5 . THE PHYLUM PROTOZOA
141
22"C, concentrations of more than 50 x lo4 organisms per ml were
reached in 3 days. Cultures maintained a t 14-22°C decline rapidly,
however, and trophozoites cannot be found within a week. I n cultures
maintained a t - 0.2 to 50"C, the organisms grow slower. The number
of flagellates reaches concentrations equal to those grown a t 14-22°C
in 3-4 weeks. It has also been determined that temperatures above
25°C are lethal to Hexamita.
Knowing the thermal death point of Hexamita, Scheltema has
proposed the following hypothesis to explain the seasonal fluctuation
of this flagellate in oysters. During the winter, when the oyster's
metabolism is very low, Hexamita is apparently capable of reproducing
rapidly enough so that it is found in a large percentage of the oysters.
On the other hand, during the late spring and early summer months,
although the flagellate reproduces more rapidly, the processes within
the oyster which remove Hexamita, i.e. phagocytosis by leucocytes,
exceed the flagellate's reproductive rate and hence its number declines.
However, if the oyster's metabolic rate is for some reason lowered, the
increased divisional rate of the flagellates results in the occurrence of
large numbers of flagellates, hence the large numbers of Hexamita in
physiologically stressed or moribund oysters. During the summer
months the temperature approaches or even reaches the thermal death
point of Hexamita and hence it is seldom found in large numbers in
oysters during this period.
Practically nothing is know about the physiology of
H . in$ata other than that its reproductive rate is dependent upon the
ambient temperature.
Thus far H . inflata has been cultured in two types of media. (1) It
has been successfully cultured in " shell liquor " (mantle fluid) from
oysters filtered through No. 42 Whatman paper and subsequently through
a 0.8 p Millipore filter. Streptomycin is added to 15 ml samples of the
filtrate in standard bacteriological culture tubes as a bacteriastatic
agent. (2) Hexamita has also been cultured in a medium consisting
of 15 ml of filtered sea water in a bacteriological culture tube to
which is added one small shucked oyster. Oysters to be used for this
purpose are first held a t 30°C for about 1 h. Streptomycin is added to
each tube to inhibit bacterial growth (Scheltema, 1962).
A detailed account of the histopathological changes
which occur in oysters associated with H . inflata has been given earlier
(see Chapter 4, Section 11, A, 6). Although rather severe alterations are
sometimes found in infected oysters, such as severe atrophy of the
surrounding Leydig tissues (Fig. 22), it is not known whether such
Physiology.
Pathology.
141
22"C, concentrations of more than 50 x lo4 organisms per ml were
reached in 3 days. Cultures maintained a t 14-22°C decline rapidly,
however, and trophozoites cannot be found within a week. I n cultures
maintained a t - 0.2 to 50"C, the organisms grow slower. The number
of flagellates reaches concentrations equal to those grown a t 14-22°C
in 3-4 weeks. It has also been determined that temperatures above
25°C are lethal to Hexamita.
Knowing the thermal death point of Hexamita, Scheltema has
proposed the following hypothesis to explain the seasonal fluctuation
of this flagellate in oysters. During the winter, when the oyster's
metabolism is very low, Hexamita is apparently capable of reproducing
rapidly enough so that it is found in a large percentage of the oysters.
On the other hand, during the late spring and early summer months,
although the flagellate reproduces more rapidly, the processes within
the oyster which remove Hexamita, i.e. phagocytosis by leucocytes,
exceed the flagellate's reproductive rate and hence its number declines.
However, if the oyster's metabolic rate is for some reason lowered, the
increased divisional rate of the flagellates results in the occurrence of
large numbers of flagellates, hence the large numbers of Hexamita in
physiologically stressed or moribund oysters. During the summer
months the temperature approaches or even reaches the thermal death
point of Hexamita and hence it is seldom found in large numbers in
oysters during this period.
Practically nothing is know about the physiology of
H . in$ata other than that its reproductive rate is dependent upon the
ambient temperature.
Thus far H . inflata has been cultured in two types of media. (1) It
has been successfully cultured in " shell liquor " (mantle fluid) from
oysters filtered through No. 42 Whatman paper and subsequently through
a 0.8 p Millipore filter. Streptomycin is added to 15 ml samples of the
filtrate in standard bacteriological culture tubes as a bacteriastatic
agent. (2) Hexamita has also been cultured in a medium consisting
of 15 ml of filtered sea water in a bacteriological culture tube to
which is added one small shucked oyster. Oysters to be used for this
purpose are first held a t 30°C for about 1 h. Streptomycin is added to
each tube to inhibit bacterial growth (Scheltema, 1962).
A detailed account of the histopathological changes
which occur in oysters associated with H . inflata has been given earlier
(see Chapter 4, Section 11, A, 6). Although rather severe alterations are
sometimes found in infected oysters, such as severe atrophy of the
surrounding Leydig tissues (Fig. 22), it is not known whether such
Physiology.
Pathology.