Reef Biodiversity, an introduction:

by Russell Moffitt and Molly Timmers

Coral reefs have been dubbed the rainforests of the sea due to their extraordinary biodiversity. They are among the most diverse and biologically complex marine ecosystems in the world even though they represent only 0.2% of the area in the ocean. Yet, the magnitude of their biodiversity is uncertain.  Estimates of the number of coral reef species range into the millions, though mankind has only identified and described a small handful. Moreover, many coral reefs are threatened by anthropogenic and environmental stressors including climate change, ocean acidification, resource exploitation, marine debris, sedimentation, invasive species, and other factors. Because even the broad dynamics of coral reef decline and recovery are poorly understood, it is difficult to predict the long-term impacts of human activities on them. Without robust knowledge of coral reef biodiversity, detecting changes in reef assemblages and investigating causes of such change will be impossible. Developing universal sampling methods and protocols is therefore imperative to establish this necessary baseline against which we can make spatial and temporal comparisons in the future.

Due to the presence of long-standing taxonomic expertise and the relative ease in sampling them, fish, corals and some macroinvertebrates have been well documented. However, this is not the case with the lesser known and cryptic marine invertebrates which compose the majority of the species that inhabit coral reefs. The difficulty in extracting these small organisms from the reef matrix has hampered broad-scale diversity investigations. Thus, methods that can successfully sample the lesser known coral reef fauna need to be developed.

An ARMS unit attached to the reef
awaiting occupants

Autonomous Reef Monitoring Structures (ARMS) were developed by CRED in conjunction with the Census of Coral Reefs Project (CReefs) of the Census of Marine Life (CoML) to serve as a method to detect the cryptic fauna on reef systems. By mimicking the structural complexity of benthic habitats, they are designed to be colonized by an array of mobile and sessile invertebrates as well as crustose and turf algae. The ARMS contain 9 layers: a green mesh layer providing “habitat” for organisms such as polychaetes, sipunculids, and acorn worms (hemichordata); four open layers for sessile organisms such as sponges, bryozoans, bivalves, and tunicates; and four semi-closed layers that attract cryptic motile fauna such as galatheid and xanthid crabs, alpheid shrimp, and nudibranchs.  Additionally, as sessile organisms colonize the structure, they create additional complexity and potential habitat for other organisms.

Divers intall an ARMS unit

Two divers install the ARMS on the seafloor by pounding stakes into the reef and then securing the ARMS unit to the stakes. Two 7 lb weights are also attached to each ARMS to help keep the unit in position during heavy currents or wave surge. Once installed, each ARMS unit remains on the sea floor for two years.  ARMS were deployed at Howland and Baker Islands during ASRAMP 2006 and will be removed from the reef during this cruise. The species that have colonized the ARMS over the past couple years will be systematically assessed using both taxonomic and molecular genetic analyses in order to compare indices of cryptic biodiversity across diverse biogeographic and habitat gradients thereby facilitating the monitoring of changes in these assemblages over time.

To date, ARMS have been deployed widely in tropical seas across the globe. Current sites include Moorea, Australia, Reunion, Brazil, Hawaii, American Samoa, the Marianas Islands, Panama, Belize, Papua New Guinea, and the U.S. Central Pacific Islands. They will soon be deployed in the Cayman Islands, Puerto Rico, Indonesia, and the Seychelles. Data from the ARMS will be used to determine the degree to which the communities recruiting to these artificial structures are representative of the reef communities in which they are deployed.  While NOAA conducts a broad suite of reef monitoring and observing techniques, the ARMS will provide insights into the components of the coral reef community that SCUBA divers cannot directly quantify.