5 . THE PHYLUM PROTOZOA
139
styles extending along length of body, not readily visible in living
specimens ; one to two contractile vacuoles in free-living forms ;
cytostome obscure ; endoplasm with refractile granules.
Life cycle. The complete life cycle of H . inJlata is not known but
that of H . salmonis, a related species found in the intestine of various
species of trout and salmon, is known (Moore, 1922, 1923 ; Davies, 1925).
I n H . salmonis, schizogony is said to occur in the lining epithelium of
the piscine host's pyloric caeca and intestine although this observation
is in need of verification. Encystment occurs, with the cyst serving as
the infective form which is transmitted from fish to fish via the ingestion
of contaminated water. Similarly, in the case of H . intestinalis, another
related species found in the intestine of frogs and of Trutta fario and
in the rectum of Motella tricirrata and M . mustela in European waters,
cysts are known to exist (Alexeieff, 1912).
As mentioned earlier, Hexamita inJlata is usually a free-living
saprobic species commonly found in the proximity of oyster beds. It
is only under unfavorable ambient conditions, when the oyster is
placed under physiological stress, that H . inJlata invades the pelecypod
and becomes a facultative parasite. Although so-called " intracellular
stages ') of H . inJlata in oysters have been reported by Mackin et al.
(1952), these have not been conclusively implicated as representing
stages in the life cycle of this flagellate. I n fact, these authors have
stated that " an effort . . . to piece the various intracellular elements
of the intestinal epithelium and the leucocytes into a cohesive life
cycle (was) without success.)'
Scheltema (1 962) has infected Crassostrea virginica experimentally.
He placed fifteen 2-year-old oysters in each of two aquaria containing 7
liters of sea water maintained at 5.8 f 0.8"C. To each aquarium was
added high concentrations of Hexamita trophozoites. It was found after
33 days that a very dense population of Hexamita grew in the aquaria
water with the numbers observed in both aquaria equaling those found
in heavily infected oyster stomachs (ca l o 4 organisms per ml). Of the
thirty oysters, only one died and among the survivors, only 37.9% were
infected. Furthermore, the infections that did occur were not heavy.
Scheltema concluded that: " Thus even under exposure to extraordinarily high concentrations of Hexamita, no significant mortality occurred
and the index of incidence remained relatively low." From this and
similar unpublished reports by others, it appears quite definite that
H . inJlata is not a highly lethal parasite as has been claimed by Mackin
et al. (1952) and Stein et al. (1961), although there is some indication
that the mortality rate of parasitized oysters maintained a t low
temperatures is greater than that of controls (Stein et al., 1961).
139
styles extending along length of body, not readily visible in living
specimens ; one to two contractile vacuoles in free-living forms ;
cytostome obscure ; endoplasm with refractile granules.
Life cycle. The complete life cycle of H . inJlata is not known but
that of H . salmonis, a related species found in the intestine of various
species of trout and salmon, is known (Moore, 1922, 1923 ; Davies, 1925).
I n H . salmonis, schizogony is said to occur in the lining epithelium of
the piscine host's pyloric caeca and intestine although this observation
is in need of verification. Encystment occurs, with the cyst serving as
the infective form which is transmitted from fish to fish via the ingestion
of contaminated water. Similarly, in the case of H . intestinalis, another
related species found in the intestine of frogs and of Trutta fario and
in the rectum of Motella tricirrata and M . mustela in European waters,
cysts are known to exist (Alexeieff, 1912).
As mentioned earlier, Hexamita inJlata is usually a free-living
saprobic species commonly found in the proximity of oyster beds. It
is only under unfavorable ambient conditions, when the oyster is
placed under physiological stress, that H . inJlata invades the pelecypod
and becomes a facultative parasite. Although so-called " intracellular
stages ') of H . inJlata in oysters have been reported by Mackin et al.
(1952), these have not been conclusively implicated as representing
stages in the life cycle of this flagellate. I n fact, these authors have
stated that " an effort . . . to piece the various intracellular elements
of the intestinal epithelium and the leucocytes into a cohesive life
cycle (was) without success.)'
Scheltema (1 962) has infected Crassostrea virginica experimentally.
He placed fifteen 2-year-old oysters in each of two aquaria containing 7
liters of sea water maintained at 5.8 f 0.8"C. To each aquarium was
added high concentrations of Hexamita trophozoites. It was found after
33 days that a very dense population of Hexamita grew in the aquaria
water with the numbers observed in both aquaria equaling those found
in heavily infected oyster stomachs (ca l o 4 organisms per ml). Of the
thirty oysters, only one died and among the survivors, only 37.9% were
infected. Furthermore, the infections that did occur were not heavy.
Scheltema concluded that: " Thus even under exposure to extraordinarily high concentrations of Hexamita, no significant mortality occurred
and the index of incidence remained relatively low." From this and
similar unpublished reports by others, it appears quite definite that
H . inJlata is not a highly lethal parasite as has been claimed by Mackin
et al. (1952) and Stein et al. (1961), although there is some indication
that the mortality rate of parasitized oysters maintained a t low
temperatures is greater than that of controls (Stein et al., 1961).