The study of animal movement has intrigued ecologists and animal enthusiasts throughout the centuries. Across the entire animal kingdom, movement plays a pivotal role in many aspects of life. Dispersal, migration, and foraging movements are just some examples of movement in the animal kingdom.

Migration is perhaps the most apparent and overt type of movement in the natural world and is defined as “the movement between two well-defined habitats on a temporary, predictable basis”.

All major animal groups, from arthropods to mammals, contain species that undertake migrations of varying extents. Some of the most famous examples include the salmon migration from the oceans to rivers, the mass movement of Christmas Island red crabs, and the seasonal migration of thousands of bird species.

All of these examples take place for reproductive purposes; however, migrations may also occur due to the availability of food or changes in the local climate.

What’s important is that migration takes place worldwide across countless species. But there is one form of migration that is lesser known, and it just so happens to be the largest daily movement of biomass on the planet.

It is the movement of the deep scattering layer.

What is the deep scattering layer?

The deep scattering layer, otherwise known as the sound scattering layer, is a region in the water column that is so densely packed with marine organisms, that sound waves are reflected.

This is how the deep scattering layer was first discovered. Ships using sonar signals to map the seafloor were often given false readings which led to the frequent misinterpretation of ocean depth. As such, the deep scattering layer has since been referred to as the ‘false’ or ‘phantom’ bottom.

A sonar scan revealing the deep scattering layer (green) above the sea floor, which often gave false depth measurements. Picture sourced from NOAA.

Sonar (sound navigation and ranging) is a widely used method of detecting objects, measuring distance, and navigating in the ocean using sound waves. The first documented use of sonar was in 1490, when Leonardo da Vinci used a hollow tube inside water to detect objects by ear.

Since then, the technology surrounding sonar has been significantly developed, driven by the outbreak of both world wars and the growing threat of submarine warfare.

It was on such an occasion, during the second world war, that sonar operators were bewildered by a false ‘sea floor’ that changed depths throughout the day. This puzzling layer was dubbed the ECR layer, named after the sonar technicians, C. F. Eyring, R. J. Christensen, and R. W. Raitt aboard the USS Jasper in 1942.

Further study soon revealed the culprits of the false bottom: millions of marine organisms.

The Deep Scattering Layer Community

The marine world is full of danger; chief of all is predation. Marine organisms are at a constant evolutionary arms race to combat predation pressures, as well as overcome effective survival adaptations from their prey.

However, before many species of marine life live to see their adaptations put to use, they must first survive through the larval stage.

The larval stage is the developmental stage in many forms of life, after birth and before the adult form is reached. Most marine organisms take full advantage of the larval stage to grow and disperse.

But being small in an ocean full of dangers leaves many organisms vulnerable to attack.

As with the majority of threats in the natural world, evolution has produced some cunning defence mechanisms. One of which is the behavioural trait known as diel vertical migrations.

As the name suggests, these movements involve the daily migration from deeper waters to surface waters, typically at night. By spending daylight hours at deeper depths, where light is scarce, planktonic organisms can avoid optically active predators and thus, increase their chances of survival.

After a day of rest, the plankton return to surface depths to feed during the night.

Planktonic species are not the only organism to undergo diel vertical migrations. This large movement of biomass naturally attracts natural predators and as such, many species of fish, squid, and other taxon undergo this migration, furthermore, increasing the daily movement of biomass that comprises the deep scattering layer.

The lanternfish (above) comprise a significant portion of the deep scattering layer biomass. Sonar is reflected off the swim bladder, thus giving the illusion of a false bottom. Picture by Christopher Glenn.

It is no wonder then, that sonar technology is able to detect this incredible natural phenomenon.

The mystery that long surrounded the deep scattering layer is one that illustrates one of the most powerful draws to ocean science: discovery.

The deep scattering layer remains the largest known migration of organisms on the entire planet and aptly demonstrates the significance of our oceans.

Written by Lucas King