5. THE PHYLUM PROTOZOA
177
Description of sta,ges in mollusc. Cyst, 14-20 p in diameter, most
commonly found in egg cytoplasm abutting nucleus (Fig. 66) but is
also found within nucleus or in undifferentiated gonadal cells ; cyst
spherical or ovoid, with smooth membrane measuring 0 . 3 ~ thick
(Fig. 67); immature cysts may include one, two, four, eight or more
nuclei ; mature cysts enclose thirty-two to sixty-four spores which tend
t o clump together near the center ; each spore (Figs. 68-76), 3-5 p in
diameter, rarely 6-8 p, spherical or elongate and commonly with large
vacuole a t one pole in cytoplasm ; vacuole contains inclusion body ;
one or two, typically two, chromatin granules present in each spore.
Other information. Although the effect of C. mytilovum on M . edulis,
especially its ova, has not been studied in any detail, there is no reason
to believe that this parasite is economically important. Sprague (19658)
has stated that : “ I n most (infected) specimens the proportion of
infected eggs to normal ones was quite small and considerable searching
was usually required to find the parasite.’’
2. Chytridiopsis ovicola LBger and Hollande, 1917
Chytridiopsis ovicola, the only other species of Chytridiopsis known
to parasitize a mollusc, the European oyster, Ostrea edulis, was originally
described by LBger and Hollande (1917) in France. It is quite similar
to C. mytilovum. It too is generally found within its host’s ova. I n
fact, as Sprague (1965a) has pointed out, the morphological differences
between the two species are slight ; however, “since they seem to have
some slight morphological differences, it would be premature to conclude that they belong to the same species.”
Description of stages in mollusc. Cysts, 14-16p in diameter, in
cytoplasm of host’s ova, closely associated with nucleus ; cyst membrane
smooth and identical in shape with that of C. mytilovum ; spores within
mature cysts very similar to those of C. mytilovum except that they are
slightly smaller, measuring 2.3 p in diameter ; each spore with one large
chromatin granule surrounded by a clear zone located at the pole
opposite that occupied by vacuole.
(Order Microsporida; family?)
F. Subphylum CILIOPHORA
Class Ciliatea
Among the ciliates, a number of species belonging to the order
Thigmotrichida have been reported associated with marine molluscs,
including some which are commercially or semi-commercially important.
A.kI.B.--L
12
177
Description of sta,ges in mollusc. Cyst, 14-20 p in diameter, most
commonly found in egg cytoplasm abutting nucleus (Fig. 66) but is
also found within nucleus or in undifferentiated gonadal cells ; cyst
spherical or ovoid, with smooth membrane measuring 0 . 3 ~ thick
(Fig. 67); immature cysts may include one, two, four, eight or more
nuclei ; mature cysts enclose thirty-two to sixty-four spores which tend
t o clump together near the center ; each spore (Figs. 68-76), 3-5 p in
diameter, rarely 6-8 p, spherical or elongate and commonly with large
vacuole a t one pole in cytoplasm ; vacuole contains inclusion body ;
one or two, typically two, chromatin granules present in each spore.
Other information. Although the effect of C. mytilovum on M . edulis,
especially its ova, has not been studied in any detail, there is no reason
to believe that this parasite is economically important. Sprague (19658)
has stated that : “ I n most (infected) specimens the proportion of
infected eggs to normal ones was quite small and considerable searching
was usually required to find the parasite.’’
2. Chytridiopsis ovicola LBger and Hollande, 1917
Chytridiopsis ovicola, the only other species of Chytridiopsis known
to parasitize a mollusc, the European oyster, Ostrea edulis, was originally
described by LBger and Hollande (1917) in France. It is quite similar
to C. mytilovum. It too is generally found within its host’s ova. I n
fact, as Sprague (1965a) has pointed out, the morphological differences
between the two species are slight ; however, “since they seem to have
some slight morphological differences, it would be premature to conclude that they belong to the same species.”
Description of stages in mollusc. Cysts, 14-16p in diameter, in
cytoplasm of host’s ova, closely associated with nucleus ; cyst membrane
smooth and identical in shape with that of C. mytilovum ; spores within
mature cysts very similar to those of C. mytilovum except that they are
slightly smaller, measuring 2.3 p in diameter ; each spore with one large
chromatin granule surrounded by a clear zone located at the pole
opposite that occupied by vacuole.
(Order Microsporida; family?)
F. Subphylum CILIOPHORA
Class Ciliatea
Among the ciliates, a number of species belonging to the order
Thigmotrichida have been reported associated with marine molluscs,
including some which are commercially or semi-commercially important.
A.kI.B.--L
12