To understand the tube growth process of Rlftia pachyptila, morphological aspects of worms and their tubes were studled In parallel, tube secl-etlon experiments were perfol-med on live animals, In pressure aquarla. Dry w e~g h t s of the secretions, along with their chitin content (a major component of the tube) were used to quantify tube productlon Our results show a variation of the gross morphology of the plume and the trunk of R. pachyptlla during ~t s growth and indicate that vestlmentum

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... ngth and tube diamctrr could b e useful indices of lndlv~dual and tube sizes of R pachyptila. The presence of clumps of freshly secreted tube material at the base of the exoskeleton as well as the new observation of bifid tubes allow us to propose a model of tube growth at both ends. In t h~s model the tube growth would exhibit a moulting-like step. Bifid shapes may help in space displacement, and the modification of the positioning in height relative to a vent could be used by a n individual to modify its access to vent fluid. Tube growth at the apical end facing the plume has been quantified as a m~n~r n a l estimate of the rate of tube production. Experiments performed on repressurized worms indicate that 1 mm2 of worm vestimentum area may secrete more than 2 pg of dry weight tube m a t e r~a l per day, leadlng to a minimum tube growth rate of 14 cm yr.'. When compared to other marine ecosystems, it is obvious that chitin productlon per unit of area of the the vent communities, based on the R. pachyptila alone, a r e the highest recorded and s~milar values were only recorded in polluted freshwater environments.