84
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
The requirement of a carbohydrate source of host origin among
intramolluscan trematode larvae appears to be one of absolute necessity.
Simoeo and Coelho (1955) have demonstrated that when Australorbis
glabratus is removed from water, the enclosed larvae of Schistosoma
mansoni stop developing but development is resumed when the snail is
returned to water. This finding is extremely significant when the work
of MagalhBes and de Almeida (1956) is considered. These workers have
reported that the hepatopancreatic glycogen of A . glabratus drops to
50% of the normal content when the snails are kept out of water and
to 10-15% just before death in 75 days. These data suggest that the
inhibition of normal development of the trematode larvae is correlated
with the amount of glycogen in the host’s digestive gland. This hypothesis appears to be supported by the results of Sindermann el al. (1957)
who have found that the emission of the cercariae of Austrobilharzia
variglandis, the marine dermatitis-causing trematode, by Nassarius
obsoletus is reduced if the gastropod is starved. But upon reinitiation of
feeding, massive cercarial emergence follows within 48 h. Similarly,
Brackett (1940) has reported that starved snails do not emit cercariae
until they are fed. These data may well be interpreted to mean that the
decrease in cercaria production in starved snails has resulted from a
decrease in stored glycogen as mentioned above, and with the reinstitution of feeding there is an accumulation of stored glycogen which
in turn becomes available for developing cercariae. Another piece of
evidencc which lends weight to this hypothesis is the work of Kendall
(1949) pertaining to the development of Fasciola hepatica in Lymnaea
trunculata. He has found a correlation between the amount of food
intake and the number of developing cercariae reaching maturity.
Other than trematodes, practically nothing is known about the
carbohydrate contents or requirements of symbionts of molluscs.
Among mesozoans, Lameere (1916) has reported that the rhombogens of
Pseudicyema truncatum in the nephridium of the squid, Xepia oflicinalis,
include somatic cells packed with glycogen. These cells often projected
conspicuously on the body surface and are termed “ verruciform ”
cells. The origin of this polysaccharide remains unknown, although
presumably it is synthesized from sugars of host origin and is utilized
as a source of energy.
b. Lipids
Little is known about lipid metabolism in intramolluscan parasites.
Even the lipid contents in those species of protozoa, mesozoa, and
arthropods found in marine molluscs remain uninvestigated. The same
can be said of the nematodes and cestodes found in molluscs. The
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
The requirement of a carbohydrate source of host origin among
intramolluscan trematode larvae appears to be one of absolute necessity.
Simoeo and Coelho (1955) have demonstrated that when Australorbis
glabratus is removed from water, the enclosed larvae of Schistosoma
mansoni stop developing but development is resumed when the snail is
returned to water. This finding is extremely significant when the work
of MagalhBes and de Almeida (1956) is considered. These workers have
reported that the hepatopancreatic glycogen of A . glabratus drops to
50% of the normal content when the snails are kept out of water and
to 10-15% just before death in 75 days. These data suggest that the
inhibition of normal development of the trematode larvae is correlated
with the amount of glycogen in the host’s digestive gland. This hypothesis appears to be supported by the results of Sindermann el al. (1957)
who have found that the emission of the cercariae of Austrobilharzia
variglandis, the marine dermatitis-causing trematode, by Nassarius
obsoletus is reduced if the gastropod is starved. But upon reinitiation of
feeding, massive cercarial emergence follows within 48 h. Similarly,
Brackett (1940) has reported that starved snails do not emit cercariae
until they are fed. These data may well be interpreted to mean that the
decrease in cercaria production in starved snails has resulted from a
decrease in stored glycogen as mentioned above, and with the reinstitution of feeding there is an accumulation of stored glycogen which
in turn becomes available for developing cercariae. Another piece of
evidencc which lends weight to this hypothesis is the work of Kendall
(1949) pertaining to the development of Fasciola hepatica in Lymnaea
trunculata. He has found a correlation between the amount of food
intake and the number of developing cercariae reaching maturity.
Other than trematodes, practically nothing is known about the
carbohydrate contents or requirements of symbionts of molluscs.
Among mesozoans, Lameere (1916) has reported that the rhombogens of
Pseudicyema truncatum in the nephridium of the squid, Xepia oflicinalis,
include somatic cells packed with glycogen. These cells often projected
conspicuously on the body surface and are termed “ verruciform ”
cells. The origin of this polysaccharide remains unknown, although
presumably it is synthesized from sugars of host origin and is utilized
as a source of energy.
b. Lipids
Little is known about lipid metabolism in intramolluscan parasites.
Even the lipid contents in those species of protozoa, mesozoa, and
arthropods found in marine molluscs remain uninvestigated. The same
can be said of the nematodes and cestodes found in molluscs. The