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Seasonal demographic vanation was studied in an estuarine population of the opportunistic polychaete Polydora lignj between May 1982 and November 1983 in Alewife Cove, Connecticut, USA. The demography of 15 cohorts settling during this period was analyzed using life cycle graph models. Cohorts settling in spnng and early summer had the shortest life spans (= 10 wk); highest survivorship over all life cycle stages and adult females produced an estimated maxlmum of 6 larval broods. Early adult stages contributed most to population growth during this period, and the potential for population growth, h, was the highest during the study. Late summer cohorts had low juvenile survivorship, life spans of = 12 wk, and females produced a maximum of 8 broods. Later stage females contributed most to population growth, and h values were usually 5 1, indicating population decline. Fall cohorts comprised 2 groups. Early fall cohorts reproduced only in late fall, as females growing into later stages during winter did not reproduce and died before reproduction resumed in spring. These cohorts had low fecundity, 4 to 6 broods female-', and h 5 1. One cohort settled at the beginning of winter and along with late fall cohorts overwintered to produce the following spring's cohorts. Overwintering cohorts had low juvenile but high adult survivorship, extended times to maturity (;= 14 to 20 wk), the highest mean fecundity, 4 to 8 broods female-' and h ;= 1. The results indicate the demography of Polydora ligni exhibits 3 seasonal phases with respect to population growth over the year: (1) a spring/early summer growth period; (2) a late summer/fall transition phase; (3) a late fall/ winter maintenance period. These demographic phases can be related to regular periodic changes in estuarine soft-bottom habitats and can influence the evolution of its life history traits. Seasonal life history patterns were consistent with predictions of life-history models centering on demographic selection. Seasonal changes in demographic selection may act to increase life-history flexibility in Polydora ligni, and in other infaunal opportunists, as a way of maintaining fitness with respect to periodic changes in habitat conditions. Such flexibility may either enhance or dim~nish its colonizing abilities. Seasonal demograph~c changes can influence the response of P. ljgni to benthic disturbances as opportunistic potential falls off after early summer due to life-history shifts.

Population ecology of Polydora ligni (Polychaeta: Spionidae). II. Seasonal demographic variation and its potential impact on life history evolution