challenges provided by the abyssal zone. Fish and invertebrates had to evolve to withstand the sheer cold and intense pressure found at this level. They also had to not only find ways to hunt and survive in constant darkness but to thrive in an ecosystem that has less oxygen and biomass, energy sources or prey items, than the upper zones. To survive in a region with so few resources and low temperatures, many fish and other organisms developed a much slower metabolism and require much less oxygen than those in upper zones. Many animals also move very slowly to conserve energy. Their reproduction rates are also very slow, to decrease competition and conserve energy. Animals here typically have flexible stomachs and mouths so that when scarce food items are found they can consume as much as possible. Other challenges faced by life in the abyssal zone are the pressure and darkness caused by the zone’s depth. Many organisms living in this zone have evolved to minimize internal air spaces, such as swim bladders. This adaptation helps to protect them from the extreme pressure, which can reach around 75 MPa (11,000 psi). The absence of light also spawned many different adaptations, such as having large eyes or the ability to produce their own light (bioluminescence). Large eyes would allow the detection and use of any light available, no matter how small. Commonly, animals in the abyssal zone are bioluminescent, producing blue light, because the blue wavelength of light is attenuated over greater travel distances than other wave lengths. Due to this lack of light, complex designs and bright colors are not needed. Most fish species have evolved to be transparent, red, or black so they better blend in with the darkness and don't waste energy on developing and maintaining bright or complex designs. Animals The abyssal zone is made up of many different types of organisms, including microorganisms, crustaceans, molluscan (bivalves, snails, and cephalopods), different classes of fishes, and possibly some animals that have yet to be discovered. Most of the fish species in this zone are characterized as demersal or benthopelagic fishes. Demersal fishes are a term that refers to fishes whose habitat is very close to (typically less than five meters) or on the seafloor. Most fish species fit into that classification because the seafloor contains most of the abyssal zone’s nutrients so the most complex food web or