HABITAT SELECTION ny AQUATIC INVERTEBRATES
315
Polyzoa are also common on seaweeds and the larval preferences of a
few species have been documented. The larvae of two Japanese
hydroids studied by Nishihira (1967a, 1968a) settled on the species of
seaweed to which they were normally attached in nature. It is
interesting to note that the larvae of one of the species, Sertularella
miurensis Stechow, although not planktonic and not likely to swim,
still showed distinct preferences (Nishihira, 1967b) ; this is reminiscent
of the larval behaviour of Spirorbis rupestris (Gee and Knight-Jones,
(1962) and it would be interesting to know of other examples. Ryland
(1959) studied the settlement preferences of four polyzoan species and
showed preferences which, as in the case of the hydroids and the
Spirorbis spp., resembled their natural distribution.
Williams (1964) and Gee (1965) have conclusively demonstrated that
the specific stimulus Spirorbis larvae receive from fucoids is chemical and
they have made some attempt to categorize the biochemical nature of
the substances involved. Experiments on polyzoan and hydroid larvae
(Crisp and Williams, 1960; Nishihira, 1968b) and on the settlement of
gastropod and bivalve larvae on the seaweed Cystoseira, closely related
to dargmsum (Kiseleva, 1966b, 1967a), indicate the existence of chemical
stimuli in these instances too.
Detailed comparison between these results is difficult because of
different experimental and biochemical procedures. However some
seaweed extracts appear to have inhibitory effects. Extracts of
Cystoseira tested by Kiseleva (1966a, 1967a) did not stimulate aettlement, but the larvae tested belonged to species having a wide distribution over a range of habitats (Mytilus galloprovincialis Lamarck,
Nereis zonata Malmgren and Platynereis dumerili (Audouin and
M. Edwards)) and so the results do not have much ecological meaning.
On the other hand, Visscher (1928) obtained experimental verification
for the rare occurrence of barnacles on seaweeds from his observations
on the unsuitability of extract treated surfaces for the settlement of
barnacle cyprids, and Nishihira (1968b) showed that larvae of the
hydroid Coryne uchidai Stechow were killed by extracts of a species of
seaweed on which they were not normally found in the sea. Indeed,
it is possible that settlement sometimes occurs not because a surface is
attractive, but because it is not unattractive, although this might be a,
semantic point.
I n contrast to the marine environment, very little is known of
habitat selection by freshwater invertebrates living on plants apart
from their plant food preferences which will be considered below. The
unusual aquatic larvae of Bellura melanopyga Grote (Lepidoptera)
burrow into and feed on the leaves of a yellow water lily, N y m p h e a
315
Polyzoa are also common on seaweeds and the larval preferences of a
few species have been documented. The larvae of two Japanese
hydroids studied by Nishihira (1967a, 1968a) settled on the species of
seaweed to which they were normally attached in nature. It is
interesting to note that the larvae of one of the species, Sertularella
miurensis Stechow, although not planktonic and not likely to swim,
still showed distinct preferences (Nishihira, 1967b) ; this is reminiscent
of the larval behaviour of Spirorbis rupestris (Gee and Knight-Jones,
(1962) and it would be interesting to know of other examples. Ryland
(1959) studied the settlement preferences of four polyzoan species and
showed preferences which, as in the case of the hydroids and the
Spirorbis spp., resembled their natural distribution.
Williams (1964) and Gee (1965) have conclusively demonstrated that
the specific stimulus Spirorbis larvae receive from fucoids is chemical and
they have made some attempt to categorize the biochemical nature of
the substances involved. Experiments on polyzoan and hydroid larvae
(Crisp and Williams, 1960; Nishihira, 1968b) and on the settlement of
gastropod and bivalve larvae on the seaweed Cystoseira, closely related
to dargmsum (Kiseleva, 1966b, 1967a), indicate the existence of chemical
stimuli in these instances too.
Detailed comparison between these results is difficult because of
different experimental and biochemical procedures. However some
seaweed extracts appear to have inhibitory effects. Extracts of
Cystoseira tested by Kiseleva (1966a, 1967a) did not stimulate aettlement, but the larvae tested belonged to species having a wide distribution over a range of habitats (Mytilus galloprovincialis Lamarck,
Nereis zonata Malmgren and Platynereis dumerili (Audouin and
M. Edwards)) and so the results do not have much ecological meaning.
On the other hand, Visscher (1928) obtained experimental verification
for the rare occurrence of barnacles on seaweeds from his observations
on the unsuitability of extract treated surfaces for the settlement of
barnacle cyprids, and Nishihira (1968b) showed that larvae of the
hydroid Coryne uchidai Stechow were killed by extracts of a species of
seaweed on which they were not normally found in the sea. Indeed,
it is possible that settlement sometimes occurs not because a surface is
attractive, but because it is not unattractive, although this might be a,
semantic point.
I n contrast to the marine environment, very little is known of
habitat selection by freshwater invertebrates living on plants apart
from their plant food preferences which will be considered below. The
unusual aquatic larvae of Bellura melanopyga Grote (Lepidoptera)
burrow into and feed on the leaves of a yellow water lily, N y m p h e a