Virtually all species of Hyperiidea have a symbiotic relationship with gelatinous zooplankton. It has been theorized that hyperiid amphipods are descendants of benthic crustaceans that have developed a unique benthic-like existence on the pelagic substratum provided by gelatinous zooplankton. But does this strategy have hidden metabolic costs? Metabolism, in the form of both direct respiration and enzyme activities, and proximate composition of Phronima were compared to pelagic and benthic crustaceans. Although less energy is needed to remain in the water column since the gelatinous zooplankton makes the Phronima more buoyant than just Phronima alone, metabolic rates of Phronima are not lower than those of other pelagic crustaceans, possibly due tothe energy needed to propel the barrel. However, Phronima have lower protein concentrations, indicating less energy is allocated to muscle production. The ratio of LDH (L-lactate dehydrogenase) activity to CS (citrate synthase) activity was >1, indicating that burst swimming is important in Phronima's metabolism. The symbiotic relationship provides the Phronima with food and a substrate for the brooding of young, but does not give them an energetic advantage over other pelagic crustaceans.
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