A variety of hyperiid amphipod crustaceans may be closely associated (especially early in their development) with various types of gelatinous zooplankton, including medusae, ctenophores, siphonophores, and salps. The degree to which each hyperiid species is host-specific (i.e., restricted to just one or a few particular host species) remains unclear. Similarly, the question of the degree to which these associations are obligate (i.e., required for the hyperiid to complete its life cycle) remains controversial. The question of how much the hyperiids harm their hosts also remains controversial and presumably varies depending on the species involved (Gasca and Haddock 2004 and references therein). Hyperia medusarum and Lestrigonus shoemakeri are two parasites that may heavily infest the Pacific Sea Nettle in both the wild and in captivity (Crossley et al. 2009).

Larvae of Cancer gracilis crabs may be found hitchiking rides on Pacific Sea Nettles (Wrobel and Mills 1998 cited in Widmer 2008).

Suchman et al. (2008) analyzed the diet of the Pacific Sea Nettle off northern California and found that the eggs of euphausiid crustaceans (krill) were consumed at a disproportionately high rate relative to their abundance in the plankton. When euphausiid eggs were absent, gelatinous zooplankton were the preferred food. Only where both krill eggs and gelatinous zooplankton were scarce did the widely abundant calanoid copepods become the primary prey of the Pacific Sea Nettle. These differences in prey "preferences" may largely be a function of the relative difficulty of catching different prey (e.g., eggs that can't escape versus fast-moving copepods).