Seafloor Ecology Spring 2020

The ROV is loaded and ready to distribute small logs along the seafloor at 500 meters (1,640 feet). Fourteen years ago, Chief Scientist Jim Barry and Postdoctoral Fellow Craig McLain randomly distributed small logs throughout 500 square meters (5,381 square feet) of seafloor to look at the fate and development of wood-fall communities in the deep ocean. In November 2006, 36 pieces of acacia wood sewn into fine mesh bags were laid out on the seafloor at approximately 3,200 meters (10,498) deep off the coast of Monterey Bay. Five years later on October 2011, those logs were recovered and painstakingly picked of fauna that colonized and consumed them. Questions arose from that work, most curious among them were the surprising variability in number, timing, and type of species that arrived to live and dine on the logs—especially between logs separated by just a few meters.

On today’s final dive of our expedition series, MBARI’s Benthic Ecology Group, working with Janet Voight (Field Museum Investigative Research Center) and Julia Sigwart (Coastal and Marine Sciences Institute, University of California, Davis) are revisiting this work.

Late Friday evening, the ROV Doc Ricketts’ large drawer was neatly loaded with forty-one, 1’ x 4” x 4” sections of Douglas fir lumber. In a smaller drawer, the vehicle carried an acoustic beacon that will be placed on the bottom and used to relocate the experiment in the future. The 3,075-meter dive (10,088-feet) site out in the fan of Monterey Canyon is where mesh-wrapped fir blocks were laid out regularly along two axes on the seafloor, about 500 meters (1,640 feet) long, one heading east, one north.

Even at nearly two miles deep, the navigating ability of the ROV allowed researchers to place wood at predetermined spots along these track lines with meter-level precision. The gridded arrangement of this wood fall experiment may help us gain insight on the role that current direction plays at these depths and how it influences the possible episodic colonization of deep-sea animal larvae.

The most abundant colonizers found in previous wood fall studies are boring clams in the family Xylophagaidae. Xylophaga are deep-water relatives of shallower, wooden-boat-fouling clams with the misplaced moniker, shipworm. Teredinidae, the shipworm family, are the termites of the sea. They can wreak havoc on wooden structures submerged in saltwater—ships, docks, and pier pilings. Xylophaga, their smaller deep-sea cousin, also excavates holes in wood, slowly burrowing by the movement of its foot and hard bivalve shell over time. The hollowed-out homes they make while feeding eventually become their wooden caskets. They grow as they bore and the small settlement opening at the wood’s surface eventually turns into a passageway they can no longer escape through. These small encapsulated “ecosystem engineers” of the deep sea contribute to the organic matter that surrounds wood falls and enhances food availability for other animals in the community. A halo of life composed of bacteria, fungi, and a diverse assemblage of deep-sea invertebrates grows up in, on, and around the wood. Yet, our previous study oddly found that some logs even in close proximity can show little to no signs of colonization. We will return to this site in a few years to take video, retrieve some lumber to sample, and see if this latest experiment helps answer some of our questions about how, when, and where these deep-sea animals colonize deep-sea wood falls.