Metabolism and chemical composition of four pelagic amphipods in the Oyashio region, western subarctic Pacific Ocean

Abstract

As a basis for estimating trophic functions of amphipods in the pelagic ecosystem, oxy- gen consumption and ammonia excretion rates were determined for 4 amphipods, Themisto pacifica, T. japonica, Primno abyssalis and Cyphocaris challengeri in the Oyashio region in the western sub- arctic Pacific Ocean. Water, ash and elemental composition (carbon, C; nitrogen, N) were also deter- mined. For the 4 amphipod species, water content ranged from 67.8 to 89.7% of wet weight, ash from 15.2 to 46.5% of dry weight, C from 27.4 to 67.1% of dry weight, and N from 5.3 to 11.8% of dry weight. Comparisons of weight-adjusted metabolic rate (AMR) among amphipods showed T. pacifica to be the most active species (15.2 µl O2 body N - 0.90 h -1 ), followed by T. japonica (10.3), P. abyssalis (10.2) and C. challengeri (5.9). The first 2 species changed order when the AMR for ammonia excre- tion was determined (T. japonica, T. pacifica, P. abyssalis and C. challengeri). The atomic ratio of O:N derived from oxygen consumption and ammonia excretion data varied by species, ranging from 18.7 (T. japonica) to 85.2 (C. challengeri ). An alternate method of assessing metabolic activity using daily metabolic losses in body C (from oxygen consumption data) and N (from ammonia excretion data) was also computed. The results led to a similar conclusion to that reached from AMR results. The interspecific differences in metabolism and body composition among these 4 sympatric amphipods are discussed in terms of species-specific vertical distribution patterns and differing food habits. The present results are compared with published data on congeneric amphipods in other regions.