The deep scattering layer, sometimes referred to as the sound scattering layer, is a layer in the ocean consisting of a variety of marine animals. It was discovered through the use of

sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances (ranging), communicate with or detect objects on o ...

, as ships found a layer that scattered the sound and was thus sometimes mistaken for the

seabed The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most o ...

. For this reason it is sometimes called the false bottom or phantom bottom. It can be seen to rise and fall each day in keeping with

diel vertical migration Diel vertical migration (DVM), also known as diurnal vertical migration, is a pattern of movement used by some organisms, such as copepods, living in the ocean and in lakes. The word ''diel'' comes from the Latin ''dies'' day, and means a 24- ...

. Sonar operators, using the newly developed sonar technology during World War II, were puzzled by what appeared to be a false sea floor 300–500 metres (980–1,640 ft) deep at day, and less deep at night. Initially, this mysterious phenomenon was called the ECR layer using the initials of its three discoverers. It turned out to be due to millions of marine organisms, most particularly small

mesopelagic The mesopelagic zone (Greek μέσον, middle), also known as the middle pelagic or twilight zone, is the part of the pelagic zone that lies between the photic epipelagic and the aphotic bathypelagic zones. It is defined by light, and begins at ...

fish, with

swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wit ...

s that reflected the sonar. These organisms migrate up into shallower water at dusk to feed on plankton. The layer is deeper when the moon is out, and can become shallower when clouds pass over the moon.Ryan
"Deep-sea creatures: The mesopelagic zone"
''Te Ara - the Encyclopedia of New Zealand''. Updated 21 September 2007. Lanternfish account for much of the biomass responsible for the deep scattering layer of the world's oceans.

Sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances (ranging), communicate with or detect objects on o ...

reflects off the millions of lanternfish swim bladders, giving the appearance of a false bottom.

Description

The phantom bottom is caused by the sonar misinterpreting as the ocean floor a layer of small seagoing creatures that congregate between below the surface. The name is derived from the fact that the first people to see these measurements erroneously reported that they had discovered sunken islands. Most mesopelagic fishes are small

filter feeder Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

s which ascend at night to feed in the nutrient rich waters of the

epipelagic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...

. During the day, they return to the dark, cold, oxygen deficient waters of the mesopelagic where they are relatively safe from predators. Most mesopelagic organisms, including mesopelagic fish,

squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fittin ...

and

siphonophore Siphonophorae (from Greek ''siphōn'' 'tube' + ''pherein'' 'to bear') is an order within Hydrozoa, which is a class of marine organisms within the phylum Cnidaria. According to the World Register of Marine Species, the order contains 175 specie ...

s, make daily vertical migrations. They ascend at night into the shallow

, often following similar migrations of

zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...

, and return to the mesopelagic depths for safety when there is daylight.Moyle and Cech, 2004, p. 585 These vertical migrations often occur over large vertical distances. Fish undertake these migrations with the assistance of a

swimbladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wit ...

. The swimbladder is inflated when the fish wants to move up, and, due to the high pressures in the mesopelegic zone, this requires significant energy. As the fish ascends, the pressure in the swimbladder must adjust to prevent it from bursting. When the fish wants to return to the depths, the swimbladder is deflated.Douglas EL, Friedl WA and Pickwell GV (1976
"Fishes in oxygen-minimum zones: blood oxygenation characteristics"
''Science'', 191 (4230) 957–959. Some mesopelagic fishes make daily migrations through the

thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...

, where the temperature changes between 10 and 20 °C, thus displaying considerable tolerances for temperature change. In 1998, sampling via deep

trawling Trawling is a method of fishing that involves pulling a fishing net through the water behind one or more boats. The net used for trawling is called a trawl. This principle requires netting bags which are towed through water to catch different spec ...

indicated lanternfish account for as much as 65% of all deep sea fish

biomass Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bio ...

. Lanternfish are among the most widely distributed, populous, and diverse of all

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...

s, playing an important

ecological Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...

role as prey for larger organisms. The previous estimated global biomass of lanternfish was 550–660 million

tonne The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton ( United State ...

s, about six times the annual tonnage captured worldwide by fisheries. However, this was revised upwards as these fish have a special gland for detecting movement from up to 30 metres away (e.g. fishing nets and fish sampling nets). In 2007 global sonar detectors indicated a more accurate figure for the global biomass was between 5,000 and 10,000 million tonnes: a truly massive weight of living mass.

References

Further references

* Bone Q and Moore RH (2008
''Biology of Fishes''
Taylor & Francis Group.

External links

Deep scattering layer
''Britannica online''.
The Deep Scattering Layer - Ocean Phenomenon
– ''YouTube'' {{hydroacoustics Marine organisms Oceanography Articles containing video clips

Description

See also

References

Further references

External links