4. ANALYSIS O F FACTORS INVOLVED I N SYMBIOSIS
125
Cercaria reynoldsi possess thicker and more heavily calcified shells than
uninfected ones. Rothschild (1936) has reported abnormal shell development in Hydrobia ulvae parasitized by Cercaria oocysta and C. ubiquita.
Relative to the “ ballooning ” of shell whorls and abnormal shell
development, Rothschild (1936) has suggested that such could be due
to pressure exerted from the interior by the parasites. Unpublished
observations of mine on several species of gastropods, including
Helisomu anceps, Littorina Eittorea, H . trivolvis, Physa gyrina and
Nassarius obsoletus, infected with trematode germinal sacs have revealed
that abnormal growth of the shell is more common in thin-shelled
freshwater species than in thick-shelled marine species. Furthermore,
my observations tend to confirm Rothschild’s belief that it is the
pressure exerted from within by parasites which causes the abnormal
whorling and “ ballooning ”. It thus follows that abnormal shell
development occurs more frequently in extremely heavily parasitized
gastropods in which the sporocysts or rediae exert pressure against
the mantle. It might be noted that examination of thousands of
parasitized pelecypods, both marine and freshwater, has not yet
revealed any abnormal shell formation. A possible explanation for
this is that endoparasites of pelecypods do not as a rule exert as much
pressure, if any at all, on the mantle, even in very heavy infections,
since the large mantle cavity serves to “ cushion ” the exerted pressure.
Relative to shell calcification in parasitized molluscs, Cheng et al.
(1966) have demonstrated that the amount of ionic calcium (0.570 f
0.170 mg) in each gram of soft tissue of Nitrocris dilatatus parasitized
by Prosthodendrium (Acanthatrium) anaplocami is significantly greater
at the 5% level than that found in each gram of soft tissue of nonparasitized snails (0.409 f 0-242 mg) of the same age and clone.
Subsequent histological and histochemical studies have revealed that
the number and size of the hepatopancreatic calcium spherites in
parasitized snails are increased and, when host cells become ruptured
by the parasites, the release of ionic calcium stored in calcium cells,
plus the breakdown of calcium phosphates which are incorporated in
the spherites, both contribute free ionic calcium to the tissues. It is
believed that some of the ionic calcium may eventually become
incorporated in the shell, much in the same manner as during shell
repair as reported by Aboling-Krogis (1960)) thus accounting for the
heavier shell of certain parasitized snails (Etges, 1961). Similarly,
one of my former students, Alan H. Anderson (unpublished), has found
that the shells of Nassarius obsoletus parasitized by Austrobilharzia
variglandis or Himasthla quissetensis sporocysts are significantly larger
and heavier than those of uninfected ones.
125
Cercaria reynoldsi possess thicker and more heavily calcified shells than
uninfected ones. Rothschild (1936) has reported abnormal shell development in Hydrobia ulvae parasitized by Cercaria oocysta and C. ubiquita.
Relative to the “ ballooning ” of shell whorls and abnormal shell
development, Rothschild (1936) has suggested that such could be due
to pressure exerted from the interior by the parasites. Unpublished
observations of mine on several species of gastropods, including
Helisomu anceps, Littorina Eittorea, H . trivolvis, Physa gyrina and
Nassarius obsoletus, infected with trematode germinal sacs have revealed
that abnormal growth of the shell is more common in thin-shelled
freshwater species than in thick-shelled marine species. Furthermore,
my observations tend to confirm Rothschild’s belief that it is the
pressure exerted from within by parasites which causes the abnormal
whorling and “ ballooning ”. It thus follows that abnormal shell
development occurs more frequently in extremely heavily parasitized
gastropods in which the sporocysts or rediae exert pressure against
the mantle. It might be noted that examination of thousands of
parasitized pelecypods, both marine and freshwater, has not yet
revealed any abnormal shell formation. A possible explanation for
this is that endoparasites of pelecypods do not as a rule exert as much
pressure, if any at all, on the mantle, even in very heavy infections,
since the large mantle cavity serves to “ cushion ” the exerted pressure.
Relative to shell calcification in parasitized molluscs, Cheng et al.
(1966) have demonstrated that the amount of ionic calcium (0.570 f
0.170 mg) in each gram of soft tissue of Nitrocris dilatatus parasitized
by Prosthodendrium (Acanthatrium) anaplocami is significantly greater
at the 5% level than that found in each gram of soft tissue of nonparasitized snails (0.409 f 0-242 mg) of the same age and clone.
Subsequent histological and histochemical studies have revealed that
the number and size of the hepatopancreatic calcium spherites in
parasitized snails are increased and, when host cells become ruptured
by the parasites, the release of ionic calcium stored in calcium cells,
plus the breakdown of calcium phosphates which are incorporated in
the spherites, both contribute free ionic calcium to the tissues. It is
believed that some of the ionic calcium may eventually become
incorporated in the shell, much in the same manner as during shell
repair as reported by Aboling-Krogis (1960)) thus accounting for the
heavier shell of certain parasitized snails (Etges, 1961). Similarly,
one of my former students, Alan H. Anderson (unpublished), has found
that the shells of Nassarius obsoletus parasitized by Austrobilharzia
variglandis or Himasthla quissetensis sporocysts are significantly larger
and heavier than those of uninfected ones.