4. ANALYSIS OF FACTORS INVOLVED I N SYMBIOSIS
51
Since in Davenport’s model several additional favorable pressures have
been introduced, Dethier’s reservations can be removed.
The examples and theoretical considerations presented above are
again not meant to detract from our main theme of marine symbioses.
On the contrary, these are presented in hopes of stimulating thinking
and perhaps suggesting new avenues of approach in its study.
One of the few studies concerned with host-preference by a marine
symbiont for a mollusc was reported by Ross (1965), who studied the
relationship between the sea anemone, Stomphia coccinea, and the ribbed
mussel, Modiolus modiolus. Specimens of Stomphia coccinea collected
by dredging in San Juan Channel of Puget Sound usually are found
attached to shells of Modiolus modiolus. This suggests that the epiphoretic anemone prefers settling on these pelecypods. This led to
experiments in which eighteen anemones were induced to swim by
placing them in contact with the starfish Dermasterias imbricata. This
swimming reaction of Stomphia coccinea to Dermasterias imbricatn and
other starfish had been discovered earlier by Yentsch and Pierce (1955)
and Sund (1958). After the anemones ceased swimming, each specimen
was placed in a separate bowl containing flat stones and a live Modiolus
modiolus, with an ample area of glass in between. The anemones were
arranged so that they lay on the glass, with the tentacles of six and the
pedal disks of another six touching the molluscs’ shells. The remaining
six anemones were touching stones. One hour later, eleven of the twelve
anemones touching shells had settled on these. Of those remaining,
five had settled on glass and two remained unattached.
Sund (1958) has reported that when Stomphia comes out of its
post-swimming torpor ”, usually it quickly settles on any available
surface by attaching its pedal disk little by little to that surface. ROSS
has found that those which settle on M . modiolus behave differently.
Some bent over shells with their tentacles and oral disk extended outwards like an umbrella (Fig. 9A). Others leaned on the shells with a few
tentacles in contact. This tentacular-oral activity was followed by
movements of the basal region. The latter was either extended horizontally like a huge plate which was turned towards the shell or swelled
up into a hemisphere which was pushed towards the shell. The events
beginning with the tentacular response and ending with the movement
of the basal disk to the shell only took about 1-2 min. It was also noted
that once the pedal disk became attached, the anemone generally slides
across the shell surface towards a final resting position (Fig. 9B).
Subsequent experiments revealed that once the pedal disk makes
contact with the shell, the tentacles and oral disk become detached and
the animal straightens itself. If the pedal disk contact is not made
“
51
Since in Davenport’s model several additional favorable pressures have
been introduced, Dethier’s reservations can be removed.
The examples and theoretical considerations presented above are
again not meant to detract from our main theme of marine symbioses.
On the contrary, these are presented in hopes of stimulating thinking
and perhaps suggesting new avenues of approach in its study.
One of the few studies concerned with host-preference by a marine
symbiont for a mollusc was reported by Ross (1965), who studied the
relationship between the sea anemone, Stomphia coccinea, and the ribbed
mussel, Modiolus modiolus. Specimens of Stomphia coccinea collected
by dredging in San Juan Channel of Puget Sound usually are found
attached to shells of Modiolus modiolus. This suggests that the epiphoretic anemone prefers settling on these pelecypods. This led to
experiments in which eighteen anemones were induced to swim by
placing them in contact with the starfish Dermasterias imbricata. This
swimming reaction of Stomphia coccinea to Dermasterias imbricatn and
other starfish had been discovered earlier by Yentsch and Pierce (1955)
and Sund (1958). After the anemones ceased swimming, each specimen
was placed in a separate bowl containing flat stones and a live Modiolus
modiolus, with an ample area of glass in between. The anemones were
arranged so that they lay on the glass, with the tentacles of six and the
pedal disks of another six touching the molluscs’ shells. The remaining
six anemones were touching stones. One hour later, eleven of the twelve
anemones touching shells had settled on these. Of those remaining,
five had settled on glass and two remained unattached.
Sund (1958) has reported that when Stomphia comes out of its
post-swimming torpor ”, usually it quickly settles on any available
surface by attaching its pedal disk little by little to that surface. ROSS
has found that those which settle on M . modiolus behave differently.
Some bent over shells with their tentacles and oral disk extended outwards like an umbrella (Fig. 9A). Others leaned on the shells with a few
tentacles in contact. This tentacular-oral activity was followed by
movements of the basal region. The latter was either extended horizontally like a huge plate which was turned towards the shell or swelled
up into a hemisphere which was pushed towards the shell. The events
beginning with the tentacular response and ending with the movement
of the basal disk to the shell only took about 1-2 min. It was also noted
that once the pedal disk became attached, the anemone generally slides
across the shell surface towards a final resting position (Fig. 9B).
Subsequent experiments revealed that once the pedal disk makes
contact with the shell, the tentacles and oral disk become detached and
the animal straightens itself. If the pedal disk contact is not made
“