4. ANALYSIS OF FACTORS INVOLVED I N SYMBIOSIS
129
represent the effect or cause of the presence of Hexamita? If these
changes are caused by the facultative parasite, the secretion of some
toxic and/or lytic substance(s) is suggested since the histopathological
changes indicate more than mechanical damage. But if one compares
the photomicrographs of Stein et a2. (1961 : Pig. 6) of the pathological
intestinal mucosa with those of Sparks and Pauley (1964 : Figs. 5-8)
of intestinal mucosa undergoing " normal )' post-mortem changes, it
is certainly suggestive that the " histopathological changes " associated
with Hexamita are not caused by the flagellates but are " normal "
post-mortem necrotic changes. The same can be said of the changes
in the Leydig tissues (viz. compare Sparks and Pauley's Figs. 10 and 11
with Fig. 7 of Stein et al.). Thus it may be that Hexamita, a saprobic
organism, are attracted to necrotic tissues in dead or dying oysters.
Parasitization of the alimentary tract. Although a number of symbionts, particularly parasites, have been reported from the alimentary
tract of marine molluscs, for example the amoeba VahlkamJia patuxent
reported by Hogue (1921) in the gut of oysters, there is essentially no
information concerning their pathogenicity. Furthermore, there is no
reason to believe that most of these are pathogenic, at least seriously
so, or that they cause any mechanical damage.
When one considers parasitization of the alimentary tract of
molluscs, it should be borne in mind that a number of parasites, such
as the post-egg stages of certain helminths, are temporarily found
within the alimentary tract. Although these transient parasites, such
as the coracidium of Tylocephalum sp., commonly do cause lesions of
varying severity while penetrating the host's gut (Cheng, 1966a), they
are not herein considered as intestinal or gastric parasites.
Examples of what appear to be true intestinal parasites of a number
of species of marine molluscs are the cyclopoid copepods of the genera
Mytilicola, Piratasta and related forms. Steuer ( 1902) first described
such a copepod, M . intestinalis, from the intestine of the mussel,
Mytilus galloprovincialis, in the Gulf of Trieste. Since then, Mytilicola
intestinalis has been repeatedly found and studied. (Other references
to this parasite are given in Chapter 9, Section I, A.) It appears to
be confined to European waters where it has been reported in the
Adriatic Sea, the Mediterranean Sea, from Germany, the northern
coast of France, Great Britain and Ireland. It was believed to be of
sufficient importance as a mortality factor among commercially
important marine pelecypods to form the subject of a conference held in
Paris. In the proceedings of that conference published in 1951, several
papers (Korringa (b) ; Korringa and Lambert ; Lambert ; Heldt ;
A.M.B.-5
10
129
represent the effect or cause of the presence of Hexamita? If these
changes are caused by the facultative parasite, the secretion of some
toxic and/or lytic substance(s) is suggested since the histopathological
changes indicate more than mechanical damage. But if one compares
the photomicrographs of Stein et a2. (1961 : Pig. 6) of the pathological
intestinal mucosa with those of Sparks and Pauley (1964 : Figs. 5-8)
of intestinal mucosa undergoing " normal )' post-mortem changes, it
is certainly suggestive that the " histopathological changes " associated
with Hexamita are not caused by the flagellates but are " normal "
post-mortem necrotic changes. The same can be said of the changes
in the Leydig tissues (viz. compare Sparks and Pauley's Figs. 10 and 11
with Fig. 7 of Stein et al.). Thus it may be that Hexamita, a saprobic
organism, are attracted to necrotic tissues in dead or dying oysters.
Parasitization of the alimentary tract. Although a number of symbionts, particularly parasites, have been reported from the alimentary
tract of marine molluscs, for example the amoeba VahlkamJia patuxent
reported by Hogue (1921) in the gut of oysters, there is essentially no
information concerning their pathogenicity. Furthermore, there is no
reason to believe that most of these are pathogenic, at least seriously
so, or that they cause any mechanical damage.
When one considers parasitization of the alimentary tract of
molluscs, it should be borne in mind that a number of parasites, such
as the post-egg stages of certain helminths, are temporarily found
within the alimentary tract. Although these transient parasites, such
as the coracidium of Tylocephalum sp., commonly do cause lesions of
varying severity while penetrating the host's gut (Cheng, 1966a), they
are not herein considered as intestinal or gastric parasites.
Examples of what appear to be true intestinal parasites of a number
of species of marine molluscs are the cyclopoid copepods of the genera
Mytilicola, Piratasta and related forms. Steuer ( 1902) first described
such a copepod, M . intestinalis, from the intestine of the mussel,
Mytilus galloprovincialis, in the Gulf of Trieste. Since then, Mytilicola
intestinalis has been repeatedly found and studied. (Other references
to this parasite are given in Chapter 9, Section I, A.) It appears to
be confined to European waters where it has been reported in the
Adriatic Sea, the Mediterranean Sea, from Germany, the northern
coast of France, Great Britain and Ireland. It was believed to be of
sufficient importance as a mortality factor among commercially
important marine pelecypods to form the subject of a conference held in
Paris. In the proceedings of that conference published in 1951, several
papers (Korringa (b) ; Korringa and Lambert ; Lambert ; Heldt ;
A.M.B.-5
10