4. ANALYSIS OF FACTORS INVOLVED IN SYMBIOSIS
43
Indeed, as stated earlier, cercariae of H . quissetensis, stimulated by the
plasma of Crassostrea virginica or C. gigas, encyst on the exterior,
specifically on the gill surfaces, and are thus prevented from penetrating
the host’s tissues.
A theoretical point which may be raised is concerned with the
effectiveness of chemical attractants in the marine environment.
Sea water, at least in certain areas, comparatively speaking, appears to
be richer in organic molecules than fresh water (Duursma, 1961;
Sutcliffe et al., 1963 ; Riley et al., 1964 ; Wangersky, 1965 ; and others).
Much of the organic materials have resulted from the plant and animal
excretions and secretions and the degradation of decaying biota.
It is possible that these organic molecules may act in competition with
the attractants of molluscs and thus diminish the effectiveness of the
latter.
It is also known that various marine animals engage in extraintestinal digestion, i.e. proteolytic secretions are poured over the food
to reduce it to a semi-liquid form. For example, to cope with large food
masses, many echinoderms evert their stomachs and pour proteases
over the food. Similarly, the Portuguese man-of-war, Physalia,
discharges ferments through the gasterozooids which adhere to the
prey, the polyclad Leptoplana initiates digestion outside its body by
exuding proteases through its everted pharynx over the food mass, and
octopi are known to predigest their prey by discharging a protease
into them (see Nicol, 1960). Thus, if the chemotactic substance is in the
form of a large protein or protein-containing molecule, there is the
possibility that it may be digested by various extra-intestinal proteases
in sea water if the secretion of these enzymes occurs in the proximity
of molluscs and during periods when symbionts are vulnerable to attraction. If such occurs, attractants would be enzymatically altered and
rendered ineffective. These theoretically possible chemical influences,
plus the continuous flow of sea water that is particularly noticeable in
estuaries, could render chemotactic substances ineffective or at least
reduce their efficiency. Hence, in considering the role of chemotaxis in
the marine environment, comparable ambient chemical and physical
factors should be taken into consideration when working models are
designed in the laboratory.
Dwelling on the question of host attraction a bit longer, it appears
appropriate to raise the question as to whether some type of attraction
exists when larger symbionts, such as the commensalistic and parasitic
crabs of the genus Pinnotheres, find and enter the mantle cavities of
marine pelecypods. Observations on the invasion of Crassostreu
virginica by Pinnotheres ostreum suggest that it is not a “ hit or miss ”
43
Indeed, as stated earlier, cercariae of H . quissetensis, stimulated by the
plasma of Crassostrea virginica or C. gigas, encyst on the exterior,
specifically on the gill surfaces, and are thus prevented from penetrating
the host’s tissues.
A theoretical point which may be raised is concerned with the
effectiveness of chemical attractants in the marine environment.
Sea water, at least in certain areas, comparatively speaking, appears to
be richer in organic molecules than fresh water (Duursma, 1961;
Sutcliffe et al., 1963 ; Riley et al., 1964 ; Wangersky, 1965 ; and others).
Much of the organic materials have resulted from the plant and animal
excretions and secretions and the degradation of decaying biota.
It is possible that these organic molecules may act in competition with
the attractants of molluscs and thus diminish the effectiveness of the
latter.
It is also known that various marine animals engage in extraintestinal digestion, i.e. proteolytic secretions are poured over the food
to reduce it to a semi-liquid form. For example, to cope with large food
masses, many echinoderms evert their stomachs and pour proteases
over the food. Similarly, the Portuguese man-of-war, Physalia,
discharges ferments through the gasterozooids which adhere to the
prey, the polyclad Leptoplana initiates digestion outside its body by
exuding proteases through its everted pharynx over the food mass, and
octopi are known to predigest their prey by discharging a protease
into them (see Nicol, 1960). Thus, if the chemotactic substance is in the
form of a large protein or protein-containing molecule, there is the
possibility that it may be digested by various extra-intestinal proteases
in sea water if the secretion of these enzymes occurs in the proximity
of molluscs and during periods when symbionts are vulnerable to attraction. If such occurs, attractants would be enzymatically altered and
rendered ineffective. These theoretically possible chemical influences,
plus the continuous flow of sea water that is particularly noticeable in
estuaries, could render chemotactic substances ineffective or at least
reduce their efficiency. Hence, in considering the role of chemotaxis in
the marine environment, comparable ambient chemical and physical
factors should be taken into consideration when working models are
designed in the laboratory.
Dwelling on the question of host attraction a bit longer, it appears
appropriate to raise the question as to whether some type of attraction
exists when larger symbionts, such as the commensalistic and parasitic
crabs of the genus Pinnotheres, find and enter the mantle cavities of
marine pelecypods. Observations on the invasion of Crassostreu
virginica by Pinnotheres ostreum suggest that it is not a “ hit or miss ”