Creatures Of The Deep Shark
Kathrine Selvage
Most are familiar with the surface layer, which extends down 650 feet (200 m) and receives the most sunlight, allowing photosynthetic organisms like phytoplankton to convert sunlight to energy. It is the home of pods of dolphins, schools of fish, and shoals of sharks. Scientists refer to this highly productive area as the epipelagic zone.
But the majority of the space in the ocean is a dark world. Dive below the epipelagic and you will enter the mesopelagic zone. Also known as the twilight zone, this area receives only faint, filtered sunlight, allowing no photosynthetic organisms to survive. Many animals have adapted to the near-darkness with large eyes and counterillumination.
The bathypelagic is between 3,300 and 13,100 feet (1,000 and 4,000 m) beneath the ocean surface. It is an area void of light (called aphotic) and at 39 degrees Fahrenheit (4 degrees Celsius), it is very cold. Moreover, the pressure is over 110 times that at sea level. Creatures in this zone must live with minimal food, so many have slow metabolisms. Many rely on marine snow as their main food source. They are also characterized by squishy bodies and slimy skin. The black hagfish, viperfish, anglerfish, and sleeper shark are common fish that call this zone home. While something like the gulper eel, with its massive expandable gullet, is a rare and amazing sight and could almost be mistaken for an alien. Vampire squid and dumbo octopus also venture to these depths.
The Abyssopelagic extends from 13,100 to 19,700 feet (4,000-6,000 m) down to the seafloor or abyssal plain. Animals that can withstand the pressures in this depth, which can reach up to 600 times what is experienced at sea level are highly specialized. Tripod fish are an oddity that can be found in this zone. Often found resting on the seafloor, tripod fish can pump fluid into their elongated fins to make them like rigid stilts (or as their name implies, a tripod), sometimes a few feet high. Rattail fish, octopuses, and sea cucumbers are also well adapted to the intense pressure here.
The abyssal plain is the relatively level deep seafloor. It is a cold and dark place that lies between 3,000 and 6,000 meters below the sea surface. It is also home to squat lobsters, red prawns, and various species of sea cucumbers. For these creatures food is scarce most of the time. Bits of decaying matter and excretions from thousands of meters above must trickle down to the seafloor, with only a small fraction escaping the hungry jaws of creatures above. Less than five percent of food produced at the surface will make its way to the abyssal plain. Most of this comes in great pulses as the result of phytoplankton blooms. When the phytoplankton are gone, the animals that grew quickly to eat them die and sink to the seafloor.
For the majority of the ocean floor large animals are scarce. The little nutrition that rains down from above in the form of marine snow is not nearly consistent enough nor substantive enough to fuel a large living creature (though there are billions of tiny ones). Whale or other large animal deaths are different.
Whale falls occur when a whale dies in surface waters and sinks to the bottom of the ocean. Trees, sharks, and large fish can also fall to the seafloor and provide food. The sudden arrival of food prompts creatures from afar to congregate and feast on the fleshy carcass. Once the flesh has been stripped and consumed by predators, bone eaters arrive so that not even the skeleton will remain. In the months and years after a whale fall the site will become the home and food source for millions of creatures.
No two whale fall communities are the same. The size of the whale, the depth of the seafloor, and the location all contribute to the types of animals that colonize the area and determine how long it takes for the skeleton to disappear. Our knowledge of whale falls comes from few and far between ROV and AUV encounters, so though whale falls are scarce, scientists estimate they exist at every 5 to 16 km in the Pacific Ocean.
Animal life at a hydrothermal vent relies on the energy produced by symbiotic bacteria. The bacteria live either inside the bodies or on the surface of their hosts. But unlike most life on earth that uses light from the sun as a source of energy, these bacteria produce energy through a chemical reaction that uses minerals from the vents.
A cold seep is a place on the ocean floor where fluids and gases trapped deep in the earth percolate up to the seafloor. A cold seep gets its name not because the liquid and gas that emerge are colder than the surrounding seawater, but because they are cooler than the scalding temperature of the similar hydrothermal vent.
Canyons are hotspots of life because they are areas of ample nutrition. A canyon acts like a funnel in the ocean, congregating decaying matter that originates from land down to the ocean depths. The geography of a canyon also creates currents of moving water that suspend the amassed nutrition into the water column, often even reaching up into shallower, sunlit depths where photosynthetic algae grow. Krill and crustaceans called amphipods thrive off the phytoplankton, and it is the masses of these zooplankton that attract tuna, swordfish, and sharks to canyons.
