HABITAT SELECTION BY AQUATIC INVERTEBRATES
285
size group of their natural sand. Animals avoid the coarse sands and
the survival experiments show that they will rapidly swim away from
these; they avoid the finest sand and silt, and the survival experiments
show how they will die if they remain on or in them. Other experiments
illustrate that animals in the most suitable sand may well remain in the
same position for days. The detrimental effect of very fine sands is
borne out by experiments in which either the fraction under 200 pm
or the silt fraction was removed from natural sand whereupon it became
considerably more attractive than before. By adding more fine sand
or silt the natural sand became concomitantly less attractive.
Webb and Hill (1958) conclude that Branchiostoma is found in its
typical sand because (a) it finds it easier to burrow in 200-300 pm sand
as opposed to coarse sand, (b) the sand is the correct size for the
functioning of the branchiae, and (c) animals that attempt to burrow
in finer sands do not do so completely and so are easily disturbed by
any tactile stimulation. As with the intertidal environment, we know
very little about the influence on habitat selection of the changing
porosity and packing that occur as particle size changes (Webb, 1958b,
1969).
To summarize, there is in general less evidence for habitat selection
by marine invertebrates than there is for intertidal forms. The reasons
for this must partly be ones of manpower, although the difficulties of
collecting and maintaining planktonic and benthic marine animals
may dissuade many workers. We saw how there were many well
documented instances of the light and pressure responses of larval
planktonic invertebrates and some evidence for adult planktonic
invertebrates. But when we turned to other variables, such as sediment characteristics, currents, salinity and gravity, to which animals
must respond as they select habitats, we found the evidence scattered,
often fragmentary, and for benthic animals almost non-existent.
Further investigation would also be profitable on the responses of
planktonic species to discontinuities of density and temperature
(Harder, 1968) and on the responses of benthic species to the surface
charge of sediments (Pravdid, 1970). When considering the intertidal
environment we commented on the very few attempts to integrate
the various factors that govern habitat selection in any one species ;
there are none for sublittoral invertebrates. Clearly the sublittoral
marine environment offers considerable potential, particularly for
those who can combine experimental studies in the laboratory with
observations and experiments on the behaviour of animals under
natural conditions in the sea. Since the results are likely to be of direct
application to the management of commercial fisheries, to pollution
285
size group of their natural sand. Animals avoid the coarse sands and
the survival experiments show that they will rapidly swim away from
these; they avoid the finest sand and silt, and the survival experiments
show how they will die if they remain on or in them. Other experiments
illustrate that animals in the most suitable sand may well remain in the
same position for days. The detrimental effect of very fine sands is
borne out by experiments in which either the fraction under 200 pm
or the silt fraction was removed from natural sand whereupon it became
considerably more attractive than before. By adding more fine sand
or silt the natural sand became concomitantly less attractive.
Webb and Hill (1958) conclude that Branchiostoma is found in its
typical sand because (a) it finds it easier to burrow in 200-300 pm sand
as opposed to coarse sand, (b) the sand is the correct size for the
functioning of the branchiae, and (c) animals that attempt to burrow
in finer sands do not do so completely and so are easily disturbed by
any tactile stimulation. As with the intertidal environment, we know
very little about the influence on habitat selection of the changing
porosity and packing that occur as particle size changes (Webb, 1958b,
1969).
To summarize, there is in general less evidence for habitat selection
by marine invertebrates than there is for intertidal forms. The reasons
for this must partly be ones of manpower, although the difficulties of
collecting and maintaining planktonic and benthic marine animals
may dissuade many workers. We saw how there were many well
documented instances of the light and pressure responses of larval
planktonic invertebrates and some evidence for adult planktonic
invertebrates. But when we turned to other variables, such as sediment characteristics, currents, salinity and gravity, to which animals
must respond as they select habitats, we found the evidence scattered,
often fragmentary, and for benthic animals almost non-existent.
Further investigation would also be profitable on the responses of
planktonic species to discontinuities of density and temperature
(Harder, 1968) and on the responses of benthic species to the surface
charge of sediments (Pravdid, 1970). When considering the intertidal
environment we commented on the very few attempts to integrate
the various factors that govern habitat selection in any one species ;
there are none for sublittoral invertebrates. Clearly the sublittoral
marine environment offers considerable potential, particularly for
those who can combine experimental studies in the laboratory with
observations and experiments on the behaviour of animals under
natural conditions in the sea. Since the results are likely to be of direct
application to the management of commercial fisheries, to pollution