I think it's great that you're doing a project on the deep sea. It's both the largest and the least explored habitat on our planet. The intense pressure at such large depths makes it hard to design instruments to study the animals living down there, and the animals are more spread out so you wouldn't get much if you towed a fishing net and what you did get wouldn't survive the trip back to the surface. Right now, the only way to study deep sea communities is with submersibles, or small vehicles tethered to ships on the sea surface that travel to the bottom of the ocean with a pilot and 1-2 scientists. The trip can take up to 8 hours.
The submersibles are too small to stand up in, and too small to have bathrooms. Still, the discomfort is a small price to pay for the opportunity to see a world that few people even know about.Two types of hadal communities exist: those species that live in the water, and those that live on the sea floor or in the mud. I would guess that scientists don't even know about most of the organisms living in the hadal region. Many of the best-known deep-sea creatures are probably the fish that live in the midwater region, such as hatchet fish and angler fish. Other organisms that live in deep waters also include arthropods (shrimp, or example) and some strange-looking jellatinous organisms such as ctenophores and pteropods. Deep sea floor communities are becoming easier to study with improved technology. This group includes clams, mussels, worms, crabs and insect-like creatures (arthropods) that burrow in the mud or swim just above the sea floor.If you think about it, the hadal environment is one of the most extreme environments on the planet. The water temperature is always just above freezing, there is never any sunlight so there's very little energy input, the weight of all that water is enormous (600 times the pressure of the atmosphere at the surface), and the only source of food is what rains down from thousands of feet above you. The major adaptations to this type of environment are to the lack of light, the pressure and the scarce and irregular food supply. How would you find a mate in the dark if there were very few of your species around AND you didn't want to give yourself away to your predators? How would you adapt to find food in inky blackness? How would you adapt your head and mouth to take advantage of any possible food that you managed to find? (Would it make sense to not have a head and a mouth? What would you eat then?) How would you deal with the crushing pressure?
Many of the animals down in the depths are able to make their own light from chemical reactions within their bodies (bioluminescence). Some uses for this light include finding a mate (many species have individual glowing patterns), attracting food (one fish has a little light source dangling like a fishing lure in front of it's mouth), and escaping from predators (ever had a flashlight pointed right in your eyes after they've adjusted to the dark?). All that bioluminescence creates another problem, though: how to hide a glowing meal when it's in your stomach.
(Answer: have pigments in your gut that absorb light and make it look black.) Many deep-sea fish have HUGE heads in relation to their bodies so that they are able to eat a wide size range of food, and the small body means they don't have a lot of muscle and organ tissue that they need to maintain. A lot of deep-sea organisms have bodies made mostly (up to 98%) of water, which is pretty much incompressible and so isn't affected by the crushing pressure. Almost all deep-sea organisms probably have incredible senses of smell, so that they can detect and find food from long distances in the dark. There are few external sounds in the deep sea, and organisms are so dilute that using sound as a way to find food (eg, sonar) or as a way to communicate wouldn't really work. My guess is that most deep sea organisms don't use sound very much. The only marine organisms that have true "ears" are marine mammals or birds, and I doubt there are any marine mammals or birds in the hadal zone since these organisms are dependent on air.
Many of the organisms that live on or in the sea floor have special bacteria living in their tissues that convert chemical energy into organic compounds, and so can make food without sunlight! (In this case, you wouldn't need a head to eat, but the organisms that survive this way - clams, mussels and worms - don't have heads anyway so the lack of a head is not really an evolutionary adaptation to the deep sea.) In general, these bacteria use sulfur compounds for energy, which can be quite toxic to the organism that harbor the bacteria, and so these organisms have adapted special circulatory systems to transport sulfur from the surrounding water to their symbiotes, which are usually concentrated in some organ or another. Imagine having bacteria in your skin or your lungs or your stomach that fed you! You wouldn't ha
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