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Amy R. Baco*, Craig R. Smith*, Andrew S. Peek**, and Robert C. Vrijenhoek
*University of Hawaii, Department of Oceanography, 1000 Pope Road, Honolulu, Hawaii 96822, Email: abaco@iniki.soest hawaii.edu
*Center for Theoretical and Applied Genetics, Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey 08903-0231
Whale skeletons on the deep-sea floor may be important habitats for organisms dependent on chemoautotrophy, acting as sulfide-rich stepping-stones for dispersal. Numerous adult and juvenile vesicomyid clams have been collected at two whale fall sites on the California Slope, one at 1240 m in the Santa Catalina Basin and one at 960 m near San Nicolas Island. To identify the clams, DNA from mitochondrial cytochrome c oxidase I (COI) was amplified and sequenced from muscle tissues of nine adults and one juvenile. Phylogenetic analysis reveals that the vesicomyids collected on the whale falls represent three clades (putative species) known from other reducing habitats. Seven adult whale clams fall within a Vesicomya gigas/Calyptogena kilmeri vent clade; the juvenile clam falls into the V. gigas/C. kilmeri seep clade; and another adult falls near Calyptogena elongata. The final adult did not associate with any clade and may represent a newly collected species. Within their clades, most whale clam individuals are within the 1-2% range observed for intraspecific mitochondrial COI variation for conspecifics from the same geographic location. All are within the observed 7-9.5% range for conspecifics from different geographic locations. These data suggest that three of the vesicomyid taxa found on whale falls are members of distinct species that have been collected previously from other deep-sea sites. With the inclusion of the three vesicomyids, a total of fourteen species found on whale falls are now known to overlap with other reducing habitats, such as vents and seeps. Based on mitochondrial COI sequence variation, vesicomyids had a last common ancestor 22~8 mya, which coincides with the appearance of large whales in the fossil record about 40 mya.
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