A seamount is an underwater mountain that can rise thousands of feet above the seafloor. Just as canyons funnel water, seamounts also influence the flow of water, often diverting deep currents. They are often found at the edges of tectonic plates where magma is able to rise through the surface crust. When dense, nutrient rich ocean currents hit the seamount they deflect up toward the surface, allowing marine life to thrive on the newly supplied food. Crabs, corals, anemones, sea stars, and many other creatures make the walls of seamounts their home. About 80 commercial species live on seamounts, and many are only found near this habitat.
In a deep, dark world anything that lights up stands out. But in fact, producing light in the deep is the norm rather than the exception. Some creatures produce their own light to snag a meal or find a mate in a process called bioluminescence.
Animals can use their light to lure prey towards their mouths, or even to light up the area nearby so that they can see their next meal a bit better. Sometimes the prey being lured can be small plankton, like those attracted to the bioluminescence around the beak of the Stauroteuthis octopus. But the light can also fool larger animals. Whales and squid are attracted to the glowing underside of the cookie-cutter shark, which grabs a bite out of the animals once they are close. The deep-sea anglerfish lures prey straight to its mouth with a dangling bioluminescent barbel, lit by glowing bacteria.
In addition to feeding, creatures of the deep use light in flashy displays meant to attract mates. Or, animals use a strong flash of bioluminescence to scare off an impending predator. The bright signal can startle and distract the predator and cause confusion about the whereabouts of its target. The light can even attract a bigger predator that will eat the attacker. If an animal needs to blend in, bioluminescence can be used to help in camouflage with the use of counterillumination, a display of light that helps them blend into the background.
And while for many creatures partaking in the migration is a way to avoid predators, others take advantage of the reliable movement of potential prey. One tiny plankton, a foraminfera, waits in the path of the migration and ensnares passing copepods, a migrating crustacean, in a web of protruding spines. A layer of these plankton create a dense mine field for the tiny crustaceans to swim through on their path each day. In the arms race of evolution, it pays to be one step ahead.
The Deep Reef Observation Project (DROP) is a Smithsonian research program launched to explore marine life and monitor changes on deep reefs in the southern Caribbean. Scientists turn to submarines to explore at depths too great for SCUBA gear. The Curasub is a 5-person manned submersible capable of descending to 1,000 feet. The state-of-the-art sub is equipped with hydraulic collecting arms that allow for the collection of marine life and the deployment of long-term monitoring devices on the deep reef.
Biological collections from the Curasub off Curaao have resulted in the discovery of numerous new and rare species of fishes, marine mollusks, echinoderms and crustaceans. This project utilizes the taxonomic expertise of more than a dozen Smithsonian scientists and employs modern molecular tools and digital photography and videography to fully document species and genetic diversity on deep reefs.
What does it take to live in the deep sea? Curator Karen Osborn wants to know how and why animals adapt in order to survive in a cold, dark, and pressurized environment. Many animals that live in this largest of the earth's habitats are very bizarre and dramatically different from their closest relatives. For example, some make an extreme effort to see, building huge bulbous eyes that can detect even the smallest glimmer of light, while others completely forfeit any form of sight and instead rely on heightened scent and touch. Since most animal groups have representatives living in the open ocean, learning about the differences in the way these animals live compared to their relatives in shallow water tells us a lot about how this environment changes and shapes the many animals that survive there.
A juvenile pancake batfish found between 200m depth and the surface, Gulf of Mexico, July 2018. The adults live on the bottom of the ocean and have been found at depths below 800m. The adults have a body that is flattened out like a pancake and they have modified fins for walking along the bottom. Science Photo Library / Dante Fenolio
Deep sea urchin from the Saint Peter and Saint Paul Archipelago in the Atlantic Ocean on the equator off the north east coast of Brazil. These sea urchins are characterised by their surprisingly bright colour pattern, usually red and white. Even more surprisingly, their tests (skeletons) are brightly coloured, too, even after drying, or sometimes fossilisation. Sea urchins move slowly, propelling themselves with their spines Solvin Zankl / Greenpeace
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