9. THE CLASS CRUSTACEA
289
responses of first copepodids, which is the invasive form, to sunlight
makes it doubtful if phototaxis is an important factor involved in
host-searching.
Grainger (1951), as the result of his maintenance studies, has
reported that crowding has a serious effect on nauplii, metanauplii and
first copepodids, especially among metanauplii. If ten to twenty
specimens are placed in a 4-5 in. Petri dish, they are maintained quite
satisfactorily ; however, if larger numbers are thus maintained, death
occurs.
Grainger (1951) has also studied the distribution of M . intestinalis
in naturally infected Mytilus edulis. He has found that the copepods
are almost exclusively limited to the recurrent portion of the intestine
and in the rectum. Moreover, the largest number can be found in that
portion of the recurrent intestine embedded in the digestive gland.
This would suggest that perhaps some nutritional requirements are
derived from the digestive gland. Grainger has also found a significant
correlation between the size of the mussel host and the number of
parasites. It has been shown that the larger mussels harbor the
greatest number of small copepods.
Relative to seasonal fluctuations in the copepod density in naturally
infected M . edulis in Ireland, Grainger has reported that there is a
significant decrease in the number of copepods per mussel between
November to December and May to June. Furthermore, although
egg-bearing females are found all year round, the lowest number of
adults in mussels of the 50-69 mm group occurs during September
while during November and December the number of immature stages
is highest. On the other hand, the number of immature stages is lowest
during May and August. I n natural populations there is always an
excess of male adults throughout the year.
Since M . intestinalis has been reported to be a parasite of both
Mytilus edulis and Ostrea edulis among other molluscs, Hepper (1956)
studied the transmission of this parasite from mussels to oysters. It is
of considerable interest t o point out that although earlier (Hepper,
1953) he was able to infect 0. edulis exposed to copepodids, when 0.
edulis and M . edulis are exposed to copepodids in the same aquarium
the mussels are readily infected but the oysters are not. The same
phenomenon has been mentioned earlier by Bolster (1954). It would
thus appear that M . intestinalis demonstrates a preference for Mytilus
edulis, but if this mussel is not available as a choice it will parasitize
Ostrea edulis.
Physiology. What is known about the physiology of M . intestinalis
has stemmed from the maintenance and culture studies of Grainger
A.M.R.-5
19
289
responses of first copepodids, which is the invasive form, to sunlight
makes it doubtful if phototaxis is an important factor involved in
host-searching.
Grainger (1951), as the result of his maintenance studies, has
reported that crowding has a serious effect on nauplii, metanauplii and
first copepodids, especially among metanauplii. If ten to twenty
specimens are placed in a 4-5 in. Petri dish, they are maintained quite
satisfactorily ; however, if larger numbers are thus maintained, death
occurs.
Grainger (1951) has also studied the distribution of M . intestinalis
in naturally infected Mytilus edulis. He has found that the copepods
are almost exclusively limited to the recurrent portion of the intestine
and in the rectum. Moreover, the largest number can be found in that
portion of the recurrent intestine embedded in the digestive gland.
This would suggest that perhaps some nutritional requirements are
derived from the digestive gland. Grainger has also found a significant
correlation between the size of the mussel host and the number of
parasites. It has been shown that the larger mussels harbor the
greatest number of small copepods.
Relative to seasonal fluctuations in the copepod density in naturally
infected M . edulis in Ireland, Grainger has reported that there is a
significant decrease in the number of copepods per mussel between
November to December and May to June. Furthermore, although
egg-bearing females are found all year round, the lowest number of
adults in mussels of the 50-69 mm group occurs during September
while during November and December the number of immature stages
is highest. On the other hand, the number of immature stages is lowest
during May and August. I n natural populations there is always an
excess of male adults throughout the year.
Since M . intestinalis has been reported to be a parasite of both
Mytilus edulis and Ostrea edulis among other molluscs, Hepper (1956)
studied the transmission of this parasite from mussels to oysters. It is
of considerable interest t o point out that although earlier (Hepper,
1953) he was able to infect 0. edulis exposed to copepodids, when 0.
edulis and M . edulis are exposed to copepodids in the same aquarium
the mussels are readily infected but the oysters are not. The same
phenomenon has been mentioned earlier by Bolster (1954). It would
thus appear that M . intestinalis demonstrates a preference for Mytilus
edulis, but if this mussel is not available as a choice it will parasitize
Ostrea edulis.
Physiology. What is known about the physiology of M . intestinalis
has stemmed from the maintenance and culture studies of Grainger
A.M.R.-5
19