NATURAL MORTALITY IN NE PACIFIC COASTAL HYDROMEDUSAE - GRAZING PREDATION, WOUND-HEALING AND SENESCENCE

Hydromedusae appear in the coastal plankton during defined species-spe cific periods. In a predictable succession, some species appear shortl y after the spring plankton bloom begins, and others follow as ecologi cal conditions continue to change. Most species also have well-defined seasonal terminations. This study explores the programs of mortality in the field other than whole-animal predation of five abundant specie s of hydromedusae in the San Juan Archipelago, north of Puget Sound in Washington State. Living specimens were regularly hand-collected and examined for injuries, disease, general deterioration, evidence of dam age by parasites or symbionts, and for gut fullness. Young spring medu sae of most hydrozoan species were in excellent physical condition. Th e proportion of damaged individuals increased with time throughout the season. The nature and source of this damage varied according to the species of hydromedusa. The large; long-lived species Aequorea victori a and Mitrocoma cellularia showed high amounts of grazing damage, caus ed mostly by hyperiid amphipods and parasitic larval sea anemones, who se negative effects accumulate late in the season. The shorter-lived s pecies Clytia gregarium and Mitrocomella polydiademata showed lower ov erall incidence of damage than species with long-lived individuals, an d were assumed to be removed largely by whole animal (rather than graz ing) predation. Senescence was seen to be the primary factor only in t he demise of the population of Gonionemus vertens.