abyssal zone adaptation aphotic zone bathyal zone bathyscaphe trieste bioluminescence camouflage deep-sea exploration eye fish flashlight fish hadal zone hermaphrodite hydrothermal vent imax jaw light marine snow mesopelagic metabolism muscle night vision ocean pelagic zone photic zone photophore predation primary production retina retroreflector rod cell sea level silhouette skeleton squid submarine landslide tapetum lucidum the blue planet thermocline