4. ANALYSIS OF FACTORS INVOLVED IN SYMBIOSIS
41
molluscan species, suggest that the stimulant given off by the molluscan
hosts may be of a general rather than a species-specific nature. Thus
in the case of Sudds’ work, the more apparent attraction of such unnatural hosts as Bulimnaea megasoma and Fossaria abrussa for Trichobilharzia elvae miracidia could very well represent non-specific
attraction, divorced from subsequent successful or unsuccessful
development of the parasite in the host. Only additional studies of
this nature will reveal whether specific and general attractions both
occur in nature.
Direct evidences which indicate that molluscs do secrete substances
to the exterior where they can influence symbionts in the immediate
proximity is still in need of confirmation. Suggestive evidences, however, are available. Wright (1959b), for example, has demonstrated by
paper chromatography that there are what he considers species-specific
substances in the body-surface mucus of a number of species of snails.
It is unfortunate that later (1959a) he should imply that such substances may serve as specific attractants, since no proof of this exists
as far as I can determine. Another example of the ability of molluscs to
secrete parasite-effecting substances has been demonstrated by Cheng
et al. (1966a,b). It was found that the plasma (hemolymph) seeping to
the exterior from the soft tissues of two species of oysters, Crassostrea
virgirtica and C. gigas, will stimulate the cercariae of Himasthla
quissetensis to encyst and thus immobilize them and render them unable
to penetrate these pelecypods. It is well known that oysters and perhaps
other pelecypods continuously lose blood during diapedesis (see review
by Galtsoff, 1964). It is not known, however, if the diapedetic rate
fluctuates and, if it does, what ambient or physiological factors influence this. Thus the secretion of chemical substances from a mollusc,
which may result in chemotaxis, could also be influenced by both the
environment and the host’s physiological state.
An evaluation of presently available data leads me to believe that
chemotaxis, operative within short distances and probably not speciesspecific, does exist in the case of mollusc-symbiont relationships.
Although earlier authors who have specifically studied miracidiamollusc relationships have attributed chemotactic attraction to mucus,
there is no direct evidence for this. In fact, misquotes and failures to
check the original literature have misled others to single out mucus
as the only attractant-incorporating exudate. Faust and Hoffman
(1934) used the loose term “ juice ”. MacInnis’s (1965) elaborate study
with known chemicals is a commendable beginning in attempting to
characterize the attractant; however, characterization of natural
chemotactic agent(s) is still wanting.
41
molluscan species, suggest that the stimulant given off by the molluscan
hosts may be of a general rather than a species-specific nature. Thus
in the case of Sudds’ work, the more apparent attraction of such unnatural hosts as Bulimnaea megasoma and Fossaria abrussa for Trichobilharzia elvae miracidia could very well represent non-specific
attraction, divorced from subsequent successful or unsuccessful
development of the parasite in the host. Only additional studies of
this nature will reveal whether specific and general attractions both
occur in nature.
Direct evidences which indicate that molluscs do secrete substances
to the exterior where they can influence symbionts in the immediate
proximity is still in need of confirmation. Suggestive evidences, however, are available. Wright (1959b), for example, has demonstrated by
paper chromatography that there are what he considers species-specific
substances in the body-surface mucus of a number of species of snails.
It is unfortunate that later (1959a) he should imply that such substances may serve as specific attractants, since no proof of this exists
as far as I can determine. Another example of the ability of molluscs to
secrete parasite-effecting substances has been demonstrated by Cheng
et al. (1966a,b). It was found that the plasma (hemolymph) seeping to
the exterior from the soft tissues of two species of oysters, Crassostrea
virgirtica and C. gigas, will stimulate the cercariae of Himasthla
quissetensis to encyst and thus immobilize them and render them unable
to penetrate these pelecypods. It is well known that oysters and perhaps
other pelecypods continuously lose blood during diapedesis (see review
by Galtsoff, 1964). It is not known, however, if the diapedetic rate
fluctuates and, if it does, what ambient or physiological factors influence this. Thus the secretion of chemical substances from a mollusc,
which may result in chemotaxis, could also be influenced by both the
environment and the host’s physiological state.
An evaluation of presently available data leads me to believe that
chemotaxis, operative within short distances and probably not speciesspecific, does exist in the case of mollusc-symbiont relationships.
Although earlier authors who have specifically studied miracidiamollusc relationships have attributed chemotactic attraction to mucus,
there is no direct evidence for this. In fact, misquotes and failures to
check the original literature have misled others to single out mucus
as the only attractant-incorporating exudate. Faust and Hoffman
(1934) used the loose term “ juice ”. MacInnis’s (1965) elaborate study
with known chemicals is a commendable beginning in attempting to
characterize the attractant; however, characterization of natural
chemotactic agent(s) is still wanting.