Because the mesopelagic zone (200–1,000 metres deep) and those beneath it provide similar habitats worldwide, some of the same species are found in both hemispheres. About 20% of the food made in the epipelagic zone makes its way down to the mesopelagic. Conversely, at night many mesopelagic organisms migrate upwards into the darkened epipelagic zone. This gives the mesopelagic creatures the best of both worlds. They can feed at night in the rich waters of the epipelagic zone and hide during the day in the dark, cold, oxygen-depleted waters of the mesopelagic, safe from most predators.
A false sea floor
When sonar became widely used during the Second World War, operators detected an apparent sea floor at a depth of 300–500 metres during the day, but nearer the surface at night. This proved to consist of millions of organisms, including fish and zooplankton, which move up through the water at dusk to feed on microscopic plants growing where the sun penetrates. On moonlit nights this layer remains deeper, although the animals within it may respond to clouds passing over the moon. Many species including jellyfish, squid and crustaceans are involved in this vertical migration, but it was mainly fish that gave rise to the echo picked up by the sonar operators. The sound was reflected off their swim bladders – gas bags that enable them to fine-tune their depth without expending a lot of energy. The phenomenon of the false sea floor became known as the deep scattering layer or DSL, because it scatters the sonar signal.
Fish adaptations to the zones
Typically, species such as lanternfish (Myctophids) and bristlemouths (Gonostomatids), which make vertical migrations each day, possess a swim bladder. They also have well-developed muscles and bones, and are usually somewhat streamlined. By contrast, fish that remain as ‘sit and wait’ predators in the mesopelagic zone lack a swim bladder, and have flabby muscles and watery flesh. Because they do not go anywhere, streamlining has either not evolved or has disappeared.
Bathypelagic fish (at depths of 1,000–4,000 metres) are unable to undertake vertical daily migrations because below a certain depth the pressure is too great and the swim bladder is unable to function – gas cannot be produced. Most of these fish lack swim bladders or, like orange roughy, have bladders that are filled with wax. Sharks do not have swim bladders but are able to move vertically. To plane up and down they rely on strong swimming and the buoyancy provided by their oily livers – oil is less dense than water.
Many mesopelagic animals possess bioluminescent (light-producing) organs known as photophores. In some species the light is produced by specialised tissues, in others by symbiotic bacteria. Many mesopelagic fish, shrimps and squid have photophores on their undersides that match surface brightness. They reduce the light output during cloudy days, and boost it when it is sunny. The photophores may be turned off at night to make the animal relatively invisible.
Bioluminescence may play other roles, as there are often sexual differences in photophore distribution. In some species the typically blue-green light may indicate the species or sex of an animal, while in others the light may attract prey or enable the predator to see its quarry. Faced with a predator, some species of shrimp and squid eject clouds of luminescent ink to produce a replica of themselves, and then swim away while the predator attacks it.
Glow in the dark
Bioluminescence (light produced by living things) happens mostly in the ocean. While a few land animals such as glow-worms use it, it is almost unknown in fresh water. It is a cold light produced by a chemical reaction inside an organism. The loosejaw fish Malacosteus niger can produce a red searchlight to spot its prey while it remains unseen.
Vision in the mesopelagic zone
Typically, mesopelagic fish have large eyes that gather ambient light. Some have tubular eyes that are upward looking and give sharp forward but poor sideways vision. A secondary retina extends up the sides of the tube and dramatically increases the field of view. Parallel evolution has produced similar optical solutions in at least one octopus (Amphitretus) and some krill (Stylocheiron). Some tubular-eyed fish have a yellow filter that enables them to distinguish between natural light and bioluminescence, thereby defeating the counter-illumination systems used by their prey.
Other mesopelagic organisms
Other mesopelagic organisms include copepods and krill (planktonic animals) that eat the faeces of epipelagic copepods and any other matter that reaches that depth. Many krill and shrimp possess light organs. Ostracods, amphipods, arrow worms (Chaetognaths), jellyfish, siphonophores, comb jellies, larvaceans and pteropods are also common. Squid, including juveniles of the giant squid Architeuthis dux and the considerably larger colossal squid, are also found here.