Cycle de développement d'une population intertidale de Nereis virens (Polychaeta Nereidae) de l'estuaire maritime du Saint-Laurent

An intensive study was conducted from 1986 to 1988 to schematize the life history of the intertidal population of the annelid Nereis virens. Spawning occurred between late April and early June on the south shore of the Lower St. Lawrence estuary. Then, epitokous males leave the sediment and move towards female burrows, displaying heteronereid movements before releasing gametes. Femalesremain in their burrow and release oocytes which adhere to sediment surface where they will be fertilized. All mature individuals die after spawning. Larvae go through conspicuous changes during suprabenthic, pelagic and benthic development (monotrochophore, trochophore, metatrochophore, nectochaete). Settlement occurs in the upper intertidal zone and depends essentially on hydrodynamic processes related to the tide. After recruitment, two growth periods have been observed. During the first two years, juveniles reach 40 to 100 chaetigerous segments following metameric growth. In their 3rd and 4th years, individuals increased in body mass (weighted growth). Three-year-old individuals can migrate downshore from upper intertidal levels and join adults. This migration is different from the one displayed by epitokous males in the spawning period. Further analysis suggested that individuals of the intertidal population of the Lower St. Lawrence estuary may have a life span superior to seven years. Moreover, a comparison between the results from east-American populations and these from European studies shows discrepancies for spatial distribution, age at maturity, maximal oocyte diameter before spawning, spawning time, life span, larval settlement, growth and spatiotemporal evolution of the cohorts. All these discrepancies, together with the weak dispersion of this species are good indicators of genetic variation between populations from the Eastern American and European coasts.