4. ANALYSIS O F FACTORS INVOLVED I N SYMBIOSIS
27
japonica and to an amino acid mixture encapsuled in a sheet of cellophane. Kawashima et al. were uncertain whether the “ host factor ”
or attractant is chemical or physical. It was also pointed out that the
host-preference of the miracidia occurs without any relation to the
suitability of the snails as hosts since it is known that A . parasitologica
is the common natural host. In fact, studies have shown that A.
parasitologica can be readily infected experimentally and that this
snail is naturally infected in endemic areas. On the other hand, A.
latericea miyazakii appears to be an incompatible host while A . japonica
can be infected experimentally but the level of infection is always low.
It is thus apparent that some additional factor must be operative in
nature to bring about the selection by the miracidia for A . parasitologica.
In a later paper (Kawashima et al., 1961b), it was found that thesalinities
occurring at the intertidal habitats of these three species of snails
influence the survival of the miracidia, with that existing at the
habitat of A. parasitologica being most favorable to the miracidia.
While studying the locomotive speed and survival of Paragonimus
ohirai miracidia in various concentrations of NaCl, it was found that
the lower the salt concentration is, the more active the miracidia
become. In fact, the authors stated that : “ It seems to be indispensable
for the eggs to hatch and for the larvae to get the host, to be kept in the
solution of less than 0.25% NaCl.”
Concurrent studies on the
“ population activities ” of the three species of snails at various
salinities revealed that the optimum salinity for Assiminea parasitologica was 0.25%, that for A . latericea miyaxakii was 0.4%, and that
for A . japonica was 0.6%. These findings explain the preferred habitats
of the snails, since A . parasitologica is usually found in the proximity
of the high tide line while the other two species are found near the low
tide line. These also serve to explain why A . parasitologica is a compatible host from the ecological viewpoint. Thus these investigators
not only have demonstrated an instance where the influence of the
host’s attractant can be superimposed by an ambient environmental
factor but also have demonstrated that attraction of miracidia to
molluscs need not mean successful subsequent development, i.e.
specific attraction. Furthermore, they have demonstrated how the
salinity tolerances of the parasite and the host can serve as natural
mechanisms responsible for host specificity.
Another study of mollusc-miracidium attraction was contributed
by Etges and Decker (1963). These investigators employed a cast-iron
maze consisting of a central cylindrical chamber (4 x 8 cm diam.)
with four cylindrical side arms (each 3.5 x 1 cm diam.). To the free
end of each side arm was attached a vertical terminal cylindrical
27
japonica and to an amino acid mixture encapsuled in a sheet of cellophane. Kawashima et al. were uncertain whether the “ host factor ”
or attractant is chemical or physical. It was also pointed out that the
host-preference of the miracidia occurs without any relation to the
suitability of the snails as hosts since it is known that A . parasitologica
is the common natural host. In fact, studies have shown that A.
parasitologica can be readily infected experimentally and that this
snail is naturally infected in endemic areas. On the other hand, A.
latericea miyazakii appears to be an incompatible host while A . japonica
can be infected experimentally but the level of infection is always low.
It is thus apparent that some additional factor must be operative in
nature to bring about the selection by the miracidia for A . parasitologica.
In a later paper (Kawashima et al., 1961b), it was found that thesalinities
occurring at the intertidal habitats of these three species of snails
influence the survival of the miracidia, with that existing at the
habitat of A. parasitologica being most favorable to the miracidia.
While studying the locomotive speed and survival of Paragonimus
ohirai miracidia in various concentrations of NaCl, it was found that
the lower the salt concentration is, the more active the miracidia
become. In fact, the authors stated that : “ It seems to be indispensable
for the eggs to hatch and for the larvae to get the host, to be kept in the
solution of less than 0.25% NaCl.”
Concurrent studies on the
“ population activities ” of the three species of snails at various
salinities revealed that the optimum salinity for Assiminea parasitologica was 0.25%, that for A . latericea miyaxakii was 0.4%, and that
for A . japonica was 0.6%. These findings explain the preferred habitats
of the snails, since A . parasitologica is usually found in the proximity
of the high tide line while the other two species are found near the low
tide line. These also serve to explain why A . parasitologica is a compatible host from the ecological viewpoint. Thus these investigators
not only have demonstrated an instance where the influence of the
host’s attractant can be superimposed by an ambient environmental
factor but also have demonstrated that attraction of miracidia to
molluscs need not mean successful subsequent development, i.e.
specific attraction. Furthermore, they have demonstrated how the
salinity tolerances of the parasite and the host can serve as natural
mechanisms responsible for host specificity.
Another study of mollusc-miracidium attraction was contributed
by Etges and Decker (1963). These investigators employed a cast-iron
maze consisting of a central cylindrical chamber (4 x 8 cm diam.)
with four cylindrical side arms (each 3.5 x 1 cm diam.). To the free
end of each side arm was attached a vertical terminal cylindrical