Revista Chilena de Historia Natural 78 (1): 3-14, 2005
RESEARCH ARTICLE
Bilateral asymmetry of shell shape and spatial position in matrices of the mussel
Perumytilus purpuratus (Lamarck, 1819) (Bivalvia: Mytilidae)
In the literature, less attention has been paid to the study of directional asymmetry (DA)
than fluctuating asymmetry (FA). Although there are good examples of DA in mollusks (e.g., Ostreidae and Pectinidae), for some taxa,
such as mussels, DA has not been reported. We examined asymmetry in two shell shape traits of the mussel Perumytilus
purpuratus, in the rocky intertidal zone of three localities in central and northern Chile. We compared shell asymmetry between
mussels occupying different positions in the matrices or mussel beds (i.e., upper versus lower stratum). We observed shell shape DA
for mussels occupying both strata, with the right valve being consistently less elongated than the left valve. However, for individuals in
the lower strata the mussel valves tended to be more elongated, and also more asymmetrical, relative to mussels from the upper layer
of the matrix. These results suggest that changes in the shape and asymmetry of the mussel valves may be modified by density and
stratification in the beds. The causal explanations of this phenomenon are open to further research.
Key words:
directional asymmetry,
fluctuating asymmetry, mussel, mytilid, shell shape
