310
P. 9. MEADOWS AND J. I. CAMPBEU
Very little is known of gregarious responses in freshwater invertebrates although they must surely be common. Aggregations of the
marine bivalve Mytilw edulis are for instance paralleled by similar
aggregations in the structurally similar but unrelated freshwater
zebra mussel Dreissena polymorpha (Yonge and Campbell, 1968,
p. 30) while the larvae of the caddis fly Potamophylax catipennis Curtis
form very distinctive aggregations on the undersurface of stones
(Campbell and Meadows, 1972). Several authors have noted spontaneous aggregations of freshwater invertebrates in the laboratory that
might be caused by gregariousness (Curtis Riley (dragonfly nymphs)
1912 ; Gee (leeches) 1913 ; Holmes (water beetles) 1905), and these are
common observations to anyone working on freshwater animals.
However, the only detailed studies are on planarian aggregations, and
gregariousness and parental care in leeches. Planarians form two types
of aggregations ; in one, animals maintain a distance equivalent to their
own breadth from their neighbours, while in the other, animals overlap
and are orientated randomly (Pearl, 1903). Pearl was able to distinguish
sluggish animals from very active ones. Active animals moved right
through an aggregation and appeared t o take no notice of it while the
sluggish ones turned towards an aggregation when a short distance
away. From these observations Pearl felt that the planarians were reacting to a chemical produced by themselves but gave no experimental evidence. Planarian aggregations caused by gregarious behaviour may be
species specific, for Reynierse (1967) noted that two species if mixed
together formed aggregations solely with their own species. More
recently, Reynierse et d. (1969), after a series of long and involved
experiments, have suggested that aggregate formation is the result of
the joint effects of chemotaxis, photokinesis and of distinctive species
morphology. However, we have found it difficult to follow their
reasoning. The young of some species of leech stay attached to their
parents for a week or two after hatching, which can be regarded as a
form of gregariousness and Hatto (1968) has studied their parental
preferences. Young a108S@0&
heteroclita (L.) cannot distinguish
between their own parent and other adults, but will not attach to adults
of a Werent species. It is interesting to note that the newly liberated
young of the intertidal amphipod Marinogarnmum obtwatw (Dahl)
behave similarly, and are able to distinguish between females of their
own and another species (Sheader and Chia, 1970). Of course, for these
experiments to have any ecological meaning the young would have to
leave their parents from time to time or run a reasonable risk of
being dislodged.
Thus gregarious behaviour occurs among a large number of settling
P. 9. MEADOWS AND J. I. CAMPBEU
Very little is known of gregarious responses in freshwater invertebrates although they must surely be common. Aggregations of the
marine bivalve Mytilw edulis are for instance paralleled by similar
aggregations in the structurally similar but unrelated freshwater
zebra mussel Dreissena polymorpha (Yonge and Campbell, 1968,
p. 30) while the larvae of the caddis fly Potamophylax catipennis Curtis
form very distinctive aggregations on the undersurface of stones
(Campbell and Meadows, 1972). Several authors have noted spontaneous aggregations of freshwater invertebrates in the laboratory that
might be caused by gregariousness (Curtis Riley (dragonfly nymphs)
1912 ; Gee (leeches) 1913 ; Holmes (water beetles) 1905), and these are
common observations to anyone working on freshwater animals.
However, the only detailed studies are on planarian aggregations, and
gregariousness and parental care in leeches. Planarians form two types
of aggregations ; in one, animals maintain a distance equivalent to their
own breadth from their neighbours, while in the other, animals overlap
and are orientated randomly (Pearl, 1903). Pearl was able to distinguish
sluggish animals from very active ones. Active animals moved right
through an aggregation and appeared t o take no notice of it while the
sluggish ones turned towards an aggregation when a short distance
away. From these observations Pearl felt that the planarians were reacting to a chemical produced by themselves but gave no experimental evidence. Planarian aggregations caused by gregarious behaviour may be
species specific, for Reynierse (1967) noted that two species if mixed
together formed aggregations solely with their own species. More
recently, Reynierse et d. (1969), after a series of long and involved
experiments, have suggested that aggregate formation is the result of
the joint effects of chemotaxis, photokinesis and of distinctive species
morphology. However, we have found it difficult to follow their
reasoning. The young of some species of leech stay attached to their
parents for a week or two after hatching, which can be regarded as a
form of gregariousness and Hatto (1968) has studied their parental
preferences. Young a108S@0&
heteroclita (L.) cannot distinguish
between their own parent and other adults, but will not attach to adults
of a Werent species. It is interesting to note that the newly liberated
young of the intertidal amphipod Marinogarnmum obtwatw (Dahl)
behave similarly, and are able to distinguish between females of their
own and another species (Sheader and Chia, 1970). Of course, for these
experiments to have any ecological meaning the young would have to
leave their parents from time to time or run a reasonable risk of
being dislodged.
Thus gregarious behaviour occurs among a large number of settling