Symbiotic associations between crustaceans and gelatinous zooplankton in deep and surface waters off California

Using a remotely operated submersible (ROV) in the sea off Monterey, California, we collected deep-living zooplankton and observed their associations with crustacean symbionts. Little is known about the nature of these symbioses. Among the most interesting findings was the description of a previously unknown modality of symbiosis of the deep-living copepod Pseudolubbockia dilatata Sars. It was recorded within the subumbrellar cavity of three specimens of the bathypelagic hydromedusa Aegina citrea Eschscholtz at depths of 606-1,098 m. One of these medusae hosted a mating pair of adult copepods along with the remains of their molts corresponding to copepodid stages CV of the female and CII, CIII, CIV, and CV of the male; another medusa had an adult female, and molts of a female CV and of male CIII, CIV and CV copepodids. Our data indicate that the medusae were occupied first by an early male copepodid, and then the female joined as a CV. The presence of an adult female alone with its CV molt in a third medusa suggests that females invade the host regardless of the presence of the male in it. The medusa represents a protected environment for these copepods during vulnerable stages or processes (molting and mating). We also observed 13 new associations between hyperiid amphipods and gelatinous zooplankton at different depths. These involve four new records among members of the Infraorder Physosomata, for which only six other associations were known, and five species of amphipods and six hosts among the gelatinous zooplankton not previously recorded as symbionts. Data are provided on three families of Hyperiidea for which symbiotic associations were hitherto unknown. The ROV represents a valuable tool for the observation and sampling of these associations, whose existence has been known for a long time, but which are still poorly understood.