The decline in benthic marine fauna following an oil spill in West Falmouth, Massachusetts, permitted us to follow the responses of a number of polychaete and other invertebrate spp to an environmental disturbance. Spp with the most opportunistic life histories increased and declined at the 2 stations with the greatest reduction in spp diversity. The stations with an intermediate reduction in diversity showed increases and declines of somewhat less opportunistic spp. Electrophoretic studies of the malate dehydrogenase loci of the most opportunistic spp, Capitella capitata, indicated short-term selection for a single genotype in the large populations present in Wild Harbor following the oil spill. The life histories of the most opportunistic spp are summarized. Initial response to disturbed conditions, ability to increase rapidly, large population size, early maturation, and high mortality are all features of opportunistic spp. Using these criteria, the spp are ranked in order of decreasing degree of opportunism as: (1) Capitella capitata, (2) polydora ligni, (3) Syllides verrilli, (4) Microphthalmus aberrans, (5) Streblospio benedicti, (6) Mediomastus ambiseta. We propose using mortality as the best single measure of degree of opportunism. A definition based on mortality emphasizes the portion of the life cycle involved in adaptation through short-term selection. 2 types of marine benthic opportunists are described: (1) a mixed strategy variety with obligate planktonic dispersal where selection within local subpopulations occurs in a single generation; (2) a response-to-selection type with direct development or settlement shortly after release from brood structures allowing selection within local populations through more than 1 generation.
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