40
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
(1) Those who expect to find evidence for the attraction theory ”
and have been disappointed have erroneously expected an immediate,
dramatic, and ‘( all or none ” manifestation of attraction. As the statistically analyzed studies of Kloetzel (1958, 1960), Etges and Decker
(1963) and MacInnis (1965) have shown, chemotaxis is not an “ all
or none ’’ phenomenon, nor is it dramatic. If it exists, as these data
suggest, it is a very subtle process which is operative only within short
ranges and can be only fully appreciated upon analysis of quantitative
data. Various studies of this nature in related areas have convincingly
shown that chemotaxis does occur. For example, host-localization
among parasitic insects as reported by Salt (1935) and Laing (1937), host
attraction of the annelid Acholoe as reviewed by Davenport (1955,
1966), chemoreception and responses to chemical stimuli in free-living
flatworms as reported by Pearl (1903), Koehler (1932), Hyman (1951),
and Fraenkel and Gunn (1961) are all now widely accepted discoveries.
These and other authors have suggested that even prior to the operation
of chemotaxis the symbiont is initially attracted to a certain type of
environment and, if such an environment coincides with the natural
habitat of the host, the first stage of host-symbiont contact is accomplished. This is what Wright (1959a, 1960) has also proposed.
(2) The problem has been oversimplified by those who expect
specific chemical attraction and host-specificity to be parts of the same
process. Thus Sudds (1960), for example, expected the attraction of
schistosome miracidia to natural hosts to be distinctly more apparent
than attraction to incompatible molluscs, and a part of his argument
against the I ‘ attraction theory ” is based on comparable attraction to
at least some unnatural hosts as determined later by histopathological
studies of host reactions to invading parasites. Although the observations of Faust and Meleney (1924), Barlow (1925), Neuhaus (1953), and
the data of Etges and Decker (1963) suggest specific attraction of
S. mansoni miracidia to A . glabratus, there is no reason to believe that
host-specificity is always manifested in the initial contact. The finding
by Cheng (1963a) that the plasma” of five different species of molluscs
will activate the quiescent cercaria of Gorgodera amplicava but in
varying degrees, depending on the molluscan species, and the report of
Cheng et al. (1966b) that the plasma and tissue extracts of seven different
species of marine pelecypods have similar effects on the cercaria of
Himasthla quissetensis, but again in varying degrees depending on the
* The fluid portion of molluscan blood has been referred to in the literature as
serum, plasma or hernolymph. As molluscan blood does not clot, this fluid is technically
not serum, but because of common usage these three terms are used interchangeably
in this review.
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
(1) Those who expect to find evidence for the attraction theory ”
and have been disappointed have erroneously expected an immediate,
dramatic, and ‘( all or none ” manifestation of attraction. As the statistically analyzed studies of Kloetzel (1958, 1960), Etges and Decker
(1963) and MacInnis (1965) have shown, chemotaxis is not an “ all
or none ’’ phenomenon, nor is it dramatic. If it exists, as these data
suggest, it is a very subtle process which is operative only within short
ranges and can be only fully appreciated upon analysis of quantitative
data. Various studies of this nature in related areas have convincingly
shown that chemotaxis does occur. For example, host-localization
among parasitic insects as reported by Salt (1935) and Laing (1937), host
attraction of the annelid Acholoe as reviewed by Davenport (1955,
1966), chemoreception and responses to chemical stimuli in free-living
flatworms as reported by Pearl (1903), Koehler (1932), Hyman (1951),
and Fraenkel and Gunn (1961) are all now widely accepted discoveries.
These and other authors have suggested that even prior to the operation
of chemotaxis the symbiont is initially attracted to a certain type of
environment and, if such an environment coincides with the natural
habitat of the host, the first stage of host-symbiont contact is accomplished. This is what Wright (1959a, 1960) has also proposed.
(2) The problem has been oversimplified by those who expect
specific chemical attraction and host-specificity to be parts of the same
process. Thus Sudds (1960), for example, expected the attraction of
schistosome miracidia to natural hosts to be distinctly more apparent
than attraction to incompatible molluscs, and a part of his argument
against the I ‘ attraction theory ” is based on comparable attraction to
at least some unnatural hosts as determined later by histopathological
studies of host reactions to invading parasites. Although the observations of Faust and Meleney (1924), Barlow (1925), Neuhaus (1953), and
the data of Etges and Decker (1963) suggest specific attraction of
S. mansoni miracidia to A . glabratus, there is no reason to believe that
host-specificity is always manifested in the initial contact. The finding
by Cheng (1963a) that the plasma” of five different species of molluscs
will activate the quiescent cercaria of Gorgodera amplicava but in
varying degrees, depending on the molluscan species, and the report of
Cheng et al. (1966b) that the plasma and tissue extracts of seven different
species of marine pelecypods have similar effects on the cercaria of
Himasthla quissetensis, but again in varying degrees depending on the
* The fluid portion of molluscan blood has been referred to in the literature as
serum, plasma or hernolymph. As molluscan blood does not clot, this fluid is technically
not serum, but because of common usage these three terms are used interchangeably
in this review.