HABITAT SELECTION BY AQUATIC INVERTEBRATES
31 1
larvae of marine sedentary invertebrates, and further studies are likely
to show that it is equally common amongst mobile adult invertebrates
both in the sea and in fresh water. A widespread occurrence of gregarious behaviour would suggest that it has a high selective advantage.
Gregarious behaviour is almost certainly of advantage to both sedentary
and mobile species, particularly in large areas of environmental uniformity, because it will assist breeding, and evidence for this is provided
by the demonstration of female sexual attractants (pheromones) in
Portunus sanguinolentus (Herbst) (Ryan, 1966) and in Cammarus
duebeni Lilljeborg (Dahl, Emanuelsson and von Mecklenburg, 1970).
It may also be important in protecting populations from predation and
in the selection of a suitable habitat ; the authors, however, are aware
of flaws in these latter two arguments. The first circumscribes the
behaviour of a predator more than is warranted by the available experimental evidence. The second assumes there would be an advantage
per se in recognizing a suitable habitat by the presence of other individuals of the same species rather than by recognizing it by its own
attributes (e.g. temperature, light, shelter, food) ; it also assumes that
the first colonizers are making the right decision. On present experimental evidence these assumptions are not justified.
C. Spacing out and aggression
Gregarious behaviour brings animals of a species close enough to
one another for them to breed, but if they are too close they will
compete unnecessarily for food and living space. Probably for these
reasons many animals have a means of limiting their gregarious
tendencies that expresses itself either as spacing out, caused by what
Crisp (1961) terms territorial behaviour, or by aggressiveness, and of
course similar considerations apply to animals on land. Spacing out
behaviour in sedentary marine invertebrates takes place as the larvae
settle and is in fact superimposed on the gregarious behaviour already
described. As a result, gregarious larvae both in the sea and in the
laboratory are more likely to settle on surfaces lightly colonized by their
own species than on heavily colonized surfaces (Knight-Jones, 1951 ;
Meadows, 1969). Wisely (1960) and Crisp (1961) have described the
behaviour of the larvae of Spirorbis borealis (Annelida) and Balanus
balanoides (Cirripedia) as they recognize and avoid newly-settled
individuals (Fig. 2)) while Knight-Jones and Moyse (1961) feel that
barnacle cyprids space out more readily to their own rather than to
other species. On the other hand, not all larvae space out, for the larvae
of Sabellaria alveolata settle on top of or very close t o one another
(Wilson, 1968, p. 428 and personal communication) and oyster spat
31 1
larvae of marine sedentary invertebrates, and further studies are likely
to show that it is equally common amongst mobile adult invertebrates
both in the sea and in fresh water. A widespread occurrence of gregarious behaviour would suggest that it has a high selective advantage.
Gregarious behaviour is almost certainly of advantage to both sedentary
and mobile species, particularly in large areas of environmental uniformity, because it will assist breeding, and evidence for this is provided
by the demonstration of female sexual attractants (pheromones) in
Portunus sanguinolentus (Herbst) (Ryan, 1966) and in Cammarus
duebeni Lilljeborg (Dahl, Emanuelsson and von Mecklenburg, 1970).
It may also be important in protecting populations from predation and
in the selection of a suitable habitat ; the authors, however, are aware
of flaws in these latter two arguments. The first circumscribes the
behaviour of a predator more than is warranted by the available experimental evidence. The second assumes there would be an advantage
per se in recognizing a suitable habitat by the presence of other individuals of the same species rather than by recognizing it by its own
attributes (e.g. temperature, light, shelter, food) ; it also assumes that
the first colonizers are making the right decision. On present experimental evidence these assumptions are not justified.
C. Spacing out and aggression
Gregarious behaviour brings animals of a species close enough to
one another for them to breed, but if they are too close they will
compete unnecessarily for food and living space. Probably for these
reasons many animals have a means of limiting their gregarious
tendencies that expresses itself either as spacing out, caused by what
Crisp (1961) terms territorial behaviour, or by aggressiveness, and of
course similar considerations apply to animals on land. Spacing out
behaviour in sedentary marine invertebrates takes place as the larvae
settle and is in fact superimposed on the gregarious behaviour already
described. As a result, gregarious larvae both in the sea and in the
laboratory are more likely to settle on surfaces lightly colonized by their
own species than on heavily colonized surfaces (Knight-Jones, 1951 ;
Meadows, 1969). Wisely (1960) and Crisp (1961) have described the
behaviour of the larvae of Spirorbis borealis (Annelida) and Balanus
balanoides (Cirripedia) as they recognize and avoid newly-settled
individuals (Fig. 2)) while Knight-Jones and Moyse (1961) feel that
barnacle cyprids space out more readily to their own rather than to
other species. On the other hand, not all larvae space out, for the larvae
of Sabellaria alveolata settle on top of or very close t o one another
(Wilson, 1968, p. 428 and personal communication) and oyster spat