4. ANALYSIS OF FACTORS INVOLVED IN SYMBIOSIS
99
sometimes portray thickened out-growth or are malformed and stunted
in size. Stauber and Christensen and McDermott have noticed gill
injuries in Crassostrea virginica caused by Pinnotheres ostreum. Stauber
has described two types of gill damage: the small-crab type, characterized by local, sharply delimited erosion of one or more demibranchs ;
and the large-crab type, characterized by an extensive shortening of one
of more demibranchs, reaching from the anterior end of the gills to a
point usually ventral to the adductor muscle. Such gill damage in
oysters harboring P. ostreum is very common. Christensen and
McDermott have reported that :
Nearly all infested oysters show some gill damage. Examination of 1502
oysters, all of the 1955 year class, collected from January 6th t o August 1st
in 1956 revealed that about 50% had light damage, about 40% had moderate
gill damage, about 9 % had heavy gill damage, and only about 1 % had no
discernible gill damage.
Relative to the consequences of such crab-inflicted damage, these
investigators have reported :
Among older oysters we found a few extreme cases of heavy gill damage
where there was hardly anything left of the gills, and such oysters were
usually also very poor in condition.
Earlier, Stauber, who has studied tissue regeneration in injured oysters,
stated :
Rapid healing and regeneration of gill tissue almost keeps pace with destruction and probably saves many oysters from death.
He further noted, however, that crab-enclosing oysters do not keep
well when maintained out of water. This could either have been due
to irritation by the crab or from greater loss of ‘ I shell liquor ” resulting
from an enlarged mantle cavity created by the presence of the crab or
crabs. From the viewpoint of the commercial oysterman, this is a
serious matter.
Despite the well-documented accounts of injury to pelecypods
inflicted by P. pisum and P. ostreum, ironically the gill lesions are not
caused by the crabs eating the hosts’ tissues. If this is so, these crabs
would qualify as parasites by every definition. It has been shown by
Coupin (1894), and later confirmed by Orton (1920) and others, that
P. pisum feeds on particulate food filtered from the water by the host’s
filtering mechanisms. Orton, who observed this crab’s feeding behavior
through “ windows ” cut in the oyster’s valves, has reported that the
pea crab picks food strings from the gill margins with its chelipeds.
Stauber (1945) has reported that P. ostreum feeds in the same manner
99
sometimes portray thickened out-growth or are malformed and stunted
in size. Stauber and Christensen and McDermott have noticed gill
injuries in Crassostrea virginica caused by Pinnotheres ostreum. Stauber
has described two types of gill damage: the small-crab type, characterized by local, sharply delimited erosion of one or more demibranchs ;
and the large-crab type, characterized by an extensive shortening of one
of more demibranchs, reaching from the anterior end of the gills to a
point usually ventral to the adductor muscle. Such gill damage in
oysters harboring P. ostreum is very common. Christensen and
McDermott have reported that :
Nearly all infested oysters show some gill damage. Examination of 1502
oysters, all of the 1955 year class, collected from January 6th t o August 1st
in 1956 revealed that about 50% had light damage, about 40% had moderate
gill damage, about 9 % had heavy gill damage, and only about 1 % had no
discernible gill damage.
Relative to the consequences of such crab-inflicted damage, these
investigators have reported :
Among older oysters we found a few extreme cases of heavy gill damage
where there was hardly anything left of the gills, and such oysters were
usually also very poor in condition.
Earlier, Stauber, who has studied tissue regeneration in injured oysters,
stated :
Rapid healing and regeneration of gill tissue almost keeps pace with destruction and probably saves many oysters from death.
He further noted, however, that crab-enclosing oysters do not keep
well when maintained out of water. This could either have been due
to irritation by the crab or from greater loss of ‘ I shell liquor ” resulting
from an enlarged mantle cavity created by the presence of the crab or
crabs. From the viewpoint of the commercial oysterman, this is a
serious matter.
Despite the well-documented accounts of injury to pelecypods
inflicted by P. pisum and P. ostreum, ironically the gill lesions are not
caused by the crabs eating the hosts’ tissues. If this is so, these crabs
would qualify as parasites by every definition. It has been shown by
Coupin (1894), and later confirmed by Orton (1920) and others, that
P. pisum feeds on particulate food filtered from the water by the host’s
filtering mechanisms. Orton, who observed this crab’s feeding behavior
through “ windows ” cut in the oyster’s valves, has reported that the
pea crab picks food strings from the gill margins with its chelipeds.
Stauber (1945) has reported that P. ostreum feeds in the same manner