above them against the faint glow of the sun. Their eyes are huge relative to their body size and contain rod cells with super sensitive rhodopsin pigments capable of detecting the smallest flash of bioluminescence.
Some also have a reflective layer of crystals at the back of the retina called the tapetum lucidum, allowing light to strike visual receptors twice. Others have multi-layered retinas to optimize the capture of sparse photons.
Hence, while open ocean predators cruise with mouths agape, any unwitting prey wandering into view will be rapidly sucked into their visible void. Quite a terrifying thought from the perspective of a deep-sea shrimp!
Threats to Open Ocean Ecosystems
Overfishing
Overfishing is the greatest threat to open ocean ecosystems. Industrial fishing fleets are taking fish out of the oceans faster than they can reproduce. Some statistics show that up to 90% of large predatory fish like tuna, marlin, and swordfish have been fished out since the 1950s.
Overfishing doesn’t just threaten the fish species themselves, it disrupts the entire food chain, leading to declines in seabirds, marine mammals, and other animals that depend on fish to survive.
The tragic collapse of the northwest Atlantic cod fishery in the 1990s is one famous example of how overfishing can destroy ocean ecosystems. Cod had sustained communities in eastern Canada for centuries, but the introduction of industrial bottom trawlers lead to a catastrophic population crash.
Despite a moratorium on cod fishing in the 1990s, the species has struggled to recover. Sadly, the story of cod has been repeated for many other overexploited species like bluefin tuna in the Mediterranean, sharks worldwide, and orange roughy in Australia and New Zealand.
Climate Change
Climate change poses multiple threats to open ocean ecosystems. Rising ocean temperatures are causing mass coral bleaching events and threatening cold water-dependent animals like krill. Acidification from increased CO2 absorption makes it difficult for creatures like corals and shellfish to form their calcium carbonate skeletons and shells.
Changes in ocean circulation patterns are shifting the ranges of marine species, disrupting predator-prey relationships. Loss of sea ice in the Arctic threatens ice-dependent species like polar bears and walruses.
Climate change acts as a threat multiplier – it exacerbates other problems like overfishing and pollution. For example, when fish populations are already stressed by overfishing, they are less resilient to changes in temperature and their food sources.
Protecting and restoring ocean ecosystems to maximize resilience is an important climate change adaptation strategy. But ultimately, reducing greenhouse gas emissions is needed to limit warming and acidification trends.
Plastic Pollution
Plastic pollution has become ubiquitous in the oceans, from the Arctic to the Antarctic. It degrades into tiny microplastic particles that are ingested by marine life, working their way up the food chain. Giant ocean garbage patches have formed in gyres where plastics accumulate.
Birds, fish, turtles, and marine mammals are harmed through ingestion and entanglement. Over 640,000 tons of abandoned fishing gear enters the ocean each year, trapping and killing countless animals.
But it’s not just large debris – microplastics less than 5mm in size are the most common type of marine litter. Microplastics come from the breakdown of larger items as well as microbeads used in cosmetics and fibers shed from synthetic fabrics.
These tiny plastic particles are found on shorelines worldwide, even in remote Arctic areas. Animals can mistake them for food, allowing microplastics to accumulate in their bodies. The impacts are still being studied, but have potential to disrupt hormones, reproduction, and even energy levels in animals that ingest them.
Conclusion
While the open ocean may seem devoid of life, it is actually home to a unique ecosystem of specially adapted fish, mammals, and invertebrates. However, human activities are increasingly threatening these fragile habitats.
With care and stewardship, we can ensure the open oceans continue thriving for generations to come.
