HABITAT SELECTION BY AQUATIC INVERTEBRATES
299
immersed the three trout species in a hatching tank known to contain
copepod larvae. After two days the brook trout but not the rainbow
or German brown trout were infected. Faust and Meleney (1924),
Herter (1928, 1929), Welsh (1930, 1931), Carton (1968a, b) and Wieser
(1955) have shown similar responses in other parasites, and many
commensals behave in the same way (Davenport, 1950; Davenport
and Hickok, 1951; Johnson, 1952; Gage, 1966b; Morton, 1962). Specific chemical responses clearly play a major role in directing parasites
and commensals towards their hosts. In contrast, no one has yet
identified any of the chemicals involved although a number of authors
have conducted preliminary experiments (Carton, 1968b; Davenport,
1963a; Davenport et al., 1961; Ross and Sutton, 1963).
Species specific chemicals either lead an animal towards its host
or “ capture ” the animal once it has encountered its host. However,
there are other ways in which a chemical might act. It might, for
instance, change a behaviour pattern to such an extent that an otherwise unsuccessful animal would be able to find its host. Welsh (1930,
1931) has studied just such an example. The freshwater mite, Unionicola ypsilophorus var. haldemani (Piers) is a parasite in the mantle
cavity of the bivalve, Anodonta cataracta Say. If it is removed from
the clam and washed it is photopositive, but it quickly becomes photonegative if exposed to mantle cavity water or to gill extract from its
host, Gill extracts of other freshwater bivalves are not effective.
According to Welsh, the reversal in light response enables Unionicola
to find its host’s mantle cavity-one presumes by seeing the bivalve’s
gape as a dark hole. It would be interesting to know of other
relationships that depend on the same sort of mechanism.
Many commensals and parasites in the sea are restricted to one
species of host. Where it has been investigated, the restriction usually
depends on a positive response to the host’s chemicals, but only a slight
one or none at all to chemicals from other species (Welsh, 1931; Davenport, 1950, 1953b; Ross and Sutton, 1961a; Kearn, 1967; Carton,
1968a). Davenport (1953a) has tested the species specificity of the
response of Acholoe to its starfish host and to related species (Table 11).
Activity is restricted to Acholoe’s normal host and to one other species
in the order Phanerozonia; species in other orders of the class
Asteroidea have a low activity (c.f. Kearn on Trematode parasites of
fish, 1967, p. 693).
On occasion, a commensal may live with one of a number of host
species. Gage (1966a) describes how the bivalves Xontacuta substriata
and M . ferruginosa may each be found with four different echinoid
hosts. Although they are most commonly found with only one of the
299
immersed the three trout species in a hatching tank known to contain
copepod larvae. After two days the brook trout but not the rainbow
or German brown trout were infected. Faust and Meleney (1924),
Herter (1928, 1929), Welsh (1930, 1931), Carton (1968a, b) and Wieser
(1955) have shown similar responses in other parasites, and many
commensals behave in the same way (Davenport, 1950; Davenport
and Hickok, 1951; Johnson, 1952; Gage, 1966b; Morton, 1962). Specific chemical responses clearly play a major role in directing parasites
and commensals towards their hosts. In contrast, no one has yet
identified any of the chemicals involved although a number of authors
have conducted preliminary experiments (Carton, 1968b; Davenport,
1963a; Davenport et al., 1961; Ross and Sutton, 1963).
Species specific chemicals either lead an animal towards its host
or “ capture ” the animal once it has encountered its host. However,
there are other ways in which a chemical might act. It might, for
instance, change a behaviour pattern to such an extent that an otherwise unsuccessful animal would be able to find its host. Welsh (1930,
1931) has studied just such an example. The freshwater mite, Unionicola ypsilophorus var. haldemani (Piers) is a parasite in the mantle
cavity of the bivalve, Anodonta cataracta Say. If it is removed from
the clam and washed it is photopositive, but it quickly becomes photonegative if exposed to mantle cavity water or to gill extract from its
host, Gill extracts of other freshwater bivalves are not effective.
According to Welsh, the reversal in light response enables Unionicola
to find its host’s mantle cavity-one presumes by seeing the bivalve’s
gape as a dark hole. It would be interesting to know of other
relationships that depend on the same sort of mechanism.
Many commensals and parasites in the sea are restricted to one
species of host. Where it has been investigated, the restriction usually
depends on a positive response to the host’s chemicals, but only a slight
one or none at all to chemicals from other species (Welsh, 1931; Davenport, 1950, 1953b; Ross and Sutton, 1961a; Kearn, 1967; Carton,
1968a). Davenport (1953a) has tested the species specificity of the
response of Acholoe to its starfish host and to related species (Table 11).
Activity is restricted to Acholoe’s normal host and to one other species
in the order Phanerozonia; species in other orders of the class
Asteroidea have a low activity (c.f. Kearn on Trematode parasites of
fish, 1967, p. 693).
On occasion, a commensal may live with one of a number of host
species. Gage (1966a) describes how the bivalves Xontacuta substriata
and M . ferruginosa may each be found with four different echinoid
hosts. Although they are most commonly found with only one of the