Utilization of salt marsh plants by post-larval brown shrimp: carbon assimilation rates and food preferences

Changes in stable carbon isotope ratios were monitored at 4 d intervals for postlarval Penaeus aztecus Ives reared on plant foods representative of those found within a Spartina alterniflora Loisel salt marsh. Plant materials fed to shrimp, individually and In combination, included Skeletonema costatum (Grevffle) Cleve, Isochrysis sp., Spartina detritus, and epiphytes that grow on Spartina. Results of carbon isotope analyses indicated that the most rapid changes m tissue 6I3C values occurred when shrimp were fed S. costatum alone or all foods combined. In both of these treatments the half-life of tissue carbon was reached before the first doubling of weight. Although shrimp fed epiphytes showed growth, significant assimilation of diet carbon was not detected. Food preferences were assessed with those materials which promoted growth (i.e. S. costatum and epiphytes) and, although there was no preference for S. costatum and epiphytes together compared to epiphytes alone, selection for both of these materials was greater than for S. costatum alone. Results indicate that (i) certain plants common in Spartina salt marshes, such as the diatom S. costatum, can be important for metabolic maintenance in postlarval P. aztecus and (ii) postlarval brown shrimp may have substrate preferences that are not related to plant food value.