specimen collected by the Johnson-Sea-Link II

A large “live rock” specimen is collected by the Johnson-Sea-Link II at St. Augustine Reef. Click image for larger view.

Life on a Rock

July 29, 2002

Dr. Leslie Sautter
Geology Department, College of Charleston
Project Oceanica

camera icon Watch a video of a "live rock specimen". (mp4, 2.8 MB)

One of our mission objectives is to investigate the rock that makes up the foundation of the underwater oases. On each dive we are reassured that where there is rock, there is life! Where no rocks occur on the seafloor (except buried beneath the surface), the seascape consists of a blanket of sand that has few visible forms of biota. The rocks that outcrop, however, are completely enveloped in invertebrate organisms, to the point that not a square centimeter of the actual rock surface is exposed.

rock photographed with a scale

Each rock is photographed with a scale before we begin to pluck the invertebrates from it. Many specimens were kept alive in our shipboard aquarium. Click image for larger view.

To collect rock samples, we use the sub’s powerful manipulator arm. Often the rock is too heavy to lift, or it is firmly attached to the underlying rock pavement. Sometimes we are able to recover a great specimen, which is then placed in the sub’s collection basket. Once we are back on deck, the rock is hauled to the wet lab for examination. I call this the “ooh-ahh phase” of the research, when we examine and photograph the incredible biotic cover of the live rock and begin documenting the specimen. It is at this point that everyone—biologists, geologists, ship’s crew, and others—all share in the excitement of the exploration. Watch the video to see the initial on-deck processing of rock samples.

Today we’ve had plenty of extra time to examine the three rocks we recovered yesterday at the St. Augustine Scarp. On each of these specimens, thick mats of brightly colored algae and encrusting forms of both bryozoa and sponge wrap the rock surface in a living cloak of vibrant color. Soft coral, bivalves (e.g., clams and mussels), barnacles, and polycheate tube worms all claim their own territory, affixed to either the rock surface or in self-made, rock-lined bored holes. A variety of small crustaceans (e.g., crabs and amphipods), echinoderms (e.g., sea urchins and brittle stars) make their homes among the pits and holes, foraging through the dense overgrowth of their surrounding environment. Worms that make specialized tubes in which to live are numerous and can be found in a variety of forms.

closeup of species on collected rock

A turkey wing mussel (Genus Arca) is found living within a bored hole, firmly attached to the rocky substrate. Click image for larger view.

All these invertebrates make their home on the available hard seafloor, or “rocky substrate,” as they require an immobile surface on which to live. The nearby Gulf Stream supplies the region with strong currents, making the shifting, sandy seafloor difficult to inhabit. This diverse group of opportunistic invertebrates takes great advantage of any exposure of such “hardground.” The rock-bound communities differ slightly from rock to rock, perhaps due to local currents or other environmental conditions. Local predators may also influence the community composition.

tiny serpulid worm tubes

Tiny tubes built as housing by serpulid tube-worms were clustered in a small forest-like grove on one of the rocks collected. These tubes are approximately 0.5 cm in length and were viewed through a dissecting microscope on board the ship. Click image for larger view.

Once we return to shore, the rocks will be dried until they no longer have an overpowering odor of decay (!). Then they will be broken into smaller pieces and analyzed for their mineralogical content (composition) and grain size. While on the ship, however, we will continue to examine them as living rocks and value them as the foundation of the deep underwater oases along the edge of the southeast U.S. continental shelf.







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