The relationship of diet to growth and ammonium excretion in salt marsh fish

Abstract

Detritus is an abundant but poor quality food source for consumers in salt marsh ecosystems. Here we present results of feeding experiments to determine the ability of Fundulus heteroclitus, Cyprinodon variegatus, and Mugil cephalus, three major detritivores in Great Sippewissett Marsh, Massachussetts, to assimilate detritus and use it for growth. C. variegatus, the sheepshead minnow, gained weight on a detrital diet, but F. heteroclitus, the marsh killifish, and juvenile M. cephalus, the striped mullet, lost weight and suffered high mortality on detrital diets. C. variegatus is a herbivore with morphological adaptations for ingesting plant material. F. heteroclitus is a carnivore poorly suited to effectively assimilate detritus from the diet. Although adult M. cephalus are adapted for ingesting and assimilating detrital material, the young may lack these adaptations and thus do not assimilate detritus. C. variegatus excretes ammonium at a lower rate than F. heteroclitus when fed animal food, or when starved for short periods of time. This protein sparing effect could be crucial for survival when detritus is the only food available at certain times of the year. While both C. variegatus and M. cephalus are visitors in the marsh and may leave to exploit food sources elsewhere, F. heteroclitus is a year-round resident and is the most abundant species of fish in the marsh. Yet when high quality food sources become scarce in late summer and fall, detritus, although plentiful, is apparently not a suitable alternative.