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The mesopelagic zone is the oceanic region through which carbon and other elements must pass in order to reach deeper waters or the sea floor. However, the food web interactions that occur in the mesopelagic zone are difficult to measure and so, despite their crucial importance to global elemental cycles, are not very well known. Recent developments in technology and new approaches have advanced the study of the variability in and controls upon the distribution and diversity of organisms in the mesopelagic zone, including the roles of respiration, recycling, and repackaging of particulate and dissolved organic material. However, there are remarkably few syntheses of the ecology and biogeochemistry of the microbes and metazoa that permanently reside or habitually visit this 'twilight zone'. Without this synthesis, it is difficult to assess the impact of ongoing changes in ocean hydrography and chemistry, due to increasing atmospheric carbon dioxide levels, on the biological carbon pump. This paper reviews what is known about the distribution of microbes and metazoa in the mesopelagic zone in relation to their activity and impact on global biogeochemical cycles. Thus, gaps in our knowledge are identified and suggestions made for priority research programmes that will improve our ability to predict the effects of climate change on carbon sequestration.

One of the least-understood parts of the sea is also one of the most important

Gelatinous organisms apparently play a central role in deep pelagic ecosystems, but lack of observational methodologies has restricted information on their behaviour. We made acoustic records of diel migrating jellyfish Periphylla periphylla forming small, ephemeral groups at the upper fringe of an acoustic scattering layer consisting of krill. Groups of P. periphylla were also documented photographically using a remotely operated vehicle (ROV). Although the adaptive value of group formation remains speculative, we clearly demonstrate the ability of these jellyfishes to locate and team up with each other.

The toothed seadevil, spiny seadevil or netbeard seadevil, (Neoceratias spinifer), is a rarely seen deep-sea anglerfish found in the mesopelagic and bathypelagic zones of the western central Pacific Ocean. It is the only species in the family Neoceratidae, and is unique amongst the deep-sea anglerfish in lacking an illicium and esca (the "fishing rod" and "lure"), and in having large teeth placed on the outside of its jaws. Description Adult female toothed seadevils have slender, elongate bodies up to 11 centimetres (4.3 in) long. They are dark red-brown to black in color, with naked skin. The mouth is large and extends well past the small eye; the jaws have an inner row of short, straight, widely spaced, immobile teeth. On the outside of the jaws, there are prominent conical outgrowths that bear 2-3 straight teeth, the longest of which reach almost 15% the length of the entire body. Each of these teeth is hinged at the base, with well-developed musculature and a tiny hook at the end. The illicium, or lure, is absent, along with the trough in which it rests in other deep...

A key question when trying to understand the global carbon cycle is whether the oceans are net sources or sinks of carbon. This will depend on the production of organic matter relative to the decomposition due to biological respiration. Estimates of respiration are available for the top layers, the mesopelagic layer, and the abyssal waters and sediments of various ocean regions. Although the total open ocean respiration is uncertain, it is probably substantially greater than most current estimates of particulate organic matter production. Nevertheless, whether the biota act as a net source or sink of carbon remains an open question.

Many large marine predators make excursions from surface waters to the deep ocean below 200 m. Moreover, the ability to access meso- and bathypelagic habitats has evolved independently across marine mammals, reptiles, birds, teleost fishes, and elasmobranchs. Theoretical and empirical evidence suggests a number of plausible functional hypotheses for deep-diving behavior. Developing ways to test among these hypotheses will, however, require new ways to quantify animal behavior and biophysical oceanographic processes at coherent spatiotemporal scales. Current knowledge gaps include quantifying ecological links between surface waters and mesopelagic habitats and the value of ecosystem services provided by biomass in the ocean twilight zone. Growing pressure for ocean twilight zone fisheries creates an urgent need to understand the importance of the deep pelagic ocean to large marine predators.

Vision may play a central role in sperm whale predation. Two complementary hypotheses regarding the detection and capture of prey items are presented, based on a review of mesopelagic ecology. The first hypothesis postulates that sperm whales locate their prey visually, either silhouetted against the midwater "sky," or by searching for bioluminescence produced by the movements of their prey. The second hypothesis postulates that sperm whales create a zone of stimulated bioluminescence around the mouth, which attracts squids and other visual predators. Studies of midwater fishes and invertebrates document the importance of vision in mesopelagic communities. If sperm whales search for silhouetted prey, they should be oriented upside-down to improve visual coverage and to facilitate the transition from search to prey capture. Prey capture events should be marked by excursions toward the surface. If they lure their prey, they should swim at a steady pace, with little rapid acceleration, and spend most of their time foraging at depths with the greatest potential for stimulated bioluminescence.

The deep sea is where the ocean becomes deeper. It is also the point of transition from continental shelves to slopes. Despite the extreme pressure, organisms called deep sea fish can survive there. For example, the anglerfish can produce its own light source called an esca. Some animals can thrive by feeding on marine snow. In 1960, a bathyscaphe called Trieste went down to the bottom of the Mariana Trench, which is the deepest point on Earth. There aren't any plants at all in these depths, so all fish in the deep are carnivores. Organisms Some organisms can live in the deep sea, such as the gulper eel, lanternfish, anglerfish, and more. They have many adaptations that can help them survive in the harsh conditions. Characteristics Sunlight Sunlight cannot reach the deep ocean, except parts of the mesopelagic zone. Pressure The pressure is many high, so many people can't survive. Exploration Exploration is very limited in the deep sea due to...