5 . THE PHYLUM PROTOZOA
151
pyriform and are the microgametes, while those resulting from the other
are macrogametes. The micro- and macrogametes fuse to form zygotes
which enlarge and become surrounded by a thin wall. Each zygote
then divides to form eight or sometimes sixteen sporozoites. When
the gametocyst wall ruptures, hundreds of sporozoites are released
into the water through the host’s anus.
Clumps of sporozoites, known as gymnospores, enter an oyster in
one of two ways. After gymnospores make contact with the oyster’s
ctenidial or mantle surface, they penetrate the epithelium by means of
a pseudopod projecting from the central cell. Gymnospores can also
become phagocytized by oyster leucocytes and taken into the body.
If the second mechanism is employed, the gymnospores are commonly
transported to various parts of the host’s body in the circulatory
system. Gymnospores, whether in leucocytes or in other tissues,
break apart and the individual sporozoites increase in size. This is
followed by the formation of a resistant spore wall around each sporozoite. These spores occur most abundantly in mantle tissues but are
also found in the adductor muscle, heart, gills, labial palps (Figs. 29
and 30), and perhaps other organs (Prytherch, 1940 ; Sprague and Orr,
1955). The spores are the infective form to crabs.
It should be noted that Prytherch (1940) has reported the occurrence
of spores of two distinct sizes in oysters, but Sprague (1949) has
demonstrated experimentally that the larger spores represent those of
another species, N . prytherchi.
Ecology. Little is known about the ecology of N . ostrearum. Landau
and Galtsoff (1951), as the result of extensive surveys, have reported
that this pa,rasite is widely distributed along the Atlantic and Gulf
coasts of the United States.
As has been pointed out, although Prytherch (1938,
1940) has considered N . ostrearum to be a cause of mass mortalities of
Crassostrea virginica, his views have not been confirmed by Landau
and Galtsoff (1951) and Sprague and Orr (1955). Histopathological
changes in infected oysters have not been studied in detail. According
to Sprague and Orr, the increase in the number of spores in oysters is
the result of repeated infections and not of intramolluscan reproduction.
Pathology.
2 . Nematopsis prytherchi Sprague, 1949
(Subclass Gregarinia; order Eugregarinia; suborder Cephalina; family
Nematopsis prytherchi was originally described by Sprague ( 1949)
who demonstrated experimentally that the larger Nematopsis spores
Porosporidae)
151
pyriform and are the microgametes, while those resulting from the other
are macrogametes. The micro- and macrogametes fuse to form zygotes
which enlarge and become surrounded by a thin wall. Each zygote
then divides to form eight or sometimes sixteen sporozoites. When
the gametocyst wall ruptures, hundreds of sporozoites are released
into the water through the host’s anus.
Clumps of sporozoites, known as gymnospores, enter an oyster in
one of two ways. After gymnospores make contact with the oyster’s
ctenidial or mantle surface, they penetrate the epithelium by means of
a pseudopod projecting from the central cell. Gymnospores can also
become phagocytized by oyster leucocytes and taken into the body.
If the second mechanism is employed, the gymnospores are commonly
transported to various parts of the host’s body in the circulatory
system. Gymnospores, whether in leucocytes or in other tissues,
break apart and the individual sporozoites increase in size. This is
followed by the formation of a resistant spore wall around each sporozoite. These spores occur most abundantly in mantle tissues but are
also found in the adductor muscle, heart, gills, labial palps (Figs. 29
and 30), and perhaps other organs (Prytherch, 1940 ; Sprague and Orr,
1955). The spores are the infective form to crabs.
It should be noted that Prytherch (1940) has reported the occurrence
of spores of two distinct sizes in oysters, but Sprague (1949) has
demonstrated experimentally that the larger spores represent those of
another species, N . prytherchi.
Ecology. Little is known about the ecology of N . ostrearum. Landau
and Galtsoff (1951), as the result of extensive surveys, have reported
that this pa,rasite is widely distributed along the Atlantic and Gulf
coasts of the United States.
As has been pointed out, although Prytherch (1938,
1940) has considered N . ostrearum to be a cause of mass mortalities of
Crassostrea virginica, his views have not been confirmed by Landau
and Galtsoff (1951) and Sprague and Orr (1955). Histopathological
changes in infected oysters have not been studied in detail. According
to Sprague and Orr, the increase in the number of spores in oysters is
the result of repeated infections and not of intramolluscan reproduction.
Pathology.
2 . Nematopsis prytherchi Sprague, 1949
(Subclass Gregarinia; order Eugregarinia; suborder Cephalina; family
Nematopsis prytherchi was originally described by Sprague ( 1949)
who demonstrated experimentally that the larger Nematopsis spores
Porosporidae)