ENVIRONMENTAL CHARACTERISTICS
The deep sea is an environment completely unfriendly to humankind; it represents one of the least explored areas on Earth. Pressures even in the mesopelagic become too great for traditional exploration methods, demanding alternative approaches for deep sea research. Baited camera stations, small manned submersibles and ROVs (remotely operated vehicles) are three methods utilized to explore the ocean's depths. Because of the difficulty and cost of exploring this zone, current knowledge is limited. Pressure increases at approximately one atmosphere for every 10 meters meaning that some areas of the deep sea can reach pressures of above 1,000 atmospheres. This not only makes great depths very difficult to reach without mechanical aids, but also provides a significant difficulty when attempting to study any organisms that may live in these areas as their cell chemistry will be adapted to such vast pressures.
The physical characteristics that deep sea life must contend with to survive are:
LIGHT
The deep sea begins below about 200 m, where sunlight becomes inadequate for photosynthesis. From there to about 1,000 m, the mesopelagic or "twilight" zone, sunlight continues to decrease until it is gone altogether. This faint light is deep blue in color because all the other colors of light are absorbed at depth. The deepest ocean waters below 1,000 m are as black as night as far as sunlight is concerned. And yet, there IS some light. People who dive deep in a submersible (with its lights off) are often mesmerized by an incredible "light show" of floating, swirling, zooming flashes of light. This is bioluminescence, a chemical reaction in a microbe or animal body that creates light without heat, and it is very common. And yet, this light is low compared to sunlight, so animals here — as well as those in the mesopelagic zone — need special sensory adaptations. Many deep-sea fish such as the stout blacksmelt have very large eyes to capture what little light exists. Other animals such as tripodfishes are essentially blind and instead rely on other, enhanced senses including smell, touch and vibration.
Scientists think bioluminescence has six different functions (not all used by any one species):
Most bioluminescence is blue, or blue-green, because those are the colors that travel farthest in water. As a result, most animals have lost the ability to see red light, since that is the color of sunlight that disappears first with depth. But a few creatures, like the dragonfish, have evolved the ability to produce red light. This light, which the dragonfish can see, gives it a secret "sniper" light to shine on prey that do not even know they are being lit up!
TEMPERATURE
Except in polar waters, the difference in temperature between the euphotic, or sunlit, zone near the surface and the deep sea can be dramatic because of thermoclines, or the separation of water layers of differing temperatures. In the tropics, for example, a layer of warm water over 20°C floats on top of the cold, dense deeper water. In most parts of the deep sea, the water temperature is more uniform and constant. With the exception of hydrothermal vent communities where hot water is emitted into the cold waters, the deep sea temperature remains between about -1 to about +4°C. However, water never freezes in the deep sea (note that, because of salt, seawater freezes at -1.8°C). If it did somehow freeze, it would just float to the surface as ice! Life in the deep is thought to adapt to this intense cold in the same ways that shallow marine life does in the polar seas. This is by having "loose" flexible proteins and unsaturated membranes which do not stiffen up in the cold. Membranes are made of fats and need to be somewhat flexible to work well, so you may be familiar with this adaptation in your kitchen. Butter, a saturated fat, is very hard in your refrigerator and would make a poor membrane in the cold, while olive oil — an unsaturated fat — is semi-solid and would make a good flexible membrane. However, as with pressure, there is a tradeoff: loose membranes and proteins of cold-adapted organisms readily fall apart at higher temperatures (much as olive oil turns to liquid at room temperature).
FOOD
- abiotic (non-living) ones, namely light (or lack thereof), pressure, currents, temperature, oxygen, nutrients and other chemicals; and
- biotic ones, that is, other organisms that may be potential predators, food, mates, competitors or symbionts.
LIGHT
The deep sea begins below about 200 m, where sunlight becomes inadequate for photosynthesis. From there to about 1,000 m, the mesopelagic or "twilight" zone, sunlight continues to decrease until it is gone altogether. This faint light is deep blue in color because all the other colors of light are absorbed at depth. The deepest ocean waters below 1,000 m are as black as night as far as sunlight is concerned. And yet, there IS some light. People who dive deep in a submersible (with its lights off) are often mesmerized by an incredible "light show" of floating, swirling, zooming flashes of light. This is bioluminescence, a chemical reaction in a microbe or animal body that creates light without heat, and it is very common. And yet, this light is low compared to sunlight, so animals here — as well as those in the mesopelagic zone — need special sensory adaptations. Many deep-sea fish such as the stout blacksmelt have very large eyes to capture what little light exists. Other animals such as tripodfishes are essentially blind and instead rely on other, enhanced senses including smell, touch and vibration.
