Spatial overlap and coexistence in a mussel-associated polychaete assemblage on a South American rocky shore

Mussel beds in the intertidal of subtropical South America are known to har bour a large number of invertebrates, particularly polychaetes. This paper presents a quantitative analysis of the patterns of spatial overlap and coe xistence in an assemblage of polychaetes associated with mussel beds in a r ocky intertidal habitat on the Pacific coast of Peru. Mussel beds are made up of two zones in the intertidal: the lower, more extensive zone formed by Semimytillus algosus and the upper zone formed by Perumytilus purpuratus, Null models were employed to assess the magnitude of spatial overlap betwee n pairs of taxa encompassing a total of seven taxonomic groups. Two taxa be longing to the same functional group tended to be spatially segregated in t he Semimytilus bed: Pseudonereis vs. Halosydna (large free-ranging predator s) and Scoloplos vs. Mediomastus (deposit feeders), with further segregatin g tendencies between Pseudonereis and Mediomastus, and between Halosydna an d Scoloplos. Small-sized, free-ranging 'Typosyllis' and the bottom-dwelling scavenging predator Lumbrineris were also spatially associated. Thus, this polychaete assemblage appears to be loosely organised around groupings of taxa which are somewhat different in ecological traits and form mosaic patt erns of distribution with reduced overlap within the Semimytilus bed. These relationships were weak to non-existent in the upper, drier Perumytilus ha bitat where polychaete density was low.