Natural Mortality in NE Pacific Coastal Hydromedusae: Grazing Predation, Wound Healing and Senescence
Abstract:Hydromedusae appear in the coastal plankton during defined species-specific periods. In a predictable succession, some species appear shortly after the spring plankton bloom begins, and others follow as ecological 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 species of hydro medusae 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 damage by parasites or symbionts, and for gut fullness. Young spring medusae of most hydrozoan species were in excellent physical condition. The 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 victoria and Mitrocoma cellularia showed high amounts of grazing damage, caused mostly by hyperiid amphipods and parasitic larval sea anemones, whose negative effects accumulate late in the season. The shorter-lived species Clytia gregarium and Mitrocomella polydiademata showed lower overall incidence of damage than species with long-lived individuals, and were assumed to be removed largely by whole animal (rather than grazing) predation. Senescence was seen to be the primary factor only in the demise of the population of Gonionemus vertens.
Document Type: Research Article
Publication date: July 1, 1993
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