22 
PACIFIC SCIENCE, Vol. XX, January 1966 
order to attempt to establish the validity of the 
hypothesis that the invasive stage crab is at- 
tracted to the smaller host mussels on the basis 
of the latter’s relatively higher metabolic rate. 
An experiment, in which first stage crabs or 
very early prehards were placed in small, previ- 
ously uninfested mussels, indicates that the 
growth rate of these mussels was sufficient to 
accommodate the growing crabs. More details 
regarding this experiment will be presented in 
a separate paper. 
While the foregoing discussion suggests some 
reasons for the propensity of smaller crabs to 
associate with the proportionately smaller mus- 
sels it does not give any indication regarding 
the almost general absence of crabs from the 
very large mussels over 85 mm in length. It has 
been reported that M. modiolus is an extremely 
slow grower in its later years and quite long 
lived (Wiborg, 1946; Coe, 1948). Wiborg re- 
ports that off the coast of Norway the horse 
mussel attains its maximum size of 118 mm at 
an age of 18 years. It is therefore quite possible 
that the hosts outlive their original symbiont 
crabs. Furthermore, since there appears to be a 
tendency toward the initial infestation of the 
small, immature bivalves, these larger host mus- 
sels might never be reinfested once their original 
symbiont crab has perished. This would be espe- 
cially true if the host mussel is selected by the 
invasive crab on the basis of relative metabolic 
activity. 
LENGTH GROUPS OF MUSSELS- MM 
Fig. 6. Histogram showing per cent of total population of M. modiolus constituted by each length group. 
