98
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
found as a saprobic member of the community of organisms normally
living at the mud-water interface in coastal areas. It has also been
found as a facultative parasite in the stomach and tissues of oysters.
This is especially true when anaerobic conditions and other environmental factors in oyster bed sediments reach such a level that the oysters
are placed under stress. The frequency with which Hexamita is found in
moribund and dead oysters has caused Mackin et al. (1952), Stein et al.
(1961) and Laird (1961) t o consider it as a pathogenic parasite. However, as Shuster and Hillman (1963) have pointed out, other evidences
(see under Hexamita in Chapter 5, pp. 138-142) indicate that Hexamita
is not a pathogen and its presence in oysters in poor condition is the
effect rather than the cause. Nevertheless, an important ecologicalparasitological principle is exemplified, that is, certain free-living
organisms can become facultative parasites when unfavorable environmental conditions cause physiological stress in the host and thus
render it more susceptible to parasitism. The pathology associated with
Hexamita is discussed later in this section and in Chapter 5.
In mantle cavity. Although a variety of microscopic and macroscopic animals are known to live within the mantle cavities of marine
molluscs, especially pelecypods, there is no reason to believe that all of
these are obligatory parasites. As the mantle cavity of molluscs,
particularly pelecypods, is naturally bathed in a mixture of sea water
and plasma (a mixture termed (‘ shell liquor ” by the shellfishermen),
it is conceivable that certain facultative invaders do become temporarily dependent upon ‘( shell liquor ” as a source of proteins and
amino acids.
Among the best known examples of symbionts found in the mantle
cavities of marine molluscs are the grapsoid crabs belonging to the genus
Pinnotheres. In her monograph of members on t,his genus, Rathbun
(1918) has compared the numerous species which have been collected
from the Americas in association with various tunicates, annelids, sea
urchins and molluscs. Like most authors, she has considered the
majority of these crabs to be commensals. As to whether all pinnotherids are commensals, or whether any of them are, in light of their
possible dependency on their molluscan hosts, must remain a moot
point until further evidence becomes available. At least two species,
P. pisum, the pea crab, and P. ostreum, the oyster crab, are believed
to be true parasites (Dean, 1892; Orton, 1920; Stauber, 1945;
Christensen and McDermott, 1958). Basing his definition of parasitism
on doing harm to the host ”, Dean considered P . ostreum a parasite
because this crab evidently annoys the oyster since the latter’s palps
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
found as a saprobic member of the community of organisms normally
living at the mud-water interface in coastal areas. It has also been
found as a facultative parasite in the stomach and tissues of oysters.
This is especially true when anaerobic conditions and other environmental factors in oyster bed sediments reach such a level that the oysters
are placed under stress. The frequency with which Hexamita is found in
moribund and dead oysters has caused Mackin et al. (1952), Stein et al.
(1961) and Laird (1961) t o consider it as a pathogenic parasite. However, as Shuster and Hillman (1963) have pointed out, other evidences
(see under Hexamita in Chapter 5, pp. 138-142) indicate that Hexamita
is not a pathogen and its presence in oysters in poor condition is the
effect rather than the cause. Nevertheless, an important ecologicalparasitological principle is exemplified, that is, certain free-living
organisms can become facultative parasites when unfavorable environmental conditions cause physiological stress in the host and thus
render it more susceptible to parasitism. The pathology associated with
Hexamita is discussed later in this section and in Chapter 5.
In mantle cavity. Although a variety of microscopic and macroscopic animals are known to live within the mantle cavities of marine
molluscs, especially pelecypods, there is no reason to believe that all of
these are obligatory parasites. As the mantle cavity of molluscs,
particularly pelecypods, is naturally bathed in a mixture of sea water
and plasma (a mixture termed (‘ shell liquor ” by the shellfishermen),
it is conceivable that certain facultative invaders do become temporarily dependent upon ‘( shell liquor ” as a source of proteins and
amino acids.
Among the best known examples of symbionts found in the mantle
cavities of marine molluscs are the grapsoid crabs belonging to the genus
Pinnotheres. In her monograph of members on t,his genus, Rathbun
(1918) has compared the numerous species which have been collected
from the Americas in association with various tunicates, annelids, sea
urchins and molluscs. Like most authors, she has considered the
majority of these crabs to be commensals. As to whether all pinnotherids are commensals, or whether any of them are, in light of their
possible dependency on their molluscan hosts, must remain a moot
point until further evidence becomes available. At least two species,
P. pisum, the pea crab, and P. ostreum, the oyster crab, are believed
to be true parasites (Dean, 1892; Orton, 1920; Stauber, 1945;
Christensen and McDermott, 1958). Basing his definition of parasitism
on doing harm to the host ”, Dean considered P . ostreum a parasite
because this crab evidently annoys the oyster since the latter’s palps