54
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
ascidian host. Again, parallel studies involving marine molluscan hosts
are still not available.
It should also be mentioned a t this point that the nature of the
substrate may influence not only metamorphosis but also the initial
settling of the larvae of marine symbionts. The investigations of
Wilson (1937, 1948, 1952) on Notomastus, Ophelia, and other marine
larvae, as well as the findings of Knight-Jones (1963) relative to
Balanus balanoides and other barnacles, serve as examples to illustrate
how the nature of the substrate can influence the settling of marine
invertebrate larvae. These investigators have demonstrated that
settling larvae of special habit are sensitive to minute differences in
their environment. Furthermore, they may actually sample the substrate before settling, being either attracted or repelled by minute
physical or chemical differences.
11. ESTABLISHMENT OF TIIE SYMBIONT
Successful establishment of a symbiont in or on its host is defined
as the ability of the symbiont to become located a t a suitable site and
to succeed in continuing its normal physiological and reproductive
processes. This implies that the symbiont, has entered an oftentimes
delicate but nevertheless physiologically balanced state with its host.
I n the case of commensals and epiphoretic organisms, successful
establishment depends upon what the host environment (the microenvironment) can provide to shelter it from the surrounding environment (the macroenvironment). I n the case of parasites and mutualists,
the relationship is more complicated since it involves overcoming the
natural internal defense mechanisms of the host. I n other words, itJ
has to overcome the resistance of the host to an invading organism.
I n the earlier literature, a host's resistance was considered as the
reciprocal of its susceptibility to the invading organism (Falk, 1928).
A highly susceptible organism is thus one lacking in resistance. More
recently Schneider (1951) and Read (195810) have correctly pointed out
that this is too simple an expression. Susceptibility and resistance,
from the modern viewpoint, must be considered distinct and separate
biological attributes. Schneider has chosen to define " susceptibility "
and " resistance " in nutritional terms. Specifically, he has defined as
" susceptible factors ') those factors in the nutrition which, when withheld from the host,decrease the extent or effect of an infection and, when
supplied increase the effect. He has defined " resistance factors " as
those nutritional factors which, when withheld, increase the extent of the
infection and, when supplied, decrease the effect. Although these are
satisfactory definitions, they are only useful when considering the
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
ascidian host. Again, parallel studies involving marine molluscan hosts
are still not available.
It should also be mentioned a t this point that the nature of the
substrate may influence not only metamorphosis but also the initial
settling of the larvae of marine symbionts. The investigations of
Wilson (1937, 1948, 1952) on Notomastus, Ophelia, and other marine
larvae, as well as the findings of Knight-Jones (1963) relative to
Balanus balanoides and other barnacles, serve as examples to illustrate
how the nature of the substrate can influence the settling of marine
invertebrate larvae. These investigators have demonstrated that
settling larvae of special habit are sensitive to minute differences in
their environment. Furthermore, they may actually sample the substrate before settling, being either attracted or repelled by minute
physical or chemical differences.
11. ESTABLISHMENT OF TIIE SYMBIONT
Successful establishment of a symbiont in or on its host is defined
as the ability of the symbiont to become located a t a suitable site and
to succeed in continuing its normal physiological and reproductive
processes. This implies that the symbiont, has entered an oftentimes
delicate but nevertheless physiologically balanced state with its host.
I n the case of commensals and epiphoretic organisms, successful
establishment depends upon what the host environment (the microenvironment) can provide to shelter it from the surrounding environment (the macroenvironment). I n the case of parasites and mutualists,
the relationship is more complicated since it involves overcoming the
natural internal defense mechanisms of the host. I n other words, itJ
has to overcome the resistance of the host to an invading organism.
I n the earlier literature, a host's resistance was considered as the
reciprocal of its susceptibility to the invading organism (Falk, 1928).
A highly susceptible organism is thus one lacking in resistance. More
recently Schneider (1951) and Read (195810) have correctly pointed out
that this is too simple an expression. Susceptibility and resistance,
from the modern viewpoint, must be considered distinct and separate
biological attributes. Schneider has chosen to define " susceptibility "
and " resistance " in nutritional terms. Specifically, he has defined as
" susceptible factors ') those factors in the nutrition which, when withheld from the host,decrease the extent or effect of an infection and, when
supplied increase the effect. He has defined " resistance factors " as
those nutritional factors which, when withheld, increase the extent of the
infection and, when supplied, decrease the effect. Although these are
satisfactory definitions, they are only useful when considering the