4. ANALYSIS O F FACTORS INVOLVED I N SYMBIOSIS
55
relationship between nutrition and invasion by mutualists, or even
more so by parasites.
Read has defined susceptibility as “ a physiological state of the host
in which the parasite is supplied with its life needs ” and insusceptibility
as “ the state in which these life needs are not satisfied.” He has defined
resistance as “ those alterations of the physiological state of the host
which represent a response to previous or present experience with the
parasite, or a chemically related entity.” According to the broader
definitions given by Read, we are really concerned with susceptibilityinsusceptibility and varying degrees of resistance when we consider
establishment of a symbiont.
When one deals with relationships between symbionts and hosts,
invariably the terms “ normal ” or “ compatible ” hosts and “ abnormal ” or “ incompatible ” hosts are mentioned. Since “ normal ”
and “ abnormal ” connote meanings beyond that implied within the
realm of host-symbiont relationships, the alternative terms ( ( compatible 7 7 or “ natural ” and “ incompatible ” or “ unnatural ” are
preferred. In using these adjectives, host-specificity is implied although
the degree of specificity may vary. Host-specificity7 in the light of
Read’s broad definitions, may be expressed as a manifestation of both
susceptibility and/or resistance, primarily the former. The role of
resistance in determining host-specificity naturally excludes acquired
resistance, since this type of resistance, for example, the development
of specific antibodiesas the result ofan initial contactwithaparasite, may
bring about complete immunityto a homologous parasite in asnbsequent
contact, and hence cannot be considered as naturally occurring
incompatibility. On the other hand, innate resistance may be in whole
or in part responsible for incompatibility. It is thus apparent that one
of the first, if not the initial, confrontation facing a symbiont which
has successfully made contact with its host is to overcome resistance.
It is generally recognized that basically resistance can be of two
types : physiological, including genetical, and behavioral, including
mechanical.
A. Physiological resistance
This type of resistance is the one defined by Read (1958b) and has
been cited above. In order to analyze critically what is known about
physiological resistance in marine molluscs, in all molluscs for that
matter, it is necessary to divide this type of resistance into two subcategories.
(1) In defining parasitism it has been stated that the parasite is
metabolically dependent on its host and that the metabolic dependency
55
relationship between nutrition and invasion by mutualists, or even
more so by parasites.
Read has defined susceptibility as “ a physiological state of the host
in which the parasite is supplied with its life needs ” and insusceptibility
as “ the state in which these life needs are not satisfied.” He has defined
resistance as “ those alterations of the physiological state of the host
which represent a response to previous or present experience with the
parasite, or a chemically related entity.” According to the broader
definitions given by Read, we are really concerned with susceptibilityinsusceptibility and varying degrees of resistance when we consider
establishment of a symbiont.
When one deals with relationships between symbionts and hosts,
invariably the terms “ normal ” or “ compatible ” hosts and “ abnormal ” or “ incompatible ” hosts are mentioned. Since “ normal ”
and “ abnormal ” connote meanings beyond that implied within the
realm of host-symbiont relationships, the alternative terms ( ( compatible 7 7 or “ natural ” and “ incompatible ” or “ unnatural ” are
preferred. In using these adjectives, host-specificity is implied although
the degree of specificity may vary. Host-specificity7 in the light of
Read’s broad definitions, may be expressed as a manifestation of both
susceptibility and/or resistance, primarily the former. The role of
resistance in determining host-specificity naturally excludes acquired
resistance, since this type of resistance, for example, the development
of specific antibodiesas the result ofan initial contactwithaparasite, may
bring about complete immunityto a homologous parasite in asnbsequent
contact, and hence cannot be considered as naturally occurring
incompatibility. On the other hand, innate resistance may be in whole
or in part responsible for incompatibility. It is thus apparent that one
of the first, if not the initial, confrontation facing a symbiont which
has successfully made contact with its host is to overcome resistance.
It is generally recognized that basically resistance can be of two
types : physiological, including genetical, and behavioral, including
mechanical.
A. Physiological resistance
This type of resistance is the one defined by Read (1958b) and has
been cited above. In order to analyze critically what is known about
physiological resistance in marine molluscs, in all molluscs for that
matter, it is necessary to divide this type of resistance into two subcategories.
(1) In defining parasitism it has been stated that the parasite is
metabolically dependent on its host and that the metabolic dependency