is a large majid crab that ingests the tubes along with soft body 

 parts (Tunnicliffe and Jensen 1987). 



All examples discussed in this paper are from sites where 

 hydrothermal venting occurs on a sediment-starved volcanic 

 terrain. Even the largest mineral deposits found in this setting 

 are much smaller than some recently discovered sediment-hosted 

 deposits where hydrothermal fluids pass through a seafloor 

 sediment cover before venting into the water column. Physical and 

 chemical interaction between sediments and hydrothermal fluids 

 forms and traps minerals within the sediment blanket. In this 

 environment biological influences on mineral deposition will 

 clearly be insignificant compared to the effect of the sediments. 

 Nonetheless, it is interesting to point out that some of the 

 mechanisms by which sediment cover enhances mineral deposition - 

 restriction of vent f luid-seawater mixing, abundance of solid 

 surfaces for mineral nucleation, etc. - are similar to those 

 proposed for biological structures. 



ACKNOWLEDGMENTS 



In addition to support from the National Undersea Research 

 Program, this work was supported by the University of Victoria, 

 the Natural Sciences and Engineering Research Council (Canada), 

 the Department of Fisheries and Oceans (Canada), IFREMER 

 (France), and the French Ministere des Relations Exterieures. We 

 thank C.L. Singla for preparation of specimens for electron 

 microscopy and Tom Gore for photographic plates. 



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