212
N. A. HOLME
cloud of suspended matter would make observation difficult. For certain specific purposes, however, where great precision in sampling is
needed, a diver will have the advantage. A recent example is the
expedition by Bristol University Underwater Club to Fanafjord,
Norway, where lines of closely-spaced cores were taken to investigate
the patchiness of the bottom fauna (Fig. 25), a task almost impossible
by other means.
V. THE SAMPLING ABILITIES OF BOTTOM-SAMPLING GEAR
We may now consider how far the instruments in current use
succeed in adequately sampling the fauna. There are two categories
of animals which escape the normal quantitative samplers such as
grabs : the fast-moving and the deep-burrowing. Fast-moving animals
living on the surface of the deposit can no doubt detect the descent
of the sampler and take avoiding action ; moreover, the design of many
grabs is such that free-swimming animals like the smaller cnrstacea
may be able to escape through an uncovered top, through the sides,
or between imperfectly closed jaws. Some of these losses can be reduced
by suitable covers or flaps, but it is generally advisable to supplement
grab hauls with Agassiz trawl collections (p. 191) if it is desired to
obtain anything approaching a comprehensive list of the surface fauna.
To what extent do animals avoid being taken by burrowing down
deep or by living permanently below the depth of operation of the
sampler? I n some instances the effective depth of sampling is greater
than that of the actual sample ; thus, siphons of lamellibranchs (like
Lzctruriu) whose shells are below the sampling depth, can often be
included in the records, but usually the sampler does not dig sufficiently
deeply to capture the entire fauna.
There is some difference of opinion on the depths to which animals
may burrow (Thorson, 1957a, p. 03) in offshore deposits, due no doubt
partly to generalizations based on local conditions with which particular workers may be familiar, and partly to comparison with the position
of animals dug up between tide marks. MacGinitie (1935, 1939), for
example, has dug up certain animals at depths of two or more feet on
intertidal flats in California, and believes the same may well hold good
for the offshore fauna.
Offshore, Molander (1928) investigated the vertical distribution of
animals in Gullmarfjord, using the Otto Petterson sampler, a square
coring tube in which the sample could be separated into layers from
different depths. Samples were taken on hard and soft clay, also on
sand mixed with clay. His conclusions were that the majority of
individuals were to be found in the top 5 or 10 cm of deposit, only
N. A. HOLME
cloud of suspended matter would make observation difficult. For certain specific purposes, however, where great precision in sampling is
needed, a diver will have the advantage. A recent example is the
expedition by Bristol University Underwater Club to Fanafjord,
Norway, where lines of closely-spaced cores were taken to investigate
the patchiness of the bottom fauna (Fig. 25), a task almost impossible
by other means.
V. THE SAMPLING ABILITIES OF BOTTOM-SAMPLING GEAR
We may now consider how far the instruments in current use
succeed in adequately sampling the fauna. There are two categories
of animals which escape the normal quantitative samplers such as
grabs : the fast-moving and the deep-burrowing. Fast-moving animals
living on the surface of the deposit can no doubt detect the descent
of the sampler and take avoiding action ; moreover, the design of many
grabs is such that free-swimming animals like the smaller cnrstacea
may be able to escape through an uncovered top, through the sides,
or between imperfectly closed jaws. Some of these losses can be reduced
by suitable covers or flaps, but it is generally advisable to supplement
grab hauls with Agassiz trawl collections (p. 191) if it is desired to
obtain anything approaching a comprehensive list of the surface fauna.
To what extent do animals avoid being taken by burrowing down
deep or by living permanently below the depth of operation of the
sampler? I n some instances the effective depth of sampling is greater
than that of the actual sample ; thus, siphons of lamellibranchs (like
Lzctruriu) whose shells are below the sampling depth, can often be
included in the records, but usually the sampler does not dig sufficiently
deeply to capture the entire fauna.
There is some difference of opinion on the depths to which animals
may burrow (Thorson, 1957a, p. 03) in offshore deposits, due no doubt
partly to generalizations based on local conditions with which particular workers may be familiar, and partly to comparison with the position
of animals dug up between tide marks. MacGinitie (1935, 1939), for
example, has dug up certain animals at depths of two or more feet on
intertidal flats in California, and believes the same may well hold good
for the offshore fauna.
Offshore, Molander (1928) investigated the vertical distribution of
animals in Gullmarfjord, using the Otto Petterson sampler, a square
coring tube in which the sample could be separated into layers from
different depths. Samples were taken on hard and soft clay, also on
sand mixed with clay. His conclusions were that the majority of
individuals were to be found in the top 5 or 10 cm of deposit, only