HABITAT SELECTION BY AQUATIC INVERTEHRATES
309
be 60 individuals (Carlisle, 1957). The pod observed by Carlisle
remained m the same position for over 10 weeks, and was joined by a
further twenty individuals during the first four weeks of this. Soft
newly moulted Haia were always found in the centre, which may
therefore protect them from predation. These, however, were preliminary observations, and too much weight should not be attached to
them. StevZiO (1971) studied aggregations (heaps) of the same species
under laboratory conditions in which 5 groups of 20 individually
marked animals were observed over 3 weeks, and established a relationship between dominance rank and individual position within the heaps.
Higher ranked animals, which were usually larger males, occurred
predominantly away from heaps and were rarely found inside them.
Prom these results, 8tevEi6 suggested that the aggregations may have a
protective function, but pointed out that they were rare in his area
(the Adriatic Sea) and evidently not obligatory (loc. cit. p. 25). It is still
not clear, therefore, exactly what function these aggregations might
have. Similar aggregations are known to occur in field populations of
two species of spiny lobster and in the king crab, Paralithodes ; in these
instances the size of the aggregation depends on the age of the animals
(Lindberg, 1955 ; Fielder, 1965 ; Powell and Nickerson, 1965).
Boaden (1963) and Crisp (1969) have analysed gregariousness in an
adult interstitial annelid and an adult intertidal gastropod respectively.
Boaden’s work has already been referred to (see Interstitial Animals).
M. Crisp (1969) attempted to assess what aspects of the behaviour of the
gastropod, Nassarius obsoletus (Say), led to the formation of its very
characteristic aggregations. These latter, she concluded, were in part due
to the species’ responses to its physical and chemical environment, but also
related to a chemical or chemicals given off by the animals themselves.
Apart from direct gregariousness, that is when animals recognize
and move towards animals of their own species, some animals in similar
circumstances change their behaviour, the change in its turn leading
to an aggregation. Unpublished work on Corophium volutator provides
an example. I n this species, animals in groups are more likely to
burrow and also more likely to prefer fine to coarse sand, and these
two behaviour changes will undoubtedly lead to aggregations in the
field. We might call this an indirect gregarious response. The change
in behaviour to light when individuals of the polychaete Nephthsy
cirrosa Ehlers are grouped together (Clark, 1956), and the lowered
activity amongst groups of the prawn Palaemon elegans Rathke
(Rodriguez and Naylor, 1972), might be other examples, but it is not
so apparent in these cases whether the changes in behaviour would
increase the chances of aggregate formation.
309
be 60 individuals (Carlisle, 1957). The pod observed by Carlisle
remained m the same position for over 10 weeks, and was joined by a
further twenty individuals during the first four weeks of this. Soft
newly moulted Haia were always found in the centre, which may
therefore protect them from predation. These, however, were preliminary observations, and too much weight should not be attached to
them. StevZiO (1971) studied aggregations (heaps) of the same species
under laboratory conditions in which 5 groups of 20 individually
marked animals were observed over 3 weeks, and established a relationship between dominance rank and individual position within the heaps.
Higher ranked animals, which were usually larger males, occurred
predominantly away from heaps and were rarely found inside them.
Prom these results, 8tevEi6 suggested that the aggregations may have a
protective function, but pointed out that they were rare in his area
(the Adriatic Sea) and evidently not obligatory (loc. cit. p. 25). It is still
not clear, therefore, exactly what function these aggregations might
have. Similar aggregations are known to occur in field populations of
two species of spiny lobster and in the king crab, Paralithodes ; in these
instances the size of the aggregation depends on the age of the animals
(Lindberg, 1955 ; Fielder, 1965 ; Powell and Nickerson, 1965).
Boaden (1963) and Crisp (1969) have analysed gregariousness in an
adult interstitial annelid and an adult intertidal gastropod respectively.
Boaden’s work has already been referred to (see Interstitial Animals).
M. Crisp (1969) attempted to assess what aspects of the behaviour of the
gastropod, Nassarius obsoletus (Say), led to the formation of its very
characteristic aggregations. These latter, she concluded, were in part due
to the species’ responses to its physical and chemical environment, but also
related to a chemical or chemicals given off by the animals themselves.
Apart from direct gregariousness, that is when animals recognize
and move towards animals of their own species, some animals in similar
circumstances change their behaviour, the change in its turn leading
to an aggregation. Unpublished work on Corophium volutator provides
an example. I n this species, animals in groups are more likely to
burrow and also more likely to prefer fine to coarse sand, and these
two behaviour changes will undoubtedly lead to aggregations in the
field. We might call this an indirect gregarious response. The change
in behaviour to light when individuals of the polychaete Nephthsy
cirrosa Ehlers are grouped together (Clark, 1956), and the lowered
activity amongst groups of the prawn Palaemon elegans Rathke
(Rodriguez and Naylor, 1972), might be other examples, but it is not
so apparent in these cases whether the changes in behaviour would
increase the chances of aggregate formation.