Various subspecies of squid are free to roam the sea at extraordinary depths, ranging from six-hundred and sixty feet to as far as two-thousand and three hundred feet.  A big challenge to organisms residing at these depths would be in maintaining cell functioning as proteins can be altered and lose proper activity.  To counteract this, squids take in large amounts of trimethylamine oxide (TMAO), aiding in maintaining cell shape.  Studies of five species of squid found that samples contained extremely high levels of TMAO (2558-8064 ppm) alongside amounts of TMA and DMA Lin & Hurng, 2002). Interestingly, TMAO is also said to be a cause of the distinctive smell of many creatures of the sea, and depth has a direct impact on this. 

One of the prominent advantages squids have at their disposal lies in the neural architecture of their visual system. Tubular eyes contain retinas that have two inner segment layers instead of the traditional one, which rhymically move across depths of fifty to a thousand meters (Chung et al., 2017).  This provides aid in detecting predators through enabling squid the ability to detect fast-moving objects at various distances.  Light intensity is determined by the time of day, moreso the availability of light, and depth, this being taken into account alongside the downwelling sunlight which is depth-dependent.  This adds an additional 10-fold drop in brightness as depth increases around every seventy-five meters (Douglas et al., 2005). Eye enlargement is also a common adaptation in deep-sea predators, as enlarged eyes ensure more photons reach the retina, squid eyes being registered at sizes as large as twentyseven centimeters (Nilsson et al., 2012).