114
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
Wales, is 16 months during which it undergoes two reproductive cycles.
He has found that Cercaria parvatrema homoeotecnum infects only
juvenile snails shortly after birth. Thus, in theory, this trematode has
practically the entire 16 months in which to affect its host. However,
because of the rapid rate of the parasite’s growth and proliferation,
some 80% of the host’s digestive gland is destroyed prior to its death
in 5 months. C. Zebouri, C. ubiquita, C. roscovita and C. littorinae rudis,
on the other hand, infect only spent adults and therefore only have
from 2 to 8 months to affect their hosts, depending on whether the
snails become parasitized after the first or second reproductive cycle.
I n the case of C. lebouri, the rediae develop slowly and consequently,
even if given 8 months, only about 15% of its host’s hepatopancreas is
destroyed. Thus this parasite apparently does not destroy its host.
In fact, both parasitized and non-parasitized specimens are reported
to die at approximately the same time.
In the case of C. ubiquita, the sporocysts develop at a relatively
moderate rate. If its host becomes infected after the second reproductive cycle, it dies from senility 4 months later at which time 80%
of its hepatopancreas is found to be destroyed. But if it is infected after
the first reproductive cycle, the entire hepatopancreas is destroyed,
causing its premature death in about 6 months.
In the instances of C. roscovita, C. Zittorinae rudis and C. parvatrema
homoeotecnum, the development of the germinal sacs is so rapid and
prolific that James has stated: “ . . . the intensity of infection and the
extent of the damage to the digestive gland is limited by the damage the
host can withstand before being killed and not by the available time for
development .” The longevity of Littorina saxatilis tenebrosa parasitized
by Cercaria parvatrema homoeotecnum has been mentioned. Those
parasitized by C. roscovita and C. littorinae rudis have their digestive
glandular tissues completely destroyed within 2 and 4 months, respectively, thus leading to their deaths.
James has noted that if parasitization occurs in juvenile snails,
such as those parasitized by C. parvatrema homoeotecnum, the hosts are
prevented from reproducing completely. If the hosts are parasitized
after the first reproductive cycle, as in the case of the other four
trematodes studied, they do not reproduce during the second cycle.
Relative to the motility of the germinal sacs, James has stated that
the occurrence or non-occurrence of motility not only influences the
nature of the initial infection but also the progress of the infection.
Thus he is in general agreement with the earlier findings of W. J. Rees
(1936a) in that if the germinal sacs (sporocysts) are only located in the
proximal region of the hepatopancreas, the “ blocking layer ” formed
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
Wales, is 16 months during which it undergoes two reproductive cycles.
He has found that Cercaria parvatrema homoeotecnum infects only
juvenile snails shortly after birth. Thus, in theory, this trematode has
practically the entire 16 months in which to affect its host. However,
because of the rapid rate of the parasite’s growth and proliferation,
some 80% of the host’s digestive gland is destroyed prior to its death
in 5 months. C. Zebouri, C. ubiquita, C. roscovita and C. littorinae rudis,
on the other hand, infect only spent adults and therefore only have
from 2 to 8 months to affect their hosts, depending on whether the
snails become parasitized after the first or second reproductive cycle.
I n the case of C. lebouri, the rediae develop slowly and consequently,
even if given 8 months, only about 15% of its host’s hepatopancreas is
destroyed. Thus this parasite apparently does not destroy its host.
In fact, both parasitized and non-parasitized specimens are reported
to die at approximately the same time.
In the case of C. ubiquita, the sporocysts develop at a relatively
moderate rate. If its host becomes infected after the second reproductive cycle, it dies from senility 4 months later at which time 80%
of its hepatopancreas is found to be destroyed. But if it is infected after
the first reproductive cycle, the entire hepatopancreas is destroyed,
causing its premature death in about 6 months.
In the instances of C. roscovita, C. Zittorinae rudis and C. parvatrema
homoeotecnum, the development of the germinal sacs is so rapid and
prolific that James has stated: “ . . . the intensity of infection and the
extent of the damage to the digestive gland is limited by the damage the
host can withstand before being killed and not by the available time for
development .” The longevity of Littorina saxatilis tenebrosa parasitized
by Cercaria parvatrema homoeotecnum has been mentioned. Those
parasitized by C. roscovita and C. littorinae rudis have their digestive
glandular tissues completely destroyed within 2 and 4 months, respectively, thus leading to their deaths.
James has noted that if parasitization occurs in juvenile snails,
such as those parasitized by C. parvatrema homoeotecnum, the hosts are
prevented from reproducing completely. If the hosts are parasitized
after the first reproductive cycle, as in the case of the other four
trematodes studied, they do not reproduce during the second cycle.
Relative to the motility of the germinal sacs, James has stated that
the occurrence or non-occurrence of motility not only influences the
nature of the initial infection but also the progress of the infection.
Thus he is in general agreement with the earlier findings of W. J. Rees
(1936a) in that if the germinal sacs (sporocysts) are only located in the
proximal region of the hepatopancreas, the “ blocking layer ” formed