LIFE-HISTORY OF THE POLYCHAETE NEREIS-VIR ENS (SARS) IN AN INTERTIDALFLAT OF THE LOWER ST-LAWRENCE ESTUARY

An intensive study was conducted from 1986 to 1988 to schematize the l ife history of the intertidal population of the annelid Nereis virens. Spawning occurred between late April and early June on the south shor e of the Lower St. Lawrence estuary. Then, epitokous males leave the s ediment and move towards female burrows, displaying heteronereid movem ents before releasing gametes. Femalesremain in their burrow and relea se oocytes which adhere to sediment surface where they will be fertili zed. All mature individuals die after spawning. Larvae go through cons picuous changes during suprabenthic, pelagic and benthic development ( monotrochophore, trochophore, metatrochophore, nectochaete). Settlemen t occurs in the upper intertidal zone and depends essentially on hydro dynamic processes related to the tide. After recruitment, two growth p eriods 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 interti dal levels and join adults. This migration is different from the one d isplayed by epitokous males in the spawning period. Further analysis s uggested that individuals of the intertidal population of the Lower St . Lawrence estuary may have a life span superior to seven years. Moreo ver, a comparison between the results from east-American populations a nd these from European studies shows discrepancies for spatial distrib ution, age at maturity, maximal oocyte diameter before spawning, spawn ing time, life span, larval settlement, growth and spatiotemporal evol ution of the cohorts. All these discrepancies, together with the weak dispersion of this species are good indicators of genetic variation be tween populations from the Eastern American and European coasts.