Biology of Fahia subquadrata — Pearce 
25 
Fig. 7. Photograph showing damage done to ctenidia of M. modiolus by a Stage V crab 9-3 mm in cara- 
pace width. Note that the ctenidia lying in the left {lower) valve are eroded dorsally to the line of suspen- 
sion. The ctenidia in the right {upper) valve are alrno t entire and serve for comparison. Cystlike anomalies 
can also be noted on the mantle tissues of the left valve. l A X- 
gill' edges to appear serrated. Indentations caused 
by immature F. subquadrata are only one-fourth 
to one-third the dorso-ventral dimension of the 
gill, whereas the gill damage caused by the adult 
crab almost always appears as a single, crescent- 
shaped erosion, often extending to the base of 
the ctenidium (Fig. 7). 
It appears likely that the erosion of a mus- 
sel’s ctenidia always starts with the small-crab 
type and, as the crab matures, the damage be- 
comes that typical of infestations with the adult 
crabs. Christensen and McDermott (1958:171) 
note that the erosion caused by P. ostreum in 
the American oyster also progresses from an 
initial small-crab type damage to the more ex- 
tensive destruction of tissues connected with the 
infestation by a large adult crab. 
It is noteworthy that only 58% of the small- 
crab infestations seen during the months of June 
and early July are accompanied by extensive 
erosion, while Stage I infestations prior to and 
following this period are almost invariably ac- 
companied by extensive, small-crab type ero- 
sions. This effect could quite possibly be due to 
the infestation of previously uninfested mussels 
following copulatory swarming by the crabs, 
which thus did not have time to cause extensive 
erosion. 
Both kinds of damage appear to be about 
equally detrimental to the individual host mus- 
sel. Because the smaller Stage I or prehard crabs 
are usually associated with smaller mussels, the 
