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Smithsonian National Museum of Natural History
An autonomous reef monitoring structure sits on a coral reef.

Autonomous Reef Monitoring Structures

A large proportion of marine life on the sea floor is sessile – attached to the bottom – and forms crusts and films on hard substrate. These organisms, such as sponges and sea squirts, in turn provide habitat and food to mobile creatures like shrimp, snails, and worms that live hidden among the nooks and crannies. Such cryptobenthic creatures are challenging for scientists to sample and study even in easy to access shallow water habitats. The challenges of studying hard to detect species are even greater in deeper water benthic habitats, so the NMNH-MDBC team, with support from the Smithsonian Global ARMS Program, is deploying Autonomous Reef Monitoring Structures (ARMS) to help capture, describe and quantify Gulf of Mexico cryptobenthic biodiversity.

ARMS are standardized 3D benthic creature-collectors comprised of stacks of plates that mimic the complex structure of the sea floor: 

ARMS are installed on the sea floor for a period of time, allowing sessile creatures to colonize the plate surfaces and mobile ones to move in. The ARMS are then recovered by a team and taken apart to see what creatures are present. ARMS provide a picture of the benthic life in a particular region, and, because ARMS are a standard size and configuration, the resulting biodiversity census allows scientists to directly compare communities at different sites or see how a site changes over time.

Deployment

Square grey devices sit on a sandy sea floor next to red and white coral colonies in the darkness of the deep sea.
ROV view of ARMS units newly deployed and ready to recruit creatures in the deep Gulf of Mexico at ~1500 m. Photo credit: NOAA, ROV Global Explorer.
A device of stacked plates is covered with encrusting creatures similar to those growing on the sea floor around it. Small fish cover next to the device.
An ARMS unit in the mesophotic Gulf of Mexico after two years of recruitment and growth. Juvenile fish use the unit as protective habitat. Photo credit: Santiago Herrera, ROV Global Explorer.
A device of stacked plates sits on the sea floor. Both the grey-brown colored device and the rocks around it are coated with splashes of pink, orange, white, green and red.
ARMS recruit the local sessile community. This ARMS unit has come to resemble the surrounding mesophotic benthic habitat in the northwest Gulf of Mexico. Photo credit: Santiago Herrera, ROV Global Explorer.
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To retrieve ARMS from the sea floor, the units are first covered with a crate to capture the mobile creatures living inside. ARMS in shallow areas can be installed and retrieved by divers, but in mesophotic and deep habitats this is handled by a Remotely Operated Vehicle (ROV), which uses a manipulator arm to cap the units with a specialty latching crate.  

Once the ARMS is on the surface and in the ship’s lab, it is taken apart for scientists to count, document and preserve everything inside (See Specimen Curation and Biodiversity Characterization). Back at NMNH, DNA from each ARMS is extracted and sequenced to derive a genetic representation of the whole community. These resulting ARMS eDNA (see eDNA Reference Library) profiles are used to compare whole benthic communities across different locations in the Gulf or through time. 

Retrieval

A man in a life vest and hard hat guides a 4-legged platform of collecting baskets onto the deck of a ship.
MDBC partner Santiago Herrera (Lehigh University) guides an elevator used to retrieve ARMS onto the deck of the R/V Point Sur. Photo credit: Chris Meyer, Smithsonian NMNH.
A device made of stacked square plates sits in a black bin filled with water. The device is covered with pink algae and colorful encrusting creatures. A hand in a green latex glove holds a piece of skeleton.
An ARMS unit covered in encrusting creatures sits in a tub of seawater, ready for disassembly and processing. Photo credit: Chris Meyer, Smithsonian NMNH.
A man and woman in an industrial looking room look at a computer screen sitting next to a camera tripod. The woman guides a tray of water containing a colorful square plate.
Chris Meyer (Smithsonian NMNH) and Nicole Pittoors (Lehigh University) photograph ARMS plate surfaces on board the R/V Point Sur. Photo credit: Sarah Tweedt, Smithsonian NMNH.
A grey square on a white background is covered in color creatures, such as red and white sea squirts, white tubular sponges, and patches of yellow or pink colonial animals.
An ARMS plate from the mesophotic northwest Gulf of Mexico. Sessile creatures including tunicates, sponges, bryozoans, and worms grow across the plate surface. This ultra high-resolution image was assembled from 15 individual photos of this plate. Photo credit: Chris Meyer, Smithsonian NMNH, Nicole Pittoors, Lehigh University.
A white tray filled with water contains red striped shrimp, dark crabs, worms, pieces of shell, and algae.
A tray holds motile creatures – including shrimp, crabs and snails – that lived inside of an ARMS. These animals are sorted, photographed, and preserved for the museum collection. Photo credit: Sarah Tweedt, Smithsonian NMNH.
A woman in a teal t-shirt stands at a counter holding a clear plastic cup containing a crab in one hand and writing notes on a data sheet with the other.
Maria Granquist (College of Charleston) sorts ARMS motile creatures by morphospecies and records data. Photo credit: Sarah Tweedt, NMNH.
A bright pink and yellow slug with knobbly projections on its back, crawling on a black background.
A colorful sea slug (genus Doto) collected from Gulf mesophotic ARMS.
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