42
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
If my conclusion that chemotaxis does occur is correct, it is necessary
to examine the possible importance of this type of attraction in aiding
the symbiont to find its molluscan host. Chernin and Dunavan’s
(1962) investigation, which in essence expresses in quantitative terms
what has been known about taxes of Schistosoma mansoni miracidia,
is an excellent example demonstrating that negative geo taxis appears
to be a more powerful determinant of miracidial behavior than positive
phototaxis. In nature these two taxes may be assumed to reinforce
each other in the host-seeking process, although Chernin and Dunavan
have carefully stated: “ However, it is now evident that these responses
of the miracidium should be thought of more as tendencies than an
(sic) inviolable characteristics of their total behavior.’’ The question
is, are these taxes dominant over the host’s chemotactic stimulation
if the forces are opposed? Etges and Decker (1963) have stated:
“ There is no doubt that both light and gravity are far more powerful
stimuli in determining the orientation of S. mansoni miracidia than the
chemical ones produced by their molluscan host.” From these statements, and from personal experience, it would appear that if the
taxes, plus other environmental factors, direct the miracidium away
from its host, the chemical attractants emitted from molluscs are
definitely insufficient to bring about host-symbiont contact. It is only
under optimum conditions, for example in the case of S. mamoni miracidia
when the combined forces of positive phototaxis and negative geotaxis
bring the miracidia into the intimate proximity of Australorbis gbbratus,
that the occurrence of the chemotaxis favors host-symbiont contact.
The detailed analysis of schistosome-snail relationship presented
above is not meant to deter the reader from marine biology. Rather, it
is meant to stimulate work along these lines using marine mollusctrematode combinations. In the marine environment, especially the
estuarine, where large numbers of semi-sedentary pelecypods serve as
intermediate hosts of trematodes, the presence or absence of chemotactic
forces may well be academic. For example, trematodes such as Cercaria
myae, which develops to the cercarial stage in N y a arenaria (see
Uzmann, 1952), or Cercaria milfordensis, which develops in M y t i l w
edulis (see Uzmann, 1953), would undoubtedly be swept into their
molluscan hosts by the strong in-current if these possess a freeswimming miracidial stage. I have observed the uptake of Himasthla
quissetensis cercariae by actively pumping oysters. The cercariae are
swiftly and unquestionably carried in beyond the boundaries of the
oysters’ valves by the in-current. On the other hand, if the mollusc,
like oysters, does not possess a siphon, mere penetration beyond the
valves need not mean successful penetration into the host’s tissues.
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
If my conclusion that chemotaxis does occur is correct, it is necessary
to examine the possible importance of this type of attraction in aiding
the symbiont to find its molluscan host. Chernin and Dunavan’s
(1962) investigation, which in essence expresses in quantitative terms
what has been known about taxes of Schistosoma mansoni miracidia,
is an excellent example demonstrating that negative geo taxis appears
to be a more powerful determinant of miracidial behavior than positive
phototaxis. In nature these two taxes may be assumed to reinforce
each other in the host-seeking process, although Chernin and Dunavan
have carefully stated: “ However, it is now evident that these responses
of the miracidium should be thought of more as tendencies than an
(sic) inviolable characteristics of their total behavior.’’ The question
is, are these taxes dominant over the host’s chemotactic stimulation
if the forces are opposed? Etges and Decker (1963) have stated:
“ There is no doubt that both light and gravity are far more powerful
stimuli in determining the orientation of S. mansoni miracidia than the
chemical ones produced by their molluscan host.” From these statements, and from personal experience, it would appear that if the
taxes, plus other environmental factors, direct the miracidium away
from its host, the chemical attractants emitted from molluscs are
definitely insufficient to bring about host-symbiont contact. It is only
under optimum conditions, for example in the case of S. mamoni miracidia
when the combined forces of positive phototaxis and negative geotaxis
bring the miracidia into the intimate proximity of Australorbis gbbratus,
that the occurrence of the chemotaxis favors host-symbiont contact.
The detailed analysis of schistosome-snail relationship presented
above is not meant to deter the reader from marine biology. Rather, it
is meant to stimulate work along these lines using marine mollusctrematode combinations. In the marine environment, especially the
estuarine, where large numbers of semi-sedentary pelecypods serve as
intermediate hosts of trematodes, the presence or absence of chemotactic
forces may well be academic. For example, trematodes such as Cercaria
myae, which develops to the cercarial stage in N y a arenaria (see
Uzmann, 1952), or Cercaria milfordensis, which develops in M y t i l w
edulis (see Uzmann, 1953), would undoubtedly be swept into their
molluscan hosts by the strong in-current if these possess a freeswimming miracidial stage. I have observed the uptake of Himasthla
quissetensis cercariae by actively pumping oysters. The cercariae are
swiftly and unquestionably carried in beyond the boundaries of the
oysters’ valves by the in-current. On the other hand, if the mollusc,
like oysters, does not possess a siphon, mere penetration beyond the
valves need not mean successful penetration into the host’s tissues.