152
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
found primarily in the gills of Crassostrea virginica are not those of N .
ostrearum but those of a distinct species, N . prytherchi.
The complete description of N .
prytherchi has never been published by Sprague. I n an abstract published
in 1949, however, he has pointed out that the main diagnostic feature
of N . prytherchi in oysters is its larger spores, measuring approximately
l o p x 1 9 p , which have a special affinity for the host’s gill tissues.
Life cycle. Again, the complete life cycle of N . prytherchi has not
been published by Sprague although he has cited the salient features in
two abstracts (Sprague, 1949, 1954) and has made a detailed account
of the life cycle available in mimeographed form (Sprague, 1962). The
cycle is essentially the same as that of N . ostrearurn.
The definitive host of N . prytherchi is the crab Menippe mercenaria
which becomes infected when oyster tissues, primarily gill tissues,
enclosing spores are ingested. Upon reaching the crab’s gut, a vermiform sporozoite emerges from each spore through a terminally located
micropyle. Such sporozoites become attached by their anterior ends to
epithelial cells lining the host’s gut and a small globose epimerite is
formed. This is followed by detachment during which some sporozoites
lose their epimerites, and the joining of two or more sporadins in linear
or bifurcated syzygy. These associated sporadins are found free in the
host’s midgut where they increase in size. During this period all of the
satellites (i.e. those syzygynous individuals other than the anteriormost
one, which is the primite) fuse to form a single multinucleated compartment. When this developniental stage is reached, the associated primite
and satellite migrate to the host’s rectum where they become a rounded
gametocyst which adheres t o the chitinous wall. Cytological changes
now occur, resulting in the formation of numerous gametes. Sexual
reproduction has not yet been reported for N . prytherchi although
bodies, tentatively identified as zygotes, have been reported by Sprague.
Each of these zygotes gives rise to eight sporozoites which are arranged
in clumps as gymnospores and which are released when the gametocyst
ruptures.
It is
known that sporozoites comprising each gymnospore are phagocytized
by the oyster’s leucocytes in small numbers, but it is not clear whether
the phagocytized sporozoites are taken into the host’s body through
the surface epithelium or are first ingested and later taken in through
the gut epithelium. Both methods have been reported for certain
species found in Europe (see Hatt, 1931). Within leucocytes, the
sporozoites (termed sporoblasts by Sprague) are spherical or oval and
Description of stages in mollusc.
Exactly how oysters become infected remains uncertain.
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
found primarily in the gills of Crassostrea virginica are not those of N .
ostrearum but those of a distinct species, N . prytherchi.
The complete description of N .
prytherchi has never been published by Sprague. I n an abstract published
in 1949, however, he has pointed out that the main diagnostic feature
of N . prytherchi in oysters is its larger spores, measuring approximately
l o p x 1 9 p , which have a special affinity for the host’s gill tissues.
Life cycle. Again, the complete life cycle of N . prytherchi has not
been published by Sprague although he has cited the salient features in
two abstracts (Sprague, 1949, 1954) and has made a detailed account
of the life cycle available in mimeographed form (Sprague, 1962). The
cycle is essentially the same as that of N . ostrearurn.
The definitive host of N . prytherchi is the crab Menippe mercenaria
which becomes infected when oyster tissues, primarily gill tissues,
enclosing spores are ingested. Upon reaching the crab’s gut, a vermiform sporozoite emerges from each spore through a terminally located
micropyle. Such sporozoites become attached by their anterior ends to
epithelial cells lining the host’s gut and a small globose epimerite is
formed. This is followed by detachment during which some sporozoites
lose their epimerites, and the joining of two or more sporadins in linear
or bifurcated syzygy. These associated sporadins are found free in the
host’s midgut where they increase in size. During this period all of the
satellites (i.e. those syzygynous individuals other than the anteriormost
one, which is the primite) fuse to form a single multinucleated compartment. When this developniental stage is reached, the associated primite
and satellite migrate to the host’s rectum where they become a rounded
gametocyst which adheres t o the chitinous wall. Cytological changes
now occur, resulting in the formation of numerous gametes. Sexual
reproduction has not yet been reported for N . prytherchi although
bodies, tentatively identified as zygotes, have been reported by Sprague.
Each of these zygotes gives rise to eight sporozoites which are arranged
in clumps as gymnospores and which are released when the gametocyst
ruptures.
It is
known that sporozoites comprising each gymnospore are phagocytized
by the oyster’s leucocytes in small numbers, but it is not clear whether
the phagocytized sporozoites are taken into the host’s body through
the surface epithelium or are first ingested and later taken in through
the gut epithelium. Both methods have been reported for certain
species found in Europe (see Hatt, 1931). Within leucocytes, the
sporozoites (termed sporoblasts by Sprague) are spherical or oval and
Description of stages in mollusc.
Exactly how oysters become infected remains uncertain.