284
P. 9. MEADOWS AND J. I. UAMPBELL
interesting to know whether small invertebrates that burrow into sediments, particularly those living in estuaries, respond to these changes.
Sediments in the sea differ from place to place, sometimes over as
short distances as in the intertidal zone (Fleming and Stride, 1967;
Krumbein, 1971) and are of great importance in animal distribution
(Holme, 1950). The finest and coarsest sediments tend to contain the
greatest proportion of specialized forms, while the finest sediments are
richest both in numbers of species and number of animals (Davis, 1923,
1925). Surprisingly, in contrast to the intertidal environment the results
of ecological surveys have not often been followed by experimental work
on habitat preferences. Williams (1958) has offered three species of
burrowing prawns a choice of beach sand, shell sand, muddy sand,
sandy mud and loose peat in a long tank. There were interspecific
differences, for Penaeus duorarum Burkenroad preferred the shell
sand, while P. aztecus Ives and P. setiferus (L.) were found most
frequently in the muddy sand, sandy mud and loose peat. Williams
states that his results agree well with the species sublittoral
distribution obtained from trawling. The gastropods, Aporrhais
pes-pelecuni (L.) and A . serresiana (Noh.), are specialized for burrowing
respectively into muddy gravel at shallow, and into soft mud at greater,
depths. When the former is placed on soft mud it flounders and becomes
clogged; the latter was unable to " shoulder " its way through muddy
gravel (Yonge, 1937). The hairy snail, Trichotropis cancellata Hinds,
on the other hand, demands a firm substratum for locomotion and clear
water for ciliary feeding; even on mixed shell gravel and mud movement was found to be greatly hampered (Yonge, 1962).
The most detailed study to date is on Branchiostoma nigeriense
Webb from Lagos Lagoon, Nigeria (Webb, 1958a; Webb and Hill,
1958). In a series of experiments Webb and Hill linked the ecological
distribution of the species with its survival in Werent grades of
sediment, and these in turn with its substrate preferences. The sand
from the natural habitat of Branchiostorna has a wide grain size range
with a maximum in the 300-200 pm band and contains less than 25%
very fine grains (under 200 pm) and less than 16% silt. The results of
24-h survival experiments are as follows : in 2 000-600 pm sand animals
were active and swam strongly when touched ; in the 600-300 pm and
300-200 pm ranges animals sometimes burrowed so that the oral end
projected above the surface; in the 200-180 pm and the 180-100 pm
ranges animals did not respond readily to touch and little attempt was
made to burrow, in addition to which in the 180-100 pm range the
animals were moribund and their oral apertures became blocked with
small sand grains and mucus. In choice experiments the 300-200 pm
range was preferred to others and this agrees with the main particle
P. 9. MEADOWS AND J. I. UAMPBELL
interesting to know whether small invertebrates that burrow into sediments, particularly those living in estuaries, respond to these changes.
Sediments in the sea differ from place to place, sometimes over as
short distances as in the intertidal zone (Fleming and Stride, 1967;
Krumbein, 1971) and are of great importance in animal distribution
(Holme, 1950). The finest and coarsest sediments tend to contain the
greatest proportion of specialized forms, while the finest sediments are
richest both in numbers of species and number of animals (Davis, 1923,
1925). Surprisingly, in contrast to the intertidal environment the results
of ecological surveys have not often been followed by experimental work
on habitat preferences. Williams (1958) has offered three species of
burrowing prawns a choice of beach sand, shell sand, muddy sand,
sandy mud and loose peat in a long tank. There were interspecific
differences, for Penaeus duorarum Burkenroad preferred the shell
sand, while P. aztecus Ives and P. setiferus (L.) were found most
frequently in the muddy sand, sandy mud and loose peat. Williams
states that his results agree well with the species sublittoral
distribution obtained from trawling. The gastropods, Aporrhais
pes-pelecuni (L.) and A . serresiana (Noh.), are specialized for burrowing
respectively into muddy gravel at shallow, and into soft mud at greater,
depths. When the former is placed on soft mud it flounders and becomes
clogged; the latter was unable to " shoulder " its way through muddy
gravel (Yonge, 1937). The hairy snail, Trichotropis cancellata Hinds,
on the other hand, demands a firm substratum for locomotion and clear
water for ciliary feeding; even on mixed shell gravel and mud movement was found to be greatly hampered (Yonge, 1962).
The most detailed study to date is on Branchiostoma nigeriense
Webb from Lagos Lagoon, Nigeria (Webb, 1958a; Webb and Hill,
1958). In a series of experiments Webb and Hill linked the ecological
distribution of the species with its survival in Werent grades of
sediment, and these in turn with its substrate preferences. The sand
from the natural habitat of Branchiostorna has a wide grain size range
with a maximum in the 300-200 pm band and contains less than 25%
very fine grains (under 200 pm) and less than 16% silt. The results of
24-h survival experiments are as follows : in 2 000-600 pm sand animals
were active and swam strongly when touched ; in the 600-300 pm and
300-200 pm ranges animals sometimes burrowed so that the oral end
projected above the surface; in the 200-180 pm and the 180-100 pm
ranges animals did not respond readily to touch and little attempt was
made to burrow, in addition to which in the 180-100 pm range the
animals were moribund and their oral apertures became blocked with
small sand grains and mucus. In choice experiments the 300-200 pm
range was preferred to others and this agrees with the main particle