48
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
taxis. He has found that this taxis-reversing factor is relatively
stable and that it resists boiling and putrefaction. As the result, Welsh
has proposed that it may be a decomposition product of mucus or some
other protein from the gills. Welsh (1931) has demonstrated that the
taxis-reversing factor is specific. By testing materials from eight
species of bivalves (Anodonta cataracta, Lampsilis radiata, Elliptio
complanatus, E. dilatatua, Sphaerium sulcatum, Cyclonais tuberculata,
Eurynia iris and Ligumia fasciola) on three species of commensal mites
(Unionicola ypsilophorus from Anodonta cataracta, Unionicola fossulata
from Cyclonais tuberculata, and an unidentified species from Lampsilis
radiata), he found that only materials from natural hosts cause tactic
reversal.
These interesting findings by Welsh indicate that the natural
taxes of a symbiont may be altered once it makes contact with the
host. In the case of the mites studied, Welsh has suggested that:
“ This reversal may be considered adaptive, for, aided by a positive
chemotropism and stereotropism, it enables the mites to enter and
remain within the host.” Furthermore, he considered the positive
phototaxis to be an adaptive and secondarily acquired characteristic.
Whether this last postulation is true or not is a moot point since Davenport (1955), speculating on this phenomenon, has stated :
We have little detailed information about the early stages of these inquiline
mites studied by Welsh. Obviously they must move from host to host;
otherwise, their spread would never be accomplished. One may wonder
whether during larval life the mites exhibit that positive response to light
which enables them to wander more widely. If so, one may suppose that
subsequently internal changes take place which “ trigger ” the reversal of
response under the influence of the host-factor. This mechanism, plus a
positive chemotaxis appearing at the same time either to the same or to
another host-factor, would insure colonization of hosts when the larvae are
in proximity to a mussel bed.
Thus, if Davenport’s reasonable postulation is correct, then the positive
phototaxis of the mites should not be considered as a secondary
adaptation but as a primary characteristic present since larval stages.
On the other hand, the negative phototaxis brought about by the
host-factor could be considered a later adaptation which enables the
adult to live in a commensalistic relationship with the molluscan host.
However, Davenport (1955), in my opinion, has correctly pointed out
that the rapid reversal from positive to negative phototaxis suggests
that “ this ability to change behavior suddenly at the first experience
of external stimuli from the host may be firmly fixed in the hereditary
constitution of symbionts ”.
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
taxis. He has found that this taxis-reversing factor is relatively
stable and that it resists boiling and putrefaction. As the result, Welsh
has proposed that it may be a decomposition product of mucus or some
other protein from the gills. Welsh (1931) has demonstrated that the
taxis-reversing factor is specific. By testing materials from eight
species of bivalves (Anodonta cataracta, Lampsilis radiata, Elliptio
complanatus, E. dilatatua, Sphaerium sulcatum, Cyclonais tuberculata,
Eurynia iris and Ligumia fasciola) on three species of commensal mites
(Unionicola ypsilophorus from Anodonta cataracta, Unionicola fossulata
from Cyclonais tuberculata, and an unidentified species from Lampsilis
radiata), he found that only materials from natural hosts cause tactic
reversal.
These interesting findings by Welsh indicate that the natural
taxes of a symbiont may be altered once it makes contact with the
host. In the case of the mites studied, Welsh has suggested that:
“ This reversal may be considered adaptive, for, aided by a positive
chemotropism and stereotropism, it enables the mites to enter and
remain within the host.” Furthermore, he considered the positive
phototaxis to be an adaptive and secondarily acquired characteristic.
Whether this last postulation is true or not is a moot point since Davenport (1955), speculating on this phenomenon, has stated :
We have little detailed information about the early stages of these inquiline
mites studied by Welsh. Obviously they must move from host to host;
otherwise, their spread would never be accomplished. One may wonder
whether during larval life the mites exhibit that positive response to light
which enables them to wander more widely. If so, one may suppose that
subsequently internal changes take place which “ trigger ” the reversal of
response under the influence of the host-factor. This mechanism, plus a
positive chemotaxis appearing at the same time either to the same or to
another host-factor, would insure colonization of hosts when the larvae are
in proximity to a mussel bed.
Thus, if Davenport’s reasonable postulation is correct, then the positive
phototaxis of the mites should not be considered as a secondary
adaptation but as a primary characteristic present since larval stages.
On the other hand, the negative phototaxis brought about by the
host-factor could be considered a later adaptation which enables the
adult to live in a commensalistic relationship with the molluscan host.
However, Davenport (1955), in my opinion, has correctly pointed out
that the rapid reversal from positive to negative phototaxis suggests
that “ this ability to change behavior suddenly at the first experience
of external stimuli from the host may be firmly fixed in the hereditary
constitution of symbionts ”.