132
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
B. Behavioral and mechanical resistance
Nothing is known at this time about behavioral and mechanical
resistance or avoidance patterns among molluscs to approaching
symbionts.
111. ESCAPE OF THE SYMBIONT
When one discusses the escape of symbionts from marine molluscs,
a definition of the verb " escape " is necessary. The term is used to
designate the processes involved during the physical departure or
severance of a symbiont or its germ-cell bearing progeny from the host.
It is thus evident that the medusa buds, discharged from the gonangia
of ectophoretic (epizootic) hydroids of molluscs, or the adult animal
itself in the case of commensalistic arthropods, are the escaping forms.
Similarly it i s the cercaria that escapes from trematode-parasitized
molluscs, the infusoriform that escapes from mesozoa-infected gastropods and cephalopods, and presumably an encysted or pre-cystic form
of an intestinal amoeba that escapes. In instances where either the
progeny or the symbiont itself effects the escape, the process may be
termed active escape. In the case of intestinal fauna which are egested
in feces, the process may be termed involuntary escape.
The examples cited above all indicate the departure of the mature
animal or a germ-cell bearing stage. In either case, they leave the host
intact and eventually enter into a similar relationship with another
host. However, still another method exists which permits the symbiont
to continue its life cycle. This involves primarily those relationships
during which there is metabolic dependence, that is, mutualists and
parasites, primarily the latter, although not all parasites escape in this
manner. This method requires the ingestion of its host by the subsequent host. Thus the escape is a passive process, herein designated
as passive escape, which does not involve active behavior on the part
of either the escaping form and/or the parental form, i.e. active escape,
nor does it involve elimination via egestion by the host, i.e. involuntary
escape. Examples of this include the encysted metacestodes of Tylocephalurn in marine pelecypods, or the encysted metacercariae of such
trematodes as Himasthla spp. also in marine pelecypods.
Although from experience with closely related species it is generally
possible to predict at what stage in the life cycle of a symbiont it
escapes from its host, there is practically nothing known about the
mechanisms involved. This is especially true of parasites and mutualists. In the case of intestina1 protozoa, it is assumed that these can be
expelled by involuntary escape. Similarly, those species, such as the
sporozoan Aggregata eberthi in the cuttlefish, Sepia oficinalis, which
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
B. Behavioral and mechanical resistance
Nothing is known at this time about behavioral and mechanical
resistance or avoidance patterns among molluscs to approaching
symbionts.
111. ESCAPE OF THE SYMBIONT
When one discusses the escape of symbionts from marine molluscs,
a definition of the verb " escape " is necessary. The term is used to
designate the processes involved during the physical departure or
severance of a symbiont or its germ-cell bearing progeny from the host.
It is thus evident that the medusa buds, discharged from the gonangia
of ectophoretic (epizootic) hydroids of molluscs, or the adult animal
itself in the case of commensalistic arthropods, are the escaping forms.
Similarly it i s the cercaria that escapes from trematode-parasitized
molluscs, the infusoriform that escapes from mesozoa-infected gastropods and cephalopods, and presumably an encysted or pre-cystic form
of an intestinal amoeba that escapes. In instances where either the
progeny or the symbiont itself effects the escape, the process may be
termed active escape. In the case of intestinal fauna which are egested
in feces, the process may be termed involuntary escape.
The examples cited above all indicate the departure of the mature
animal or a germ-cell bearing stage. In either case, they leave the host
intact and eventually enter into a similar relationship with another
host. However, still another method exists which permits the symbiont
to continue its life cycle. This involves primarily those relationships
during which there is metabolic dependence, that is, mutualists and
parasites, primarily the latter, although not all parasites escape in this
manner. This method requires the ingestion of its host by the subsequent host. Thus the escape is a passive process, herein designated
as passive escape, which does not involve active behavior on the part
of either the escaping form and/or the parental form, i.e. active escape,
nor does it involve elimination via egestion by the host, i.e. involuntary
escape. Examples of this include the encysted metacestodes of Tylocephalurn in marine pelecypods, or the encysted metacercariae of such
trematodes as Himasthla spp. also in marine pelecypods.
Although from experience with closely related species it is generally
possible to predict at what stage in the life cycle of a symbiont it
escapes from its host, there is practically nothing known about the
mechanisms involved. This is especially true of parasites and mutualists. In the case of intestina1 protozoa, it is assumed that these can be
expelled by involuntary escape. Similarly, those species, such as the
sporozoan Aggregata eberthi in the cuttlefish, Sepia oficinalis, which