4. ANALYSIS O F FACTORS INVOLVED I N SYMBIOSIS
47
Laing further stated :
Not only, then, do some parasites find environments first and hosts later,
they may often use quite different senses for the perception of the two and
make quite different movements t o reach them. What those senses and
movements are, however, will differ greatly with different parasites, and
must be especially determined in each particular case,
Wright (1959a, 1960), in considering host-finding by trematode
miracidia, reiterated Laing’s theory. He suggested that three stages are
involved of which the first is concerned with the location of the hoethabitat by response to physical stimuli. Subsequent to this, random
movement, which ends when the parasite encounters a chemotactic
zone, was also postulated. The third stage involves chemotaxis. Thus
it would appear that the ‘‘ location ” of the host-habitat by a symbiont
occurs prior to the operation of any chemotactic force.
Not all of the mechanisms involved in host-habitat location are yet
known; however, the natural taxes of both symbiont and host, where
such exist, undoubtedly serve as major forces in bringing the partners
into each other’s proximity. Thus, for example, the negative geotaxis
and positive phototaxis of Schistosoma rnansoni miracidia mentioned
in the preceding section serve to bring these ciliated larvae to the
habitat of their molluscan host which a~ a rule is found in the subsurfacial region, clinging to the underside of vegetation. Although other
instances of taxes among miracidia are known (Takahashi, 1927;
Suzuki, 1931; Yasuraoka, 1954; Ingalls et a,?., 1949; and others),
such information is lacking for most marine species. Similarly, phototaxis among certain larval parasitic nematodes has been reported
(Veglia, 1915; Croll, 1965; and others) but not among marine species.
Furthermore, natural taxes among other categories of marine symbionts
remain essentially unexplored.
It is of interest at this point to mention the studies of Welsh
(1930, 1931) on tactic responses which revealed that once hostsymbiont contact is made the original response may become altered and
serve to further advantage in permitting successful establishment of the
symbiont.
Working with the commensal (2) mite, Unionicola ypsilophorus var.
huldemani, that inhabits the mantle cavity and gills of the freshwater
mussel, Anodonta cataracta, he has found that the tactic responses of
the mite could be reversed by a host factor. Specifically, Welsh has
demonstrated that if Unionicola ypsilophorus, removed from its host,
is washed free of a factor of host origin, it is positively phototactic.
If the host factor is added to an aquarium containing positively
phototactic mites, these immediately reversed to negative photo-
47
Laing further stated :
Not only, then, do some parasites find environments first and hosts later,
they may often use quite different senses for the perception of the two and
make quite different movements t o reach them. What those senses and
movements are, however, will differ greatly with different parasites, and
must be especially determined in each particular case,
Wright (1959a, 1960), in considering host-finding by trematode
miracidia, reiterated Laing’s theory. He suggested that three stages are
involved of which the first is concerned with the location of the hoethabitat by response to physical stimuli. Subsequent to this, random
movement, which ends when the parasite encounters a chemotactic
zone, was also postulated. The third stage involves chemotaxis. Thus
it would appear that the ‘‘ location ” of the host-habitat by a symbiont
occurs prior to the operation of any chemotactic force.
Not all of the mechanisms involved in host-habitat location are yet
known; however, the natural taxes of both symbiont and host, where
such exist, undoubtedly serve as major forces in bringing the partners
into each other’s proximity. Thus, for example, the negative geotaxis
and positive phototaxis of Schistosoma rnansoni miracidia mentioned
in the preceding section serve to bring these ciliated larvae to the
habitat of their molluscan host which a~ a rule is found in the subsurfacial region, clinging to the underside of vegetation. Although other
instances of taxes among miracidia are known (Takahashi, 1927;
Suzuki, 1931; Yasuraoka, 1954; Ingalls et a,?., 1949; and others),
such information is lacking for most marine species. Similarly, phototaxis among certain larval parasitic nematodes has been reported
(Veglia, 1915; Croll, 1965; and others) but not among marine species.
Furthermore, natural taxes among other categories of marine symbionts
remain essentially unexplored.
It is of interest at this point to mention the studies of Welsh
(1930, 1931) on tactic responses which revealed that once hostsymbiont contact is made the original response may become altered and
serve to further advantage in permitting successful establishment of the
symbiont.
Working with the commensal (2) mite, Unionicola ypsilophorus var.
huldemani, that inhabits the mantle cavity and gills of the freshwater
mussel, Anodonta cataracta, he has found that the tactic responses of
the mite could be reversed by a host factor. Specifically, Welsh has
demonstrated that if Unionicola ypsilophorus, removed from its host,
is washed free of a factor of host origin, it is positively phototactic.
If the host factor is added to an aquarium containing positively
phototactic mites, these immediately reversed to negative photo-