Population Dynamics of the Polychaete Nephtys Caeca in an Intertidal Estuarine Environment (Quebec, Canada)

Abstract

An extensive study of an intertidal population of Nephtys caeca Fabricius (Annelida: Polychaeta) was conducted from 1986 to 1988 in I'Anse-à-l'Orignal on the southern shore of the lower St Lawrence Estuary. The sampling, carried out at three levels in the intertidal zone, was designed to study spatio-temporal variations in oocyte diameter (1988), age (1986), density (1986–1987) and individual body weight (1986–1987). The highest oocyte diameters were found in early May when breeding occurred. A relatively long period (from 20 June to 23 August), when oocytes were not visible with a binocular microscope, was also observed. Oocyte maturation began in late summer / early autumn. Settlement did not seem to take place in the intertidal zone, suggesting sublittoral larval recruitment. Peak densities occurred at the intermediate tidal level only in June. Nephtys caeca density varied significantly according to tidal levels, but only in 1987. The number of larger individuals (>320 mg in partial weight) increased markedly with decreasing level on the shore. At the lower tidal level a significant temporal variation in body weight was observed, while mean weight reached its maximum value in July or August 1986. Body weight later decreased progressively (July-November). In June and July 1986 arrivals of juveniles were seen to be greater at every level than during 1987. Six growth-rings were found in the pharyngeal jaws of the individuals in this intertidal population (November 1986). The decomposition of the polymodal weight-frequency distributions of individuals >75 or >85 mg in partial weight isolated four or five size-groups (MacDonald & Pitcher, 1979).