Feeding ecology of Rhizophysa eysenhardti, a siphonophore predator of fish larvae1

Analysis of the gastrozooids (stomachs) of 646 colonies of the cystonect siphonophore Rhizophysa eysenhardti collected by hand in the Gulf of California showed that these specimens had fed only on fish larvae. Siphonophore density during bloom conditions averaged 0.038 ± 0.004 individuals · m ‒3 in an open‐water channel, and 0.91 ± 0.55 in a shallow cove. Siphonophore feeding was greater in the cove where fish larvae were more abundant than in the channel, and during dawn and dusk in the cove when prev were more available. Gastrozooid analysis and behavioral experiments showed that siphonophores fed only in the light. Digestion of fish larvae required 3–7 h, increasing with prey size. Food was assimilated by the siphonophores with 72% efficiency. Each siphonophore was estimated to consume an average of 8.8 fish larvae during a day in the cove. The heavy predation on fish larvae by R. eysenhardti suggests that both fish mortality and siphonophore population growth may be strongly affected.