overlying
water. In similar studies, nitrate respiration has also been reported
in the endosymbionts from the vestimentiferan Riftia
pachyptila (27) and the solemyid Solemya reidi (47). Because
the symbionts found in these three systems have been shown to
respire nitrate, it has been proposed that nitrate may be an
important electron acceptor among chemoautotrophic bacteria
involved in this type of symbiosis (25). Previous studies have
reported the presence of nitrate reductase activity in the gill
tissue of C. orbicularis as well (4, 16).