What are ARMS?

ARMS, short for Autonomous Reef Monitoring Structures, are stacks of 9 PVC plates that create complex structure for organisms to live in. ARMS are deployed by hammering the structures into the ocean floor with long metal stakes and letting them sit for months to years. Over time, invertebrates settle onto (or “move in” to) the ARMS, leading some to nickname them invertebrate hotels. After organisms have had time to settle, researchers retrieve the ARMS by covering them with a lined milk crate to stop animals from escaping and taking the covered units back to land.

Back in the lab, the units are taken apart and every organism attached to or living in the ARMS is identified. Animals that move around (i.e.motile organisms) are sorted based on size. If animals are larger than two millimeters (mm), they’re sorted by shape and photographed. If animals are smaller than 2mm, they’re lumped together and sieved into two different size fractions (500 μm and 106 μm). Each of these fractions are analyzed using genetic techniques to identify organisms that cannot be identified visually. Animals that can’t move around (i.e. sessile organisms) are accounted for by: (1) photographing the PVC plates and (2) scraping everything off the plates, blending it up, and using genetic techniques again to identify what was living on the ARMS.


Prior to the development of ARMS, studies that measured how many and which organisms were living at a site (otherwise known as the biodiversity of a site) were difficult to compare because methodologies varied considerably from study to study. For instance, take the example of scientists interested in determining whether oyster reef biodiversity is changing in Beaufort, North Carolina as a result of climate change and human development. To compare how diverse the reefs used to be to how they are now, researchers might want to compare studies that have been quantifying biodiversity in Beaufort over the past few decades. However, because each study uses a different method to measure diversity, it is impossible to tell whether the changes they see in the data are because of real changes in community composition, or because of changes in the methodologies used. ARMS were developed in response to this problem.

The back of the car loaded up with ARMS

Because the ARMS procedure is so standardized, biodiversity around the world can be compared wherever scientists deploy ARMS. The Smithsonian Institution is using ARMS to catalogue and study marine biodiversity across the globe (learn more at: http://oceanarms.org/) and the National Oceanic and Atmospheric Administration (NOAA) uses ARMS in their National Coral Reef Monitoring Program (NCRMP) and their Ocean Acidification Program. Researchers have deployed ARMS in a range of sub-tidal ecosystems, including deploying ARMS at 200m using a human-operated submarine, deploying ARMS on coral reefs using SCUBA, and deploying ARMS on shallow oyster reefs using only waders.

Jason tying buoys and temperature loggers to ARMS at the Duke University Marine Lab


As part of our Bass Connections project, we deployed ARMS on three reefs in Beaufort, North Carolina. We’re going to use the ARMS to get robust, standardized measures of biodiversity for each reef and compare those measurements to acoustic measures of diversity to test whether bioacoustics could be used to monitor biodiversity. We’ll also upload our ARMS data into the global Smithsonian database of marine biodiversity, which will increase our global understanding of marine biodiversity and potentially inspire new projects looking at changes in biodiversity over time.

Post by Julianna Renzi, Biodiversity team