Methodology

Are we casting the right net? How sampling device influences our understanding of the open ocean

Reviewing: Hetherington, E. D., Choy, C. A., Thuesen, E. V. & Haddock, S. H. D. Three Distinct Views of Deep Pelagic Community Composition Based on Complementary Sampling Approaches. Front. Mar. Sci. 9, 1–10 (2022). DOI: 10.3389/fmars.2022.864004

 

Scientists generally say we know more about the surface of our moon than we do about Earth’s Ocean. This notion shouldn’t come as a surprise; the ocean is larger than the entire surface area of the moon while also invisible beyond its blue surface. There are still many discoveries to be made as scientists work to describe who lives in this vast habitat. Learn how our interpretation of this unknown world is fundamentally shaped by the tools we use to see into its dark depths.

The tools of the trade:

Scientists use two major tools to see who lives in the ocean: nets and remote-operated vehicles (ROVs). Each type has its own set of pros and cons. Nets are dragged through the water to capture animals in aggregate – particularly small plankton that can’t avoid the net’s movement. The downside to this method is that we can’t observe animals undisturbed or in their natural habitat, and the net can be a bit damaging to animals that are more delicate, like jellyfish or salps. However, with a physical specimen in hand, scientists can more easily identify animals, describe their morphology, and even perform DNA sequencing. Scientists also choose between different types of nets, which differ in the size of animals they can catch, and the depths at which they deploy.

ROVs, are much like robotic submarines. They are equipped with cameras and have grasping arms. Their ability to physically collect animals is quite limited, instead relying almost entirely on camera footage. The benefit of this method is that its non-intrusive — scientists are able to observe animals undisturbed, including their feeding, swimming, and reproduction. However, scientists often can’t achieve detailed images of the animals, which can limit their ability to identify the species, nor can they DNA sequence any of the individuals. Cameras also have a hard time observing the many small plankton that dominate the pelagic system and are instead more adept at observing nekton, like fish and large shrimp, as well as delicate animals, like ctenophores and octopus.

Left: Net being deployed off the A-frame of a research vessel. Right: remote-operated vehicle being deployed off the A-frame of a research vessel. Figures courtesy of Jessie Perelman, and Gabrielle Stedman.

A showdown to show down deep

Both nets and ROVs are valuable tools. Frankly, we wouldn’t be able to observe the pelagic ocean without them. Considering that each tool is biased in the types of animals observed, it’s important for scientists to understand if these tools are comparable. In other words, can scientists make the same kinds of descriptions of who lives in the pelagic ocean with each type of tool?

Dr. Hetherington and fellow marine scientists sought to answer this exact question. To do this, they compared the pelagic community description at Monterrey Bay, CA using two types of nets dragged from ships (trawls: one long net, and a “MOCNESS”: multiple nets opened at discrete depths) compared to ROVs equipped with cameras. Differences would be determined by the abundance and diversity of animals observed from each method.

Graphic depiction of the three sampling types and how they would look being deployed in the water. M: MOCNESS net, T: Trawl, V: remote-operated vehicle. Figure courtesy of Hetherington et al. 2022.

Bring the right tool for the job

The research scientists learned that these sampling tools gave very different descriptions of the pelagic ecosystem. Interestingly the trawl nets and ROV were the similar then the MOCNESS net and trawl net were too each other. This stark difference was driven by the disproportionate observations of jellyfish and other gelatinous animals from camera footage – in fact, only the camera methods could observe these delicate creatures.  It seems that MOCNESS nets limit our ability to observe jellyfish, but are a good option if we want to collect a lot of crustaceans and know the depths at which they were found. Thus, scientists should carefully consider their choice of sampling method.

In a sea of unknowns, how we understand the pelagic ecosystem is shaped by the tools we use to see places beyond our eyes. Sampling the pelagic ecosystem is challenging and no single sampling tool is sufficient to capture the diversity of the entire community. The best solution is for scientists to use multiple sampling methods to put all the pieces together.

 

 

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