Phenotypic variations in the gills of the symbiont-containing bivalve Lucinoma aequizonata

The marine bivalve Lucinoma aequizonata (Lucinidae) maintains a population of sulfide-oxidizing chemoautotrophic bacteria in its gill tissue. These ar e housed in large numbers intracellularly in specialized host cells, termed bacteriocytes. In a natural population of L. aequizonata, striking variati ons of the gill colors occur, ranging from yellow to grey, brown and black. The aim of the present study was to investigate how this phenomenon relate s to the physiology and numbers of the symbiont population, Our results sho w that in aquarium-maintained animals, black gills contained fewer numbers of bacteria as well as lower concentrations of sulfur and total protein. Ni trate respiration was stimulated by sulfide (but not by thiosulfate) 33-fol d in homogenates of black gills and threefold in yellow gill homogenates. T he total rates of sulfide-stimulated nitrate respiration were the same. Oxy gen respiration could be measured in animals with yellow gills but not in a nimals with black gills. The cumulative data suggest that black-gilled clam s maintained in the aquarium represent a starvation state. When collected f rom their natural habitat black gills contain the same number of bacteria a s yellow gills. Also, no significant difference in glycogen concentrations of the host tissues was observed. Therefore, starvation is unlikely the cau se of black gill color in a natural population. Alternative sources of nutr ition to sulfur-based metabolism are discussed. Denaturing gradient gel ele ctrophoresis (DGGE) performed on the different gill tissues, as well as on isolated symbionts, resulted in a single gill symbiont amplification produc t, the sequence of which is identical to published data. These findings pro vide molecular evidence that one dominant phylotype is present in the morph ologically different gill tissues. Nevertheless, the presence of other phyl otypes cannot formally be excluded. The implications of this study are that the gill of L. aequizonata is a highly dynamic organ which lends itself to more detailed studies regarding the molecular and cellular processes under lying nutrient transfer, regulation of bacterial numbers and host-symbiont communication.