COMPARISON OF SPATIAL OVERLAP BETWEEN THE POLYCHAETES NEREIS-VIRENS AND NEPHTYS-CECA IN 2 INTERTIDAL ESTUARINE ENVIRONMENTS

An intensive study of spatial overlap between the polychaetes Nereis v irens (Sars) and Nephtys caeca (Fabricius) was conducted in 1992 on tw o tidal sand flats, which differ by their exposure to dominant winds a nd residual currents in the Lower St. Lawrence Estuary (Quebec, Canada ). Results showed that spatial overlap (Lloyd's index and spatial dist ribution) was higher among adults of both species at the lower tidal e levation of l'Anse-a-l'Orignal (north-east oriented). Results also sug gest weaker interspecific interactions among juveniles of both species because of limited spatial overlap, In Baie-du-Ha!Ha! (south-west ori ented), spatial overlap was greater than that observed in 1'Anse-8-l'O rignal and appeal-ed important in adults as well as in juveniles. Levi ns' directional measure of competition indicated an asymmetric spatial overlap between N. virens and N. caeca in Baie-du-Ha!Ha! and a symmet ric overlap in l'Anse-a-l'Orignal. The intraspecific encounter values, estimated from Lloyd's mathematical expression, were significantly hi gher than values of interspecific interactions only in l'Anse-a-l'Orig nal. Moreover, no vertical stratification was found inside the sedimen t, with no effect of the densities and individual body weights of the polychaetes. Juveniles of both species mainly inhabited the organic-ri ch upper portion of the sediments (O to 12 cm), while adults colonised greater depths( > 25 cm) where organic matter content was lower. A co mplementary field experiment was conducted in 1993 to investigate inte rspecific interactions (predation and competition) existing between N. virens and N. caeca. Results from this experiment depend on which spe cies was first-introduced and showed an important variation in mortali ty rates between allopatric and sympatric conditions. The influence of competition and predation on the structure of these populations is al so discussed in relation to their spatial overlap.