350
P. 9. MEADOWS AND J. I. OAMPBELL
two Montacuta species studied by Gage (1966b) and the polynoid
commensals belonging to the genus Arctonoe (Davenport, 1950;
Davenport and Hickok, 1951). There is therefore clear evidence for
closely related species having different habitat preferences which will
separate them spatially in the field. Where these species are obviously
sympatric, as are the two Heptagenia, the three Littorim species, the
two Eurydice species, and some of the Spirorbis species, the differences
may well be important in mitigating interspecific competition, and
also have been important in the past, in preventing inter-breeding as
the species diverged from each other.
However the behaviour of some sympatric species is not so widely
different that it accounts for their spatial separation into different
microhabitats. Larvae of the polyzoans Alcyonidium hirsutum (Fleming)
and A . polyoum (RassaIl), for instance, both settle in largest numbers
on Pucus serratus (Ryland, 1959). Spatial separation here might be
maintained by the preferences of the larvae for slightly different
convexities or for slightly different parts of the Fuczcs frond
(Ryland, 1959, Tables 7 and 9). On the other hand spatial separation
could be achieved by interspecific competition itself, although this does
not appear to be common, and may therefore have less selective advantage than direct behavioural recognition of an appropriate habitat.
Interspecific competition between closely related sympatric species has
been observed in the laboratory between two species of the intertidal
crab Uca and between two species of the freshwater crayfish Orwnectes
(Teal, 1958; Bovbjerg 1964, 1970 respectively), and also in the field
between two species of the barnacle BaZanus (Meadows 1969). Orconectes
virilis (Hagen) and Orconectes immunis (Hagen) (Bovbjerg loo. cit.)
are two North American species of crayfish having similar geographical
ranges but different habitats-0. virilis inhabits rock or gravel in
streams and lake margins, and 0. immunis inhabits mud in ponds and
slow moving streams. I n the middle stretches of the Little Sioux River
(Minnesota and Iowa) where the two species are sympatric, each is
restricted to its own habitat. Bovbjerg’s experiments prove that this
is the result of interspecific competition. If each species is separately
offered a choice of rock, gravel, and mud, they both prefer the rock,
whereas if the two species are mixed and then offered the same choice
0. virilis prefers the rock as before but drives most of the 0. immunia
onto the mud.
Having considered differences between sympatric species, it may
now be asked whether sympatric subspecies show differences in their
selection of habitats. We have only been able to find one instance. The
two subspecies of Littorim obtusata, L. 0. olivacea and L. 0. citrina, live
P. 9. MEADOWS AND J. I. OAMPBELL
two Montacuta species studied by Gage (1966b) and the polynoid
commensals belonging to the genus Arctonoe (Davenport, 1950;
Davenport and Hickok, 1951). There is therefore clear evidence for
closely related species having different habitat preferences which will
separate them spatially in the field. Where these species are obviously
sympatric, as are the two Heptagenia, the three Littorim species, the
two Eurydice species, and some of the Spirorbis species, the differences
may well be important in mitigating interspecific competition, and
also have been important in the past, in preventing inter-breeding as
the species diverged from each other.
However the behaviour of some sympatric species is not so widely
different that it accounts for their spatial separation into different
microhabitats. Larvae of the polyzoans Alcyonidium hirsutum (Fleming)
and A . polyoum (RassaIl), for instance, both settle in largest numbers
on Pucus serratus (Ryland, 1959). Spatial separation here might be
maintained by the preferences of the larvae for slightly different
convexities or for slightly different parts of the Fuczcs frond
(Ryland, 1959, Tables 7 and 9). On the other hand spatial separation
could be achieved by interspecific competition itself, although this does
not appear to be common, and may therefore have less selective advantage than direct behavioural recognition of an appropriate habitat.
Interspecific competition between closely related sympatric species has
been observed in the laboratory between two species of the intertidal
crab Uca and between two species of the freshwater crayfish Orwnectes
(Teal, 1958; Bovbjerg 1964, 1970 respectively), and also in the field
between two species of the barnacle BaZanus (Meadows 1969). Orconectes
virilis (Hagen) and Orconectes immunis (Hagen) (Bovbjerg loo. cit.)
are two North American species of crayfish having similar geographical
ranges but different habitats-0. virilis inhabits rock or gravel in
streams and lake margins, and 0. immunis inhabits mud in ponds and
slow moving streams. I n the middle stretches of the Little Sioux River
(Minnesota and Iowa) where the two species are sympatric, each is
restricted to its own habitat. Bovbjerg’s experiments prove that this
is the result of interspecific competition. If each species is separately
offered a choice of rock, gravel, and mud, they both prefer the rock,
whereas if the two species are mixed and then offered the same choice
0. virilis prefers the rock as before but drives most of the 0. immunia
onto the mud.
Having considered differences between sympatric species, it may
now be asked whether sympatric subspecies show differences in their
selection of habitats. We have only been able to find one instance. The
two subspecies of Littorim obtusata, L. 0. olivacea and L. 0. citrina, live