38
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
Barbosa and Carneiro (1957) who reported that under laboratory
conditions some of the South American and African species hybridize
freely and produce viable young.
The type of confusion which exists relative to schistosomes and their
molluscan hosts has not yet plagued studies on marine parasites and
their molluscan hosts. This does not mean that such problems will not
arise, especially since the taxonomy of marine molluscs shows indications of becoming just as complex, but the lack of such confusion merely
reflects the less intensive nature of research in marine parasitology.
Another factor which evidently has an important influence on
miracidium-mollusc contact but which, in my opinion, has not received
sufficient attention by those interested in the presence or absence of
chemotactic response has to do with the age of the miracidium. Differences in the ability of miracidia to make contact with their hosts as a
function of age was discovered by Campbell and Todd (1955a). These
investigators studied contact between Fascioloides magna miracidia
and Xtagnicola refZexa. They divided their miracidia into three age
groups. The ( ( young ” group included those ranging from 10 to 40 min
post-hatching, the ( ( medium ” group included those ranging from
92 to 104 min post-hatching, and the members of the ( ( old ” group
ranged from 8 h 40 min to 8 h 43 min post-hatching. A total of 102
miracidia and forty-eight small snails of approximately the same size
were used. Each snail was exposed to a known number of miracidia of
a specific group. The snail was examined at time intervals and the
number of miracidia not attached to the snail was recorded. I n some
instances a miracidium became detached from the snail, hence the
number of detached miracidia at any time may have been slightly more
than at the previous count. It was also noted that in each series of
exposures more miracidia became attached to snails during the first
15 min than during the succeeding 55 min. Campbell and Todd’s
results are tabulated in Table IV. From these they concluded that
(‘ the miracidia are more infective when between 1.5 and 2 h old than
when either very young (less than one hour) or very old (eight hours) ”.
Their results also suggest that while the members of the ( ( medium ”
group are the most effective attackers, members of the ‘I young ”
group are more effective than those of the ‘( old ”. It is of importance
to note that Campbell and Todd have stated that : ( ( The effect of age
on the attacking power of miracidia may help to explain inconsistencies found among previous reports on the behavior of miracidia in
the presence of a snail.”
Although Campbell and Todd found no consistent behavior pattern
which suggested chemotaxis, they did state that :
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
Barbosa and Carneiro (1957) who reported that under laboratory
conditions some of the South American and African species hybridize
freely and produce viable young.
The type of confusion which exists relative to schistosomes and their
molluscan hosts has not yet plagued studies on marine parasites and
their molluscan hosts. This does not mean that such problems will not
arise, especially since the taxonomy of marine molluscs shows indications of becoming just as complex, but the lack of such confusion merely
reflects the less intensive nature of research in marine parasitology.
Another factor which evidently has an important influence on
miracidium-mollusc contact but which, in my opinion, has not received
sufficient attention by those interested in the presence or absence of
chemotactic response has to do with the age of the miracidium. Differences in the ability of miracidia to make contact with their hosts as a
function of age was discovered by Campbell and Todd (1955a). These
investigators studied contact between Fascioloides magna miracidia
and Xtagnicola refZexa. They divided their miracidia into three age
groups. The ( ( young ” group included those ranging from 10 to 40 min
post-hatching, the ( ( medium ” group included those ranging from
92 to 104 min post-hatching, and the members of the ( ( old ” group
ranged from 8 h 40 min to 8 h 43 min post-hatching. A total of 102
miracidia and forty-eight small snails of approximately the same size
were used. Each snail was exposed to a known number of miracidia of
a specific group. The snail was examined at time intervals and the
number of miracidia not attached to the snail was recorded. I n some
instances a miracidium became detached from the snail, hence the
number of detached miracidia at any time may have been slightly more
than at the previous count. It was also noted that in each series of
exposures more miracidia became attached to snails during the first
15 min than during the succeeding 55 min. Campbell and Todd’s
results are tabulated in Table IV. From these they concluded that
(‘ the miracidia are more infective when between 1.5 and 2 h old than
when either very young (less than one hour) or very old (eight hours) ”.
Their results also suggest that while the members of the ( ( medium ”
group are the most effective attackers, members of the ‘I young ”
group are more effective than those of the ‘( old ”. It is of importance
to note that Campbell and Todd have stated that : ( ( The effect of age
on the attacking power of miracidia may help to explain inconsistencies found among previous reports on the behavior of miracidia in
the presence of a snail.”
Although Campbell and Todd found no consistent behavior pattern
which suggested chemotaxis, they did state that :