Scientists think bioluminescence has six different functions (not all used by any one species):
- headlights, such as the forward-facing light organs (called photophores) of lantern fish;
- social signals such as unique light patterns for attracting mates;
- lures to attract curious prey, such as the dangling "fishing lures" of anglerfish;
- counterillumination, in which rows of photophores on the bellies of many mesopelagic fish produce blue light exactly matching the faint sunlight from above (making the fish invisible to predators below them);
- confusing predators or prey, such as bright flashes that some squid make to stun their prey, and decoys that divert attention, such as the glowing green blobs ejected by green bomber worms; and
- "burglar alarms" in which an animal being attacked illuminates its attacker (the "burglar") so that an even bigger predator (the "police") will see the burglar and go after it. Some swimming sea cucumbers even coat their attackers with sticky glowing mucus so the "police" predators can find them many minutes later.
Most bioluminescence is blue, or blue-green, because those are the colors that travel farthest in water. As a result, most animals have lost the ability to see red light, since that is the color of sunlight that disappears first with depth. But a few creatures, like the dragonfish, have evolved the ability to produce red light. This light, which the dragonfish can see, gives it a secret "sniper" light to shine on prey that do not even know they are being lit up!
TEMPERATURE
Except in polar waters, the difference in temperature between the euphotic, or sunlit, zone near the surface and the deep sea can be dramatic because of thermoclines, or the separation of water layers of differing temperatures. In the tropics, for example, a layer of warm water over 20°C floats on top of the cold, dense deeper water. In most parts of the deep sea, the water temperature is more uniform and constant. With the exception of hydrothermal vent communities where hot water is emitted into the cold waters, the deep sea temperature remains between about -1 to about +4°C. However, water never freezes in the deep sea (note that, because of salt, seawater freezes at -1.8°C). If it did somehow freeze, it would just float to the surface as ice! Life in the deep is thought to adapt to this intense cold in the same ways that shallow marine life does in the polar seas. This is by having "loose" flexible proteins and unsaturated membranes which do not stiffen up in the cold. Membranes are made of fats and need to be somewhat flexible to work well, so you may be familiar with this adaptation in your kitchen. Butter, a saturated fat, is very hard in your refrigerator and would make a poor membrane in the cold, while olive oil — an unsaturated fat — is semi-solid and would make a good flexible membrane. However, as with pressure, there is a tradeoff: loose membranes and proteins of cold-adapted organisms readily fall apart at higher temperatures (much as olive oil turns to liquid at room temperature).
FOOD
Deep sea creatures have evolved some fascinating feeding mechanisms because food is scarce in these zones. In the absence of photosynthesis, most food consists of detritus — the decaying remains of microbes, algae, plants and animals from the upper zones of the ocean — and other organisms in the deep. Scavengers on the seafloor that eat this "rain" of detritus include sea cucumbers (the most common benthic animal of the deep), brittle stars (seen in the photo above), and grenadier or rattail fish. The corpses of large animals such as whales that sink to the bottom provide infrequent but enormous feasts for deep sea animals and are consumed by a variety of species. This includes jawless fish such as hagfish, which burrow into carcasses, quickly consuming them from the inside out; scavenger sharks; crabs; and a newly discovered group of worms (called Osedax, meaning bone-eater) which grow root-like structures into the bone marrow!
Deep-sea pelagic fish such as gulper eels have very large mouths, huge hinged jaws and large and expandable stomachs to engulf and process large quantities of scarce food. Many deep-sea pelagic fish have extremely long fang-like teeth that point inward. This ensures that any prey captured has little chance of escape. Some species, such as the deep sea anglerfish and the viperfish, are also equipped with a long, thin modified dorsal fin on their heads tipped with a photophore lit with bioluminescence used to lure prey. Many of these fish don't expend much energy swimming in search of food; rather they remain in one place and ambush their prey using clever adaptations such as these lures. Others, such as rattails or grenadiers (pictured below) cruise slowly over the seafloor listening and smelling for food sources failing from above, which they engulf with their large mouths.
Deep-sea pelagic fish such as gulper eels have very large mouths, huge hinged jaws and large and expandable stomachs to engulf and process large quantities of scarce food. Many deep-sea pelagic fish have extremely long fang-like teeth that point inward. This ensures that any prey captured has little chance of escape. Some species, such as the deep sea anglerfish and the viperfish, are also equipped with a long, thin modified dorsal fin on their heads tipped with a photophore lit with bioluminescence used to lure prey. Many of these fish don't expend much energy swimming in search of food; rather they remain in one place and ambush their prey using clever adaptations such as these lures. Others, such as rattails or grenadiers (pictured below) cruise slowly over the seafloor listening and smelling for food sources failing from above, which they engulf with their large mouths.
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