56
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
may be in the form of nutritional requirements, digestive enzymes,
development stimuli, or control of maturation. Among these, the
contribution of developmental stimuli by the host is known to be of
critical importance during the initial stages of successful establishment by a parasite. Thus alterations in the quality or quantityof
potentially important stimulatory substances could effectively serve
as a form of physiological resistance, I n conjunction with this, the
secretion of inhibitory substances by the host could prevent establishment by the parasite.
(2) I n addition to physiological factors which may inhibit or deter
normal development of the parasite, another set of factors may have
the same effect. These could be considered as “ physiological factors ”
in the broad sense ; however, these are generally considered as distinct
and are termed “ defense mechanisms.” I n order to facilitate the
presentation of a clearer review, defense mechanisms are considered as
distinct from other types of physiological resistance although without
doubt this is to some degree artificial.
1. Developmental stimuli
The development of the science of symbiosis has been such that
those concerned with commensalism and phoresis in the marine
environment have independently followed one avenue of development
and most of their published results have been of a descriptive and
ecological nature, while those concerned with parasitism and mutualism
have examined primarily the relationship between definitive vertebrate
hosts and their parasites or mutualists. The studies of Cleveland,
Hungate, Kirby, and others on the mutualistic flagellates of insects
are exceptions as are the studies of Richards and Brooks on the
bacteroid mutualists of roaches. As a result, practically nothing is
known about developmental stimuli contributed by marine molluscs,
or any mollusc, to their symbionts. Nevertheless, such must exist. It
is known from classical life history studies of parasites that molluscs,
except in a few rare cases, serve as the first or only intermediate host
of digenetic trematodes and in some instances of certain cestodes and
nematodes. It is also known that trematodes undergo both morphogenetic change and delayed polyembryony (asexual reproduction)
and nematodes undergo molts while in their molluscan hosts. Practically
nothing is known about host factors which govern or influence these
essential developmental phases other than certain aspects of nutritional
requirements which are considered in a later section (p. 80). Thus
information comparable to what is known about the influence of
vertebrate hosts on the excystment of larval tapeworms (see review by
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
may be in the form of nutritional requirements, digestive enzymes,
development stimuli, or control of maturation. Among these, the
contribution of developmental stimuli by the host is known to be of
critical importance during the initial stages of successful establishment by a parasite. Thus alterations in the quality or quantityof
potentially important stimulatory substances could effectively serve
as a form of physiological resistance, I n conjunction with this, the
secretion of inhibitory substances by the host could prevent establishment by the parasite.
(2) I n addition to physiological factors which may inhibit or deter
normal development of the parasite, another set of factors may have
the same effect. These could be considered as “ physiological factors ”
in the broad sense ; however, these are generally considered as distinct
and are termed “ defense mechanisms.” I n order to facilitate the
presentation of a clearer review, defense mechanisms are considered as
distinct from other types of physiological resistance although without
doubt this is to some degree artificial.
1. Developmental stimuli
The development of the science of symbiosis has been such that
those concerned with commensalism and phoresis in the marine
environment have independently followed one avenue of development
and most of their published results have been of a descriptive and
ecological nature, while those concerned with parasitism and mutualism
have examined primarily the relationship between definitive vertebrate
hosts and their parasites or mutualists. The studies of Cleveland,
Hungate, Kirby, and others on the mutualistic flagellates of insects
are exceptions as are the studies of Richards and Brooks on the
bacteroid mutualists of roaches. As a result, practically nothing is
known about developmental stimuli contributed by marine molluscs,
or any mollusc, to their symbionts. Nevertheless, such must exist. It
is known from classical life history studies of parasites that molluscs,
except in a few rare cases, serve as the first or only intermediate host
of digenetic trematodes and in some instances of certain cestodes and
nematodes. It is also known that trematodes undergo both morphogenetic change and delayed polyembryony (asexual reproduction)
and nematodes undergo molts while in their molluscan hosts. Practically
nothing is known about host factors which govern or influence these
essential developmental phases other than certain aspects of nutritional
requirements which are considered in a later section (p. 80). Thus
information comparable to what is known about the influence of
vertebrate hosts on the excystment of larval tapeworms (see review by