4. ANALYSIS OF FACTORS INVOLVED IN SYMBIOSIS
19
apparatus (Nelson, 1938, 1960) could become established as endosymbionts or, if they do not, certain species could become attached to the
exterior of the soft tissues as ectosymbionts. The thigmotrichous
ciliates, often found in the mantle cavity or on the gills and palps of
estuarine pelecypods, belong to the second group, while the ciliate
Trichodina myicola, found in the alimentary canal of M y a arenaria,
belongs to the first.
If the host, as in the case of many marine gastropods, is an active
detritus feeder, various symbionts could be included in the food.
Various protozoa and helminths, including cysts and eggs, could be
introduced into the host in this manner. Although examples have not
yet been found among marine molluscs, it is possible that cannibalistic
molluscs could become parasitized while feeding on other molluscs that
are parasitized. Cheng and Alicata (1965) have reported that the
transfer of the third-stage larvae of Angiostrongylus cantonensis from
one land snail, A c h t i n a fulica, to another can be effected by this
method.
C. Contact inpuenced by chemotaxis
In addition to the two methods of host-symbiont contact given
above, many symbionts, ranging from ectocommensals to endoparasites, in varying degrees, seek and contact their hosts. It is primarily
with such active symbionts that specific attraction to the host is
suspected. Unfortunately, information pertaining to the chemotactic
response of symbionts to marine molluscs is extremely sparse. Although
specially designed studies have been carried out to determine the
attraction for symbionts, primarily commensal polychaetes, to nonmolluscan hosts (Davenport, 1950 ; Davenport and Hickok, 1951), the
attraction of the sea anemone Stoichactis for the pomacentrid fish
Amphiprion percula (Davenport and Norris, 1958), the attraction of the
east coast pinnotherid crab, Dissodactylus mellitae, to its echinoid host,
Mellita (Johnson, 1952), and the attraction between Anodonta implicata
glochidia and the alewife, Pomolobus pseudoharengus (Davenport and
Warmuth, 1965), parallel studies involving marine molluscan hosts are
few (see reviews by Davenport, 1955, 1966).
Among commensalistic relationships, Ross (1960), who studied the
relationship between the actinian anemone, Calliactis parasitica, and the
hermit crab, Eupagurus bernhardus, with the latter within the shell of
the whelk, Buccinum undatum, has demonstrated that Calliactis parasitica will readily settle on shells of living Buccinum undatum in the
laboratory and will not desert these for shells occupied by crabs.
Similarly, Calliactis parasitica will with equal frequency become
19
apparatus (Nelson, 1938, 1960) could become established as endosymbionts or, if they do not, certain species could become attached to the
exterior of the soft tissues as ectosymbionts. The thigmotrichous
ciliates, often found in the mantle cavity or on the gills and palps of
estuarine pelecypods, belong to the second group, while the ciliate
Trichodina myicola, found in the alimentary canal of M y a arenaria,
belongs to the first.
If the host, as in the case of many marine gastropods, is an active
detritus feeder, various symbionts could be included in the food.
Various protozoa and helminths, including cysts and eggs, could be
introduced into the host in this manner. Although examples have not
yet been found among marine molluscs, it is possible that cannibalistic
molluscs could become parasitized while feeding on other molluscs that
are parasitized. Cheng and Alicata (1965) have reported that the
transfer of the third-stage larvae of Angiostrongylus cantonensis from
one land snail, A c h t i n a fulica, to another can be effected by this
method.
C. Contact inpuenced by chemotaxis
In addition to the two methods of host-symbiont contact given
above, many symbionts, ranging from ectocommensals to endoparasites, in varying degrees, seek and contact their hosts. It is primarily
with such active symbionts that specific attraction to the host is
suspected. Unfortunately, information pertaining to the chemotactic
response of symbionts to marine molluscs is extremely sparse. Although
specially designed studies have been carried out to determine the
attraction for symbionts, primarily commensal polychaetes, to nonmolluscan hosts (Davenport, 1950 ; Davenport and Hickok, 1951), the
attraction of the sea anemone Stoichactis for the pomacentrid fish
Amphiprion percula (Davenport and Norris, 1958), the attraction of the
east coast pinnotherid crab, Dissodactylus mellitae, to its echinoid host,
Mellita (Johnson, 1952), and the attraction between Anodonta implicata
glochidia and the alewife, Pomolobus pseudoharengus (Davenport and
Warmuth, 1965), parallel studies involving marine molluscan hosts are
few (see reviews by Davenport, 1955, 1966).
Among commensalistic relationships, Ross (1960), who studied the
relationship between the actinian anemone, Calliactis parasitica, and the
hermit crab, Eupagurus bernhardus, with the latter within the shell of
the whelk, Buccinum undatum, has demonstrated that Calliactis parasitica will readily settle on shells of living Buccinum undatum in the
laboratory and will not desert these for shells occupied by crabs.
Similarly, Calliactis parasitica will with equal frequency become