Diel Vertical Migration: The Most Impressive Animal Migration on Earth
Today I’ll be discussing the process that got me hooked on the ecology of the open ocean, diel vertical migration (shortened as DVM)! DVM is the process of deep-pelagic (deeper than 200 meters [1 meter = ~3 feet]) animals ascending several hundreds of meters in the water column at night in search of food under the guise of the dark night sky. Once daylight approaches, these animals descend back into the dark, deep ocean where they are less likely to be eaten. This process occurs everywhere in the open ocean every single day. Many of the animals that practice DVM (like the lanternfish below) are no larger than the palm of your hand! It’s not just about fishes. Squids, zooplankton, siphonophores, pyrosomes, and many others practice DVM as well. This means that within one 24-hour cycle, migrating animals experience water temperatures greater than 10 degrees Celsius (> 20 degrees Fahrenheit) and pressure differences amounting to dozens of atmospheres of pressure. Physiologically, this is remarkable! If we consider the distance traveled relative to the size of the animal and the overall magnitude of animals migrating twice each day, this has been considered by many as the largest animal migration on Earth.
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Most of the animals that undergo DVM are a part of a layer of the ocean called the deep scattering layer (DSL). The DSL is an area of the ocean where the most dense collection of animals exists, usually between 400–600 meters deep in the water column. The United States Navy became interested in the DSL after submarines began using sonar to search for German U-boats during World War II. Sonar uses sound to search for objects in water, such as submarines and undersea mountains. Unfortunately for the US Navy, certain sound frequencies also bounce off of animals. The DSL is such a thick aggregation of animals that the captain of one submarine thought they were approaching the seabed at 400 meters depth when the actual bottom depth was closer to 3,000 meters and no undersea mountains were in sight. For this reason, the DSL has also been termed a “false bottom” in naval culture.
Motivations behind studying DVM
It is not enough for us to just know that this process exists.
We want to learn why these animals would expend the energy to migrate
long distances. After all, they may be moving into a region of the ocean where
there is more food, but there are also more predators, indicating that this behavior
is a risk. It is also important to note that many species do not vertically
migrate and they have incredibly successful evolutionary lineages, suggesting that
lifestyle can work. So, is the reward of greater food availability in shallow
water worth the possibility of being eaten? I do not have an answer for this,
but the answer is likely, “It depends”. Nevertheless, the animals that do
undergo DVM are eating at shallow depths, and then they descend as digestion is
occurring within them. An intestine is essentially a long tube that can only have
so much material at one time. Animals in the ocean do not exactly have a reason
to “hold it”, so this means DVM participants are excreting nutrients at deep
depths, effectively transporting these nutrients hundreds of meters much faster
than they would naturally sink. In the context of global climate change, this
process is called carbon sequestration and could greatly help buffer our
oceans from future change!
Challenges associated with studying DVM
There are a few difficulties with studying DVM, many of
which are logistical issues with studying the deep sea in general. One of the
most common ways of studying DVM or the DSL is through the employment of acoustic
echosounders (i.e., colossal fish finders). Similar to the fish finder on a
small vessel, this does not give us species-level identifications. However, different
animals will bounce back different frequencies according to their
characteristics. For example, fishes with a swim bladder (air-filled sac
that controls buoyancy) respond to a different frequency than fishes devoid of
a swim bladder. We can identify certain taxa (groups of
related organisms) based on the acoustic frequency their bodies bounce back to
the echosounder. This information is still incredibly useful as researchers can identify migration rates of different types of animals. Some
studies have conducted transects (sampling method of collecting data
while moving in a straight line) with acoustics and then immediately resampled the same
location with trawl nets, but this is logistically challenging and expensive.
Broader Impacts
Other than a general curiosity of understanding the biology of some of the most abundant organisms on Earth, there are global implications to learning about diel vertical migration. Protected and commercially important species (ex., whales, tunas, dolphins) eat deep-sea organisms as a significant fraction of their diet, many of which becoming available to the predator because of DVM. Knowing which species practice DVM, how many animals are moving daily, and which depths these animals occupy allows us to estimate the prey available to predators. Therefore, we can refine computer models that are used to estimate future population levels of protected and commercially important species. These population size estimates tell us whether a species is threatened, vulnerable, endangered, or not currently threatened, so it is important for conservation that we do everything we can to provide accurate estimates. More so towards the DVM participants themselves, the active transport of nutrients (carbon or otherwise) from the surface into the abyss accelerates the rate that sustenance reaches other deep-sea animals that are confined to the bottom of the ocean.
Suggested Peer-Reviewed Literature (May not be open access)
Boswell, K.M., D’Elia, M., Johnston, M.W., Mohan, J.A., Warren, J.D., Wells, R.J.D., Sutton, T.T., 2020. Oceanographic Structure and Light Levels Drive Patterns of Sound Scattering Layers in a Low-Latitude Oceanic System. Front Mar Sci 7, 51. https://doi.org/10.3389/fmars.2020.00051
Brierley, A.S., 2014. Diel vertical migration. Curr Biol 24, R1074–R1076. https://doi.org/10.1016/j.cub.2014.08.054
D’Elia, M., Warren, J.D., Rodriguez-Pinto, I., Sutton, T.T., Cook, A., Boswell, K.M., 2016. Diel variation in the vertical distribution of deep-water scattering layers in the Gulf of Mexico. Deep Res Part I Oceanogr Res Pap 115, 91–102. https://doi.org/10.1016/j.dsr.2016.05.014
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