HABITAT SELEOTION BY AQUATIC INVERTEBRATES
297
drilus s ~ ~ b i o t i c u s ,
by Gray. The results published to date suggest
that temperature, light, salinity, oxygen content, sand permeability,
and pore size and particle size, are significant to many species, and
perhaps further research should be concentrated in these areas.
111. COMMENSAL AND PARASITIC ASSOCIATIONS
There are many well known examples of commensal and parasitic
associations in marine and fresh waters (Caullery, 1952) ; we shall limit
ourselves to experimental studies that attempt to discover those
behavioural mechanisms which promote and maintain such associations. Our examples are mostly of commensal associations, because
there are few parasitic associations that have been analysed experimentally.
It seems almost stating the obvious to say that at some stage of their
life history commensals and parasites will have a repertoire of
responses to stimuli from the physical and chemical environment which
parallels those of free-living animals. However, these responses often
receive only passing reference or an aside to make clear that the author
is aware of their existence. Are they only generalized environmental
responses, or do they play a specific role in leading an animal to its
host? Too little is known to make a generalization from facts that are
available. Temperature preferences, responses to light, and reactions
to current, contact and gravity, usually appear to be nothing more
than one might expect of free-living species (Fasten, 1913; Davenport
and Hickok, 1951; Davenport et al., 1960; Morton, 1962; Ronald,
1960). On the other hand, it is possible on occasion for preferences of
this sort to aid an animal in localizing its host (freshwater leeches,
Herter, 1928, 1929; marine bivalves, Gage 196613). The burrowing
bivalve Montacuta substriata (Montagu) is geonegative so will stay
near the sand surface where its host Spatangus purpureus 0. F.
Muller lives; its close relative Montacuta ferriginosa (Montagu) is geopositive and so is likely to burrow further into sediments where it will
encounter its deeper living host, Echinocardium cordatum (Pennant)
(Gage, 196613). The changing light responses of the larvae of the
trematode, Discocotyle sagittata Leuckart, might constitute another
example (Paling, 1969).
Specific stimuli from a host to its parasite or commensal, are
probably the most frequent method by which these relationships are
established and maintained, and their study has occupied a number of
workers. The specific stimulus from a host may take a number of forms,
some of which may act in unison. There appear to be no investigations
of visual recognition of a host in sea or fresh water, although
no doubt instances exist. Hermit crabs are not exactly commensal
297
drilus s ~ ~ b i o t i c u s ,
by Gray. The results published to date suggest
that temperature, light, salinity, oxygen content, sand permeability,
and pore size and particle size, are significant to many species, and
perhaps further research should be concentrated in these areas.
111. COMMENSAL AND PARASITIC ASSOCIATIONS
There are many well known examples of commensal and parasitic
associations in marine and fresh waters (Caullery, 1952) ; we shall limit
ourselves to experimental studies that attempt to discover those
behavioural mechanisms which promote and maintain such associations. Our examples are mostly of commensal associations, because
there are few parasitic associations that have been analysed experimentally.
It seems almost stating the obvious to say that at some stage of their
life history commensals and parasites will have a repertoire of
responses to stimuli from the physical and chemical environment which
parallels those of free-living animals. However, these responses often
receive only passing reference or an aside to make clear that the author
is aware of their existence. Are they only generalized environmental
responses, or do they play a specific role in leading an animal to its
host? Too little is known to make a generalization from facts that are
available. Temperature preferences, responses to light, and reactions
to current, contact and gravity, usually appear to be nothing more
than one might expect of free-living species (Fasten, 1913; Davenport
and Hickok, 1951; Davenport et al., 1960; Morton, 1962; Ronald,
1960). On the other hand, it is possible on occasion for preferences of
this sort to aid an animal in localizing its host (freshwater leeches,
Herter, 1928, 1929; marine bivalves, Gage 196613). The burrowing
bivalve Montacuta substriata (Montagu) is geonegative so will stay
near the sand surface where its host Spatangus purpureus 0. F.
Muller lives; its close relative Montacuta ferriginosa (Montagu) is geopositive and so is likely to burrow further into sediments where it will
encounter its deeper living host, Echinocardium cordatum (Pennant)
(Gage, 196613). The changing light responses of the larvae of the
trematode, Discocotyle sagittata Leuckart, might constitute another
example (Paling, 1969).
Specific stimuli from a host to its parasite or commensal, are
probably the most frequent method by which these relationships are
established and maintained, and their study has occupied a number of
workers. The specific stimulus from a host may take a number of forms,
some of which may act in unison. There appear to be no investigations
of visual recognition of a host in sea or fresh water, although
no doubt instances exist. Hermit crabs are not exactly commensal