PSM V20 D769 Longitudonal section of a bleak

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 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 of ...

. 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. 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 language, 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 ...

fish, with

swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ (anatomy), organ in bony fish that functions to modulate buoyancy, and thus allowing the fish to stay at desired water depth without having to maintain lift ...

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 Lanternfish (or myctophids, from the Greek language, Greek μυκτήρ ''myktḗr'', "nose" and ''ophis'', "serpent") are small mesopelagic fish of the large family (biology), family Myctophidae. One of two families in the order Myctophiformes, ...

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 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 aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a s ...

s which ascend at night to feed in the nutrient rich waters of the epipelagic zone. 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 A squid (: squid) is a mollusc with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also ...

and

siphonophore Siphonophorae (from Ancient Greek σίφων (siphōn), meaning "tube" and -φόρος (-phóros), meaning "bearing") is an order within Hydrozoa, a class of marine organisms within the phylum Cnidaria. According to the World Register of Marine ...

s, make daily vertical migrations. They ascend at night into the shallow epipelagic zone, often following similar migrations of

zooplankton Zooplankton are the heterotrophic component of the planktonic community (the " zoo-" prefix comes from ), having to consume other organisms to thrive. Plankton are aquatic organisms that are unable to swim effectively against currents. Consequent ...

, 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 swimbladder is inflated when the fish wants to move up, and, due to the high pressures in the mesopelagic 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 distinct layer based on temperature within a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) with a high gradient of distinct te ...

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

trawling Trawling is an industrial 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 di ...

indicated

lanternfish Lanternfish (or myctophids, from the Greek language, Greek μυκτήρ ''myktḗr'', "nose" and ''ophis'', "serpent") are small mesopelagic fish of the large family (biology), family Myctophidae. One of two families in the order Myctophiformes, ...

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

biomass Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...

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

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

s, playing an important

ecological Ecology () is the natural science of the relationships among living organisms and their environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere levels. Ecology overlaps with the closely re ...

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 1,000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton in the United States to distinguish it from the non-metric units of the s ...

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 and 10 billion 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''. {{Physical oceanography Marine organisms Oceanography Articles containing video clips

Description

See also

References

Further references

External links