18
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
I. HOST-SYMBIONT CONTACT
As Davenport (1955) has pointed out, symbiosis may be considered
as an example of specialized behavior, grading from casual association
to relationships in which the symbiont makes an active search for its
host. This implies that the factors influencing host-symbiont contact
range from none to one of very definite attraction, although even in the
case of “ casual associations ” there may or may not be the involvement
of certain factors, other than the physical properties of the ambient
environment, which influence the contact.
A. Accidental contact
The first type of host-symbiont contact may be thought of as a
casual and accidental one. There is no reason to believe that many
epiphoretic organisms found clinging to the shells of marine molluscs
had actively sought out and became attached to their hosts, nor is there
experimental evidence to suggest that there is host-specificity in the
choice of hosts by all epiphoronts. Although Dales (1957) has chosen to
interpret the association of the hydroid Clytia bakeri with certain
molluscs on the surf-swept beaches of southern California as reported by
Torrey (1904) to be beneficial for the survival of the hydroids, and
probably correctly so, there is no reason to believe that C. bakeri has a
specific affinity for the clam, Donax gouldi, the Pismo clam, Tivela
stultorum, or the olive shell, Olivella, on all three of which species it has
been found. The abundance of these molluscs, coupled with their
availability when the planula larvae were settling, most probably
accounts for their relationships with these more or less sedentary
molluscs. Because of the sedentary habits of these molluscs, the
hydroids are prevented from being swept away with the tide. In
addition, Dales has suggested that the hydroids probably benefit
from their position near the feeding currents of the clams. When found
on Olivella, the hydroid gains mobility and its usual position near the
host’s siphon may further improve its chances for food. If the hydroids
indeed do share the hosts’ food, then the relationship should be considered as commensalism, but at this time no direct evidence is available.
A list of some hydroids known to be symbiotic on marine molluscs is
given in Table 11.
B. Contact dependent upon hosts’ feeding mechanisms
The second type of mechanism involves the active feeding habits of
the hosts. In the case of sedentary filter-feeding molluscs, smaller
organisms, comprising the zooplankton, are drawn into the host and if
these successfully pass through the selective process of the host’s gill
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
I. HOST-SYMBIONT CONTACT
As Davenport (1955) has pointed out, symbiosis may be considered
as an example of specialized behavior, grading from casual association
to relationships in which the symbiont makes an active search for its
host. This implies that the factors influencing host-symbiont contact
range from none to one of very definite attraction, although even in the
case of “ casual associations ” there may or may not be the involvement
of certain factors, other than the physical properties of the ambient
environment, which influence the contact.
A. Accidental contact
The first type of host-symbiont contact may be thought of as a
casual and accidental one. There is no reason to believe that many
epiphoretic organisms found clinging to the shells of marine molluscs
had actively sought out and became attached to their hosts, nor is there
experimental evidence to suggest that there is host-specificity in the
choice of hosts by all epiphoronts. Although Dales (1957) has chosen to
interpret the association of the hydroid Clytia bakeri with certain
molluscs on the surf-swept beaches of southern California as reported by
Torrey (1904) to be beneficial for the survival of the hydroids, and
probably correctly so, there is no reason to believe that C. bakeri has a
specific affinity for the clam, Donax gouldi, the Pismo clam, Tivela
stultorum, or the olive shell, Olivella, on all three of which species it has
been found. The abundance of these molluscs, coupled with their
availability when the planula larvae were settling, most probably
accounts for their relationships with these more or less sedentary
molluscs. Because of the sedentary habits of these molluscs, the
hydroids are prevented from being swept away with the tide. In
addition, Dales has suggested that the hydroids probably benefit
from their position near the feeding currents of the clams. When found
on Olivella, the hydroid gains mobility and its usual position near the
host’s siphon may further improve its chances for food. If the hydroids
indeed do share the hosts’ food, then the relationship should be considered as commensalism, but at this time no direct evidence is available.
A list of some hydroids known to be symbiotic on marine molluscs is
given in Table 11.
B. Contact dependent upon hosts’ feeding mechanisms
The second type of mechanism involves the active feeding habits of
the hosts. In the case of sedentary filter-feeding molluscs, smaller
organisms, comprising the zooplankton, are drawn into the host and if
these successfully pass through the selective process of the host’s gill