Assimilation of trace elements and carbon by the mussel Mytilus edulis: Effects of food composition

Assimilation efficiency (AE) is an important physiological parameter in determining trace element influx from food sources into aquatic animals. We used radiotracer techniques to examine the influence of diet [seven species of algae (two diatoms, two chlorophytes, a prasinophyte, and two dinoflagellates) and glass beads] on the assimilation of seven trace elements (Ag, Am, Cd, Co, Cr, Se, Zn) and C in the mussel Mytilus edulis. Trace element assimilation was related to C assimilation and cytoplasmic distribution in the algae and to gut passage time in the mussels. Mussels displayed different C AEs for the different algal diets; the chlorophytes, which had highly refractory cell walls, were the least digestible food. Assimilation of Cd, Se, and Zn was directly correlated with C assimilation; for Am, Ag, and Cr, no relationship with C assimilation was apparent. For each species except the chlorophytes, AEs of all elements significantly correlated with their cytoplasmic distribution within each algal cell. Among all species, AEs of Am, Co, and Se also increased with elemental penetration into the cytoplasm; however, this relationship was not statistically significant for Ag, Cd, or Zn. With the exception of Cr, AEs of elements increased with gut passage time, implying more efficient digestion- absorption when the element was retained longer in the digestive tract. In waters containing large mussel populations, unassimilated particle- reactive elements should be removed from suspension and enriched in biodeposits in sediments, whereas assimilated metals should be enriched in mussel tissues.