296
P. 5. MEADOWS AND J. I. UAMPBELL
he worked with were collected on intertidal beaches from the Baltic
coast where the water is brackish. He describes (Jansson, 1967) a
vertical core of sand whose sand grains had an overall mean diameter of
500 pm, but in which the means of sub-samples varied between 100 pm
and 800 pm, and deduces from this that the selection of certain grades of
sand by interstitial animals is likely to influence markedly their local
distribution. But he then points out that a species of turbellarian
which moves by ciliary creeping, and a species of oligochaete which
burrows by peristaltic movement, have no significant preference for
graded sand fractions over the range 74 to 1000 pm-a paradox indeed.
However, he suggests that this might be due to the two species’ methods
of locomotion, and quotes as evidence the clear grain size preferences
of two other interstitial forms, a species of oligochaete and a species
of copepod, each of which moves with a sliding type of locomotion.
Further comparison with other species would be worthwhile.
Pore size and permeability in different types of sand are almost
certainly of great importance to interstitial animals, and there are now
available some details of the way in which they can vary (Webb,
1958b; 1969). It certainly seems that the movement of interstitial
terrestrial nematodes at least is controlled by some similar factors.
Wallace (1958) after detailing the physics of water movement through
a sandy loam, considers how nematodes move through sand fractions
of differing size and moisture content. A much greater proportion
of the larvae of the beet eel worm Heterodera schuchtii Schmidt migrate
through 150-500 pm sands than through 20-150 pm sands, and through
sand the interstices of which are half full of water rather than through
sand where the interstices are full or empty. While studying the
movement of larvae in single layers of grains, he was able to show
that in 75-150 pm sand many of the interstices were too small for the
larvae, that in 150-250 pm sand the interstices were wide enough for
the larvae to travel in straight lines, but that in 250-500 pm sand
the interstices were so large that the larvae slowed down. Wallace also
watched larvae moving in water films on glass and on alginate jelly.
They moved fastest when the film was 2-5 pm thick, and progressively
slower as the film thickness was increased to 50 pm, while they did not
move at all if the film was less than 1 pm thick. The details of these
latter experiments however should be treated with a little caution, as
Wallace’s methods of obtaining films were approximate.
It is evident, therefore, that a great deal more work is needed
before we can define the distribution of interstitial animals in terms of
their behaviour. While there are many thousands of species in the interstitial environment, only one has been studied in any detail, Proto-
P. 5. MEADOWS AND J. I. UAMPBELL
he worked with were collected on intertidal beaches from the Baltic
coast where the water is brackish. He describes (Jansson, 1967) a
vertical core of sand whose sand grains had an overall mean diameter of
500 pm, but in which the means of sub-samples varied between 100 pm
and 800 pm, and deduces from this that the selection of certain grades of
sand by interstitial animals is likely to influence markedly their local
distribution. But he then points out that a species of turbellarian
which moves by ciliary creeping, and a species of oligochaete which
burrows by peristaltic movement, have no significant preference for
graded sand fractions over the range 74 to 1000 pm-a paradox indeed.
However, he suggests that this might be due to the two species’ methods
of locomotion, and quotes as evidence the clear grain size preferences
of two other interstitial forms, a species of oligochaete and a species
of copepod, each of which moves with a sliding type of locomotion.
Further comparison with other species would be worthwhile.
Pore size and permeability in different types of sand are almost
certainly of great importance to interstitial animals, and there are now
available some details of the way in which they can vary (Webb,
1958b; 1969). It certainly seems that the movement of interstitial
terrestrial nematodes at least is controlled by some similar factors.
Wallace (1958) after detailing the physics of water movement through
a sandy loam, considers how nematodes move through sand fractions
of differing size and moisture content. A much greater proportion
of the larvae of the beet eel worm Heterodera schuchtii Schmidt migrate
through 150-500 pm sands than through 20-150 pm sands, and through
sand the interstices of which are half full of water rather than through
sand where the interstices are full or empty. While studying the
movement of larvae in single layers of grains, he was able to show
that in 75-150 pm sand many of the interstices were too small for the
larvae, that in 150-250 pm sand the interstices were wide enough for
the larvae to travel in straight lines, but that in 250-500 pm sand
the interstices were so large that the larvae slowed down. Wallace also
watched larvae moving in water films on glass and on alginate jelly.
They moved fastest when the film was 2-5 pm thick, and progressively
slower as the film thickness was increased to 50 pm, while they did not
move at all if the film was less than 1 pm thick. The details of these
latter experiments however should be treated with a little caution, as
Wallace’s methods of obtaining films were approximate.
It is evident, therefore, that a great deal more work is needed
before we can define the distribution of interstitial animals in terms of
their behaviour. While there are many thousands of species in the interstitial environment, only one has been studied in any detail, Proto-