Santa Monica clam and worm reef
May 21, 2013
Today, we benefited again from earlier MBARI efforts that mapped, and then surveyed, the Santa Monica South Canyon. Detailed bathymetric maps, generated by Dave Caress and Hans Thomas of the Mapping Autonomous Underwater Vehicle (AUV) Team, and processed by Krystle Anderson and Eve Lundsten of the Continental Margins Lab, allowed us to target large mounds that might signify active methane seeps.
The scientists on the previous leg of this expedition, led by Peter Brewer, sent us images of seeps with clams and worms that allowed us to target very specific locations for biological sampling. We were amazed at the immense field of clams and worm tubes at one of these mounds. When we explored the site we found extraordinary numbers of small elongate clams (typically less than five centimeters in length) so densely packed that they were jostling for position in the sulfide-enriched sediments.
These Ectenagena elongata clams were different from the larger clam species observed at the earlier seeps and Pedro’s whale fall. The clams in this family, the Vesicomyidae, host bacterial symbionts in their gills. These bacteria oxidize sulfides extracted by the clams from fluids seeping from the surrounding sediments. Because the symbionts are transmitted vertically through the clams’ eggs, each clam species in this family hosts a symbiont strain that has diversified with the host during the past 45 million years, a pattern called host-symbiont co-speciation.
We also sampled large numbers of vacant tubes that team member Greg Rouse (Scripps Institution of Oceanography) believes were occupied by serpulid annelids. Many of the tubes were encased in soft mudstones of unknown age and were partially fossilized. Interestingly, the reef structure found at this site was completely composed of these subfossil serpulid tubes. Below the serpulid tube reefs we found large, subfossil vesicomyid clams. Atop these two layers were modern, living Ectenagena elongata clams in extraordinary abundance. These exposed layers were visible within fractured regions of the seafloor (identified on the map by the black arrow) and were at least two meters thick. Using images recorded by the HD camera, we were able to create a high-resolution mosaic image which clearly shows the different layers which make up the Santa Monica clam and worm reef.
We collected samples from the subfossil layers in an effort to characterize the habitats found buried here. We will send subfossil clam and worm samples to taxonomists to determine what species they belonged to and we will also use aging techniques to estimate the time that they were alive. Using species, age, and high-resolution seafloor maps we will try to reconstruct the past with an aim of better understanding and predicting the future of the ocean.
This dive illustrates the nature of high-tech research in the deep sea. Seafloor maps created by mapping AUVs allow us to accurately target areas of scientific interest. These areas were surveyed using remotely operated vehicles (ROVs) during a previous research mission whose aims were chemical and geological in nature. Using information gleaned from those previous science efforts, we were able to pinpoint areas of biological interest, which allowed us to observe a habitat that was unlike anything we’ve ever seen in Monterey Bay or the Gulf of California. This combination of state-of-the-art tools, interdisciplinary research efforts, and collegial collaboration between scientists allows MBARI to continue its tradition of excellence in studying the deep.
A special thanks goes out to the pilots and crew for a successful cruise!
— Bob Vrijenhoek and Lonny Lundsten