Homeostasis in the essential amino acid composition of the marine copepod Euterpina acutifrons

The essential amino acid composition (EAA) of females and eggs of Euterpina acutifrons and their food was analyzed on copepods fed with different microalgae and collected from the field. There were not significant differences in the EAA of females, but the EAA of eggs varied according to food source. As predicted by homeostasis theory, a higher reproductive success was observed as the EAA of females and food became more similar. This chemical homeostasis explains the selective retention of amino acids observed in copepods, tells why conversion efficiencies of ingested nitrogen may vary according to the food source, and shows the importance of food quality on reproductive success in copepods. The most important biotic factors affecting herbivorous copepod production are considered to be: food concentration, capture efficiency (mainly determined by particle size and relative retention abilities), presence of toxic or inhibitory substances, and food quality. Food quality can be defined as the proportion and composition of biomolecules such as proteins, carbohydrates, lipids, etc., or elements. Although some specific fatty acids have been shown to be important for copepod production (Prahl et al. 1984; Jónasdóttir 1994; Jónasdóttir et al. 1995; Jónasdóttir and Kiørboe 1996; Pond et al. 1996), nitrogen (protein) also seems to be a limiting factor (Checkley 1980; Ambler 1986). This protein limitation could explain why copepods maximize nitrogenous ingestion by selectively consuming cells with a higher protein content (Cowles et al. 1988) and why amino acids are so efficiently digested (Cowie and Hedges 1996). However, besides the importance of the proportion of protein in the diet, studies carried out with copepods fed algal species with similar C/N ratios, in which protein was the limiting factor for egg production, showed differences in conversion efficiencies of ingested nitrogen to egg nitrogen (Støttrup and Jensen 1990). This result could be explained if: (1) the intraspecific composition of amino acids is as rigid as has been observed with the C : N : P ratio in major members of zooplankton (Andersen and Hessen 1991), and (2) a higher production efficiency of herbivorous zooplankton is obtained as composition of the diet becomes closer to the composition of the consuming organism, as is predicted by the homeostasis theory (Sterner 1990). To test these two hypotheses, experiments were conducted to determine whether reproductive success in the egg-sac carrier copepod E. acutifrons fed with different algal species and in copepods collected directly from the field varied ac-