Feeding and metabolism of the siphonophore Sphaeronectes gracilis

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Abstract
The in situ predation rate of the siphonophore S. gracilis was estimated from gut content analysis of hand-collected siphonophores and from laboratory data on digestion rates of prey organisms. At daytime prey densities of 0.25 copepods l-1, S. gracilis was estimated to consume 8.1-15.4 prey day-1 siphonophore-1. From data on abundances of siphonophores and copepods, S. gracilis was estimated to consume 2-4% of the copepods daily. In laboratory experiments, ingestion rates averaged 13.8 prey day-1 siphonophore-1 at prey densities of 5 copepods l-1 and 36.9 at 20 copeods l-1. This was equivalent to a specific ingestion rate (for both C and N) of .apprx. 17% day-1 and 45% day-1, respectively, while specific ingestion in situ was only 2% day-1. Ammonium excretion averaged 0.095 .mu.g-atom siphonophore-1 day-1 at 5 prey l-1, and 0.162 at 20 prey l-1. The specific respiration (C) and specific excretion (N as ammonium) were calculated to be 3% day-1 at the lower experimental food level, and 5% day-1 at the higher food level.

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