HABITAT SELEUTION BY AQUATIC INVERTEBRATES
295
move by one means or another in the sand interstices. Most of them are
confined to the topmost 5 cm of lake beaches and the topmost 12 cm
of marine beaches, while variations in their population densities in time
and space are more pronounced on freshwater than on marine beaches
(Pennak, 1951). FaurB-Fremiet (1950) divides the marine interstitial
ciliates into the mesoporal fauna-those that live in coarse sand and
which are not limited to the interstitial environment, and into the
microporal fauna-those that are only found in the interstices between
sand grains and are highly specialized. The distinction might
profitably be applied to the invertebrate interstitial fauna.
Our knowledge of the way these organisms respond to their
environment is scant, and depends almost entirely on the researches
of Boaden, Gray and Jansson. What evidence there is indicates that
interstitial animals use environmental clues to select their habitats
in the same way as do larger aquatic invertebrates.
Boaden (1962) observed the rate at which sands of differing particle
size were recolonized on an intertidal shore and concluded that the rate
of recolonization depended on particle size. The study could well be
extended to other interstitial habitats. In a later paper Boaden (1963)
concentrated on one species, the archiannelid Trilobodrilus heideri
Remane which lives near high water in moist sand and shell gravel of
about 350-650 pm particle size. It is strongly photonegative, and is
rheopositive in full strength sea water but unresponsive below about
lo%,. The latter behaviour will keep it away from fresh waters and the
former from the surface layers of sand where dry air might desiccate it.
Trilobodrilus is gregarious, and periods of aggregation occur twice in
each tidal cycle, a t about low and high tide. Exactly how this
behaviour affects their distribution is not clear, although Boaden (1963,
p. 249) has some suggestions. Gray (1965; 1966a, b, c, d) has studied
various aspects of the habitat selection of another archiannelid Protodrilus symbioticus (Giard) that also lives intertidally. It prefers 15°C
in a temperature gradient of 6OG25"C but shows no salinity preference,
which is surprising, and onIy a slight reaction to current. It will
occur near the surface of sands as it prefers high oxygen concentrations,
but not at the surface because it avoids high light intensities and high
temperatures. Protodrilw symbioticus, in common with two other
interstitial species, reacts to the numbers and types of microorganisms on sand grains (Gray, 1966d, 1967, 1968), but we shall
return to this subject later (p. 321).
Jansson (1962, 1967) has adopted a more comparative approach,
and in a comprehensive paper has attempted to link salinity
preferences with mortality limits (Jansson, 1968). Most of the species
295
move by one means or another in the sand interstices. Most of them are
confined to the topmost 5 cm of lake beaches and the topmost 12 cm
of marine beaches, while variations in their population densities in time
and space are more pronounced on freshwater than on marine beaches
(Pennak, 1951). FaurB-Fremiet (1950) divides the marine interstitial
ciliates into the mesoporal fauna-those that live in coarse sand and
which are not limited to the interstitial environment, and into the
microporal fauna-those that are only found in the interstices between
sand grains and are highly specialized. The distinction might
profitably be applied to the invertebrate interstitial fauna.
Our knowledge of the way these organisms respond to their
environment is scant, and depends almost entirely on the researches
of Boaden, Gray and Jansson. What evidence there is indicates that
interstitial animals use environmental clues to select their habitats
in the same way as do larger aquatic invertebrates.
Boaden (1962) observed the rate at which sands of differing particle
size were recolonized on an intertidal shore and concluded that the rate
of recolonization depended on particle size. The study could well be
extended to other interstitial habitats. In a later paper Boaden (1963)
concentrated on one species, the archiannelid Trilobodrilus heideri
Remane which lives near high water in moist sand and shell gravel of
about 350-650 pm particle size. It is strongly photonegative, and is
rheopositive in full strength sea water but unresponsive below about
lo%,. The latter behaviour will keep it away from fresh waters and the
former from the surface layers of sand where dry air might desiccate it.
Trilobodrilus is gregarious, and periods of aggregation occur twice in
each tidal cycle, a t about low and high tide. Exactly how this
behaviour affects their distribution is not clear, although Boaden (1963,
p. 249) has some suggestions. Gray (1965; 1966a, b, c, d) has studied
various aspects of the habitat selection of another archiannelid Protodrilus symbioticus (Giard) that also lives intertidally. It prefers 15°C
in a temperature gradient of 6OG25"C but shows no salinity preference,
which is surprising, and onIy a slight reaction to current. It will
occur near the surface of sands as it prefers high oxygen concentrations,
but not at the surface because it avoids high light intensities and high
temperatures. Protodrilw symbioticus, in common with two other
interstitial species, reacts to the numbers and types of microorganisms on sand grains (Gray, 1966d, 1967, 1968), but we shall
return to this subject later (p. 321).
Jansson (1962, 1967) has adopted a more comparative approach,
and in a comprehensive paper has attempted to link salinity
preferences with mortality limits (Jansson, 1968). Most of the species