4. ANALYSIS O F FACTORS INVOLVED I N SYMBIOSIS
17
other behavior pattern ; (3) it could be influenced by chemotaxis ; (4) it
could be influenced by other tactic responses of both the host and the
symbiont, or (5) it could be influenced by combinations of some or all of
these factors.
Two additional factors may well be involved when one considers
ectosymbionts : (6) for example, for hydroids that become attached to
the shells of marine molluscs, selectivity on the part of the settling
larvae could also influence the contact; (7) furthermore, in those
associations in which the symbiont has to be in continuous contact with
a solid or semi-solid substrate while approaching the host, the chemical
and physical nature of the substrate could be of importance.
At least seven factors could be considered subordinate to the major
heading “ establishment of the symbiont ”. (1) For commensalism
and phoresis, successful attachment at a suitable site is of prime
importance, although this is also important for the establishment of
parasitic and mutualistic relationships. (2) I n many instances even
prior to successful attachment, the symbiont must overcome the host’s
defense mechanisms, be these in the form of physical defense or a5
internal immunity. (3) In the case of parasites and mutualists, the
availability of sufficient and proper nourishment, derived from the host,
as well as (4) digestive enzymes for those unable to synthesize their own,
are essential. I n the case of commensals and phoronts, the availability
of nutrients, although not directly of host origin, is also necessary.
(5) The role of developmental stimuli contributed by the host as well as
(6) factors that control maturation are of great importance. The
occurrence of developmental stimuli and control of maturation implies
metabolic dependence, and hence occurs only during mutualism and
parasitism. (7) Finally, a tolerable level of pathological change in the
host appears to be a prerequisite for successful establishment over a
satisfactory period of time. This is particularly true during parasitic
relationships. For example, if a parasite is highly pathogenic and causes
the death of the host within a relatively brief period, the relationship,
and therefore the establishment, from the standpoint of the parasite,
could hardly be considered a biologically successful one.
Eventually either the symbiont or its germ cell-bearing progeny
must successfully escape from the host to perpetuate a similar association with another or the same host. From what is known about the
severance of symbionts from hosts, various mechanisms could be
involved to effect the escape.
The conceptual factor analysis of the phases involved in symbiosis
presented above serves as the guide line along which the following review
is based.
A.M.B.--S
3
17
other behavior pattern ; (3) it could be influenced by chemotaxis ; (4) it
could be influenced by other tactic responses of both the host and the
symbiont, or (5) it could be influenced by combinations of some or all of
these factors.
Two additional factors may well be involved when one considers
ectosymbionts : (6) for example, for hydroids that become attached to
the shells of marine molluscs, selectivity on the part of the settling
larvae could also influence the contact; (7) furthermore, in those
associations in which the symbiont has to be in continuous contact with
a solid or semi-solid substrate while approaching the host, the chemical
and physical nature of the substrate could be of importance.
At least seven factors could be considered subordinate to the major
heading “ establishment of the symbiont ”. (1) For commensalism
and phoresis, successful attachment at a suitable site is of prime
importance, although this is also important for the establishment of
parasitic and mutualistic relationships. (2) I n many instances even
prior to successful attachment, the symbiont must overcome the host’s
defense mechanisms, be these in the form of physical defense or a5
internal immunity. (3) In the case of parasites and mutualists, the
availability of sufficient and proper nourishment, derived from the host,
as well as (4) digestive enzymes for those unable to synthesize their own,
are essential. I n the case of commensals and phoronts, the availability
of nutrients, although not directly of host origin, is also necessary.
(5) The role of developmental stimuli contributed by the host as well as
(6) factors that control maturation are of great importance. The
occurrence of developmental stimuli and control of maturation implies
metabolic dependence, and hence occurs only during mutualism and
parasitism. (7) Finally, a tolerable level of pathological change in the
host appears to be a prerequisite for successful establishment over a
satisfactory period of time. This is particularly true during parasitic
relationships. For example, if a parasite is highly pathogenic and causes
the death of the host within a relatively brief period, the relationship,
and therefore the establishment, from the standpoint of the parasite,
could hardly be considered a biologically successful one.
Eventually either the symbiont or its germ cell-bearing progeny
must successfully escape from the host to perpetuate a similar association with another or the same host. From what is known about the
severance of symbionts from hosts, various mechanisms could be
involved to effect the escape.
The conceptual factor analysis of the phases involved in symbiosis
presented above serves as the guide line along which the following review
is based.
A.M.B.--S
3