Hidden Diversity

By Scott Godwin

I have had the privilege of participating in expeditions to the Northwestern Hawaiian Islands every year since 1999. My participation has always involved documenting and collecting the hidden animals that make up a majority of the diversity on coral reefs. I am a marine invertebrate zoologist but I do not focus on corals. I instead look at the variety of invertebrate organisms evolved to use coral reefs as a home. Since these animals hide or are camouflaged, most people do not have many chances to see them while diving on coral reefs. While my colleagues are counting fish and documenting corals I usually am looking under rocks or have my head stuck in a hole. Most everyone is familiar with crabs, snails and sponges but do not realize the important role they play in the existence of coral reefs and that some of them represent our closest relatives in the ocean. Furthermore the beauty that these organisms possess can rarely be seen since they are able to hide so well.

A set of ARMS underwater at French Frigate Shoals, Northwest Hawaiian Islands. Photo by Scott Godwin

It is always a challenge to document and collect these hidden organisms with the short time allowed underwater on SCUBA. I was asked to participate on a project on this research cruise, which is being run by the National Oceanic and Atmospheric Administration, Pacific Island Fisheries Science Center, Coral Reef Ecosystem Division (CRED). The CRED is partnering with the Census of Marine Life (CoML), Census of Coral Reef Ecosystems (CReefs) scientists in the development of Autonomous Reef Monitoring Structures (ARMS) to provide a systematic, consistent, and comparable method to monitor these hidden organisms. These ARMS mimic the complex habitat present on a coral reef that is used by a variety of crabs, worms, sponges and other organisms. The ARMS are discrete structures composed of a series of stacked plates that can be placed on the seafloor, left for a certain time and then retrieved. This allows the collection and documentation of many different species that take up residence in the ARMS without damage to coral reef habitat. These organisms will be identified and documented through a combination of taxonomy and genetic methods. I am assisting CRED scientific staff in the retrieval and deployment of ARMS, as well as taxonomic documentation of specific coral reef organisms that are collected by the structures.

An ARMS plate with colonial sea squirts and bivalve mollusks. Photo by Kerry Grimshaw

I have spent many years identifying crabs in coral reef habitats throughout the tropical Pacific. We are attempting to use the variety of crab species documented from ARMS as an indicator of diversity in various coral reef habitats here in the Northwestern Hawaiian Islands. Crabs are some of my favorite species to work with and I get to see species that are normally hard to collect under usual circumstances. I identify crab species collected everyday and help process all the other material. Crabs are one of the most diverse marine invertebrate groups found on coral reefs. They range from large carnivores that prowl the reef to small species that live within the arms of coral colonies.

The sponge Leucetta on natural reef habitat. Photo by Scott Godwin

There are a great number of other species that take up permanent residence on the ARMS but never move again; also known as sessile species. The most common species are tunicates, or sea squirts. These can exist as solitary species or multiple individuals that form a colony. Colonial tunicates are brightly colored and appear as a thin slimy layer on the ARMS plates. Sea squirts are in the Phylum Chordata, which also includes humans. These species along with sponges, clams and other sessile fauna consume food from the water column and are prey for other coral reef residents. Species like sea slugs, marine worms and hermit crabs consume sessile fauna as they move across surfaces. Sea slugs can be found on ARMS since they are attracted to the food provided by the sponges and colonial sea squirts. Other species like sea cucumbers and sea stars are found with the ARMS because ARMS provide a place of refuge. Since the ARMS are meant to mimic reef habitat a variety of mobile and sessile organism take up residence.

The crab Trapezia tigrina living within the branches of a coral. Photo by Scott Godwin

The ARMS team leader, Kerry Grimshaw, oversees the entire process to make sure we stick to set protocols and provide concise data. A typical day for the CRED ARMS team begins by diving on a site and retrieving three ARMS and replacing them with new ones. The ARMS are returned to the lab on the Hi`ialakai for processing. They must be dismantled and all the plates photographed before we begin removing target species. Everything is preserved in jars of ethanol at the end so that further analysis can be done by genetic researchers at a later date. This process begins in the early morning and takes all day. Many of the CRED researchers from other projects come by to help out when they return from diving. We see cool stuff every day and this helps make the long hours in the lab much easier.

The sea slug Chromodoris tinctoria. Photo by Scott Godwin

The species being documented by the CRED ARMS project represent the hidden diversity of a coral reef. The variety of animals mentioned above can be considered to be maintenance engineers for the coral reef. They consume excess organic material, provide food for many species, and integrate and recycle minerals important to reef structural integrity. The diversity and abundance of these hidden organisms can be considered a gauge of the condition of a coral reef. This is the reason behind the interest of the CRED in these organisms and the development of the ARMS program.