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Natural History, Limiting Factors and Energetics of the Opisthobranch Navanax Inermis

Paine, Robert T.
Ecology 1965 v.46 no.5 pp. 603-619
cannibalism, carnivores, coasts, correlation, digestive system, energy, energy content, habitats, ingestion, natural history, oxygen consumption, population density, population growth, protoplasm, reproduction, California
Populations of the carnivorous opisthobranch Navanax inermis, living in the vicinity of San Diego, California, were studied from September 1961 to August 1962. Three sampling stations were established, one on the open coast and two in relatively sheltered bays. For all stations, population density and prey content of each gut at time of capture are known. Other data, including caloric content of principal prey, rates of Navanax oxygen consumption, and turnover time of food in gut have permitted the energy available to the field population for growth to be estimated. Laboratory studies, including a complete energy budget in which calories of ingestion, egestion, growth, and reproduction are known, gave data on assimilation efficiency (62%) and maximum possible growth rates (small individuals, 7 to 9% wet wt/day; large, 2.1 to 3.8%). Potential growth in the field, determined by assuming complete conversion to new protoplasm of any caloric excess remaining after consideration of the daily metabolic costs, was compared to these maximum laboratory rates and related to density. In bays, an optimum habitat, density and caloric surplus are positively correlated density—dependent cannibalism characterizes populations of small individuals; and all sized individuals are usually food limited. In marginal, open coast habitats, population density appears to be more closely related to events in the physical environment.