ARMS units provide a sampling of seabed life

ARMS units provide a sampling ...
An ARMS unit, loaded up with marine organisms
An ARMS unit, loaded up with marine organisms
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An ARMS unit, loaded up with marine organisms
An ARMS unit, loaded up with marine organisms

While it's important for biologists to know which organisms have colonized the seabed in a given area, excavation is both labor-intensive and ecologically unsound. That's where the Autonomous Reef Monitoring Structures – or ARMS – are made to come in.

The ARMS program is an initiative of the Smithsonian Institution, hosted at Washington DC's National Museum of Natural History.

Each individual ARMS unit consists of one square PVC base plate, with nine other slightly smaller plates stacked horizontally on top of it. They're held in that arrangement via stainless steel hardware, with spacers creating gaps between them. The resulting three-dimensional structure is designed to mimic hard bottom marine substrates.

Networks of ARMS units are anchored to the seafloor across the region being studied, then left in place for months or even years. During this time, they're colonized by the same organisms that are colonizing the surrounding seabed – such organisms could include coral, algae, crustaceans and molluscs.

After the observation period is over, the units are pulled up to the surface and analyzed in a laboratory. Utilizing a combination of genetic analysis, image analysis and plain ol' eyeballing, researchers are able to determine which species are present, and in what quantities.

"On a single structure in Crete, we were able to identify the presence of 15 non-indigenous species," says Assoc. Prof. Matthias Obst, from Sweden's University of Gothenburg. "We knew that the region was under extreme pressure from maritime traffic from the Red Sea, but were really surprised to see that the number was this high."

Obst is the lead author of paper recently published in the journal Frontiers in Marine Science, describing a research project in which 130 ARMS units were deployed on the ocean floor throughout Europe.

Sources: AquaTT, ARMS

I'm not entirely convinced that these ARMS units accurately reflect what is found on the surrounding seafloor. Since these horizontal units are stacked, they provide a sheltered version of what is out there, which may attract organisms to colonize there as opposed to the open seabed thus giving a skewed impresssion.
If I'm not mistaken, PVC is not natural. How do they compensate for organisms that have evolved to live on natural surfaces and won't get near PVC?