LAKE DWELLING TRICLADS
23
found on N. Uist and Harris (R. C. Connolly, private communication);
none in several lakes of the more northerly Shetlands (A. Milne, private
communication). It seems unlikely that all these lakes were so unproductive that they were unable to support P. nigra a t least. Although details
of water chemistry are available for only a few, several had calcium
values exceeding 10 mg/litre.
Any hypothesis advanced to explain the present-day distribution of
triclads must be consistent with the following facts. First, the scarcity of
triclads in the extreme north shows that these organisms have not yet
occupied to anything like the full extent, the suitable habitats available,
and are presumably still extending their range northwards. This is a
similar situation to that found for several other common freshwater
species, as shown for Gammarus pulex (Reid, 1951; Hynes, 1954); for
Asellus species (Williams, 1962b); for gastropods (Hunter, 1957) and
for a fresh-water mite (Efford, 1962). Second, it must explain the similarity of the British mainland fauna and that of Ireland; finally, it must
account for the restriction of the more southerly western islands to
virtually one lake species. Before detailed arguments are attempted, it
is necessary to consider the mode of dispersal in triclads. Ullyott (1936)
has denied the role of natural transport such as birds, although Thienemann (1950) reported some examples of this and also inferred from the
distributions of Crenobia alpina and Pulycelis felina that birds may hare
aided dispersal. Arguments for and against birds were also presented by
Klie (1926) and Arndt (1924) respectively. The recent studies of Maguire
(1963) on dispersal do not offer much support for transport of organisms
like triclads by other animals. It appears that the balance of evidence,
especially the clear-cut patterns discussed above, is against birds being
an important factor in dispersal although they may have been responsible for some introductions (p. 24). It would appear that triclads mainly
achieve dispersal by their own activities (Gislbn, 1946; Dahm, 1949);
although the latter attributed some importance to passive transport.
The most significant feature of the geographical distribution of lakedwelling triclads in the British Isles is the similarity of the Irish and
mainland faunas, and also the fact that eight of the nine species comprising the entire British fauna are found also in Ireland (Southern,
1936; Reynoldson, 1958a). Bdellocephala punctata is the only absentee.
The origin of the Irish fauna, especially the Lusitanian element, has been
critically reviewed by Corbet recently (1962) who discussed three main
theories to account for it:
(a) survival in southern Ireland during the last glacial phase or from
(b) post-glacial migration to Ireland via land connections,
(c) introduction by man’s trading activities.
an earlier time,
23
found on N. Uist and Harris (R. C. Connolly, private communication);
none in several lakes of the more northerly Shetlands (A. Milne, private
communication). It seems unlikely that all these lakes were so unproductive that they were unable to support P. nigra a t least. Although details
of water chemistry are available for only a few, several had calcium
values exceeding 10 mg/litre.
Any hypothesis advanced to explain the present-day distribution of
triclads must be consistent with the following facts. First, the scarcity of
triclads in the extreme north shows that these organisms have not yet
occupied to anything like the full extent, the suitable habitats available,
and are presumably still extending their range northwards. This is a
similar situation to that found for several other common freshwater
species, as shown for Gammarus pulex (Reid, 1951; Hynes, 1954); for
Asellus species (Williams, 1962b); for gastropods (Hunter, 1957) and
for a fresh-water mite (Efford, 1962). Second, it must explain the similarity of the British mainland fauna and that of Ireland; finally, it must
account for the restriction of the more southerly western islands to
virtually one lake species. Before detailed arguments are attempted, it
is necessary to consider the mode of dispersal in triclads. Ullyott (1936)
has denied the role of natural transport such as birds, although Thienemann (1950) reported some examples of this and also inferred from the
distributions of Crenobia alpina and Pulycelis felina that birds may hare
aided dispersal. Arguments for and against birds were also presented by
Klie (1926) and Arndt (1924) respectively. The recent studies of Maguire
(1963) on dispersal do not offer much support for transport of organisms
like triclads by other animals. It appears that the balance of evidence,
especially the clear-cut patterns discussed above, is against birds being
an important factor in dispersal although they may have been responsible for some introductions (p. 24). It would appear that triclads mainly
achieve dispersal by their own activities (Gislbn, 1946; Dahm, 1949);
although the latter attributed some importance to passive transport.
The most significant feature of the geographical distribution of lakedwelling triclads in the British Isles is the similarity of the Irish and
mainland faunas, and also the fact that eight of the nine species comprising the entire British fauna are found also in Ireland (Southern,
1936; Reynoldson, 1958a). Bdellocephala punctata is the only absentee.
The origin of the Irish fauna, especially the Lusitanian element, has been
critically reviewed by Corbet recently (1962) who discussed three main
theories to account for it:
(a) survival in southern Ireland during the last glacial phase or from
(b) post-glacial migration to Ireland via land connections,
(c) introduction by man’s trading activities.
an earlier time,