Biology Reference
In-Depth Information
more gill rakers, larger eyes, and shorter upper jaws than normal smelt,
correlated with feeding on plankton. Normal smelt feed to a greater
extent on small fish. In Lake Utopia, New Brunswick, the dwarf and nor-
mal forms spawn in different streams and at different times. Mitochondr-
ial DNA and nuclear minisatellite DNA analyses also indicate that these
forms are quite distinct. The pattern of genetic differentiation of dwarf
and normal fish in other lakes is less clear. Altogether, the evidence sug-
gests that dwarf and normal fish have evolved independently in the vari-
ous lakes in which both occur. In cases such as Lake Utopia, these forms
appear to be behaving as distinct biological species.
Lake whitefishes (
Coregonus
spp.) have developed up to five ecological
races in various northern lakes in Europe and North America. Most fre-
quent are benthic and open-water morphs.These forms differ in various
morphological features, including the number and shape of gill rakers, an
adaptation for plankton feeding (Lindsey 1981). Many of these forms are
genetically distinct and appear to have differentiated in sympatry in indi-
vidual lakes (Kirkpatrick and Selander 1979; Bodaly et al. 1992).A partic-
ularly interesting situation exists in the Allegash River basin of northern
Maine (Kirkpatrick and Selander 1979). Here, a dwarf form of the wide-
spread lake whitefish (
Coregonus clupeiformis
) exists alone or together with
the normal form of the species in 22 lakes. Ancestral whitefish have col-
onized this region within the past 12,000 yr, following release from con-
tinental glaciation.The dwarf and normal forms are quite distinct in mor-
phology and life history. The dwarf form reaches sexual maturity in 1 or
2 years but rarely lives longer than 4 yr.The normal form does not mature
until its fourth year but continues to reproduce annually until a maximum
