USE OF DETRITAL FOODS AND ASSIMILATION OF NITROGEN BY COASTAL DETRITIVORES

Two killifish common in east coast U.S.A. salt marshes, Cyprinodon variegatus Lacepede and Fundulus heteroclitus Walbaum, differ in their ability to assimilate nitrogen from and grow on detritus. C. variegatus grew on a diet of detritus of Spartina alterniflora Loisel, while F. heteroclitus did not. In addition, when the fish were fed15N-labeled S. alterniflora detritus,15N:14N ratios in C. variegatus were higher than were ratios in F. heteroclitus. Therefore, even though both species ingest large amounts of detritus, C. variegatus makes more effective use of this portion of its diet. These dietary differences are corroborated by anatomical differences that suggest that C. variegatus should make better use of detrital or plant tissues than F. heteroclitus. In the label experiment, the degree of label in both fish was directly proportional to the degree of label in the food treatments. In previously published experiments designed to compare plant substrate with attached microbes as nitrogen sources for detritivores, %15N incorporated by a polychaete was also directly proportional to %15N in the detrital food. Therefore, it is difficult to distinguish between plant substrate and microbes as nitrogen sources for this detritivore. © 1990 Estuarine Research Federation.