In this work we present the first documented record of a morphological anomaly
in polychaetes in Chile observed in specimens of Perinereis falklandica (RAMSAY, 1914) obtained in
aggregations of the mytilid Perumytilus purpuratus (LAMARCK, 1819) in the intertidal zone, at Las
Cruces beach, central Chile (33°29’46” S, 71°37’39” W). Among all the observed specimens of
different families of polychaetes collected in the aggregations of mytilids we were able to detect 3
individuals among 291 specimens of Perinereis falklandica examined, with the two prostomial
antennae fused into a single triangular-shaped structure. A similar condition has been described in
several regions of the Brazilian coast for two other species of nereidids. It is recorded for the first time
in communities, in the same locality, that were studied regularly for at least nine years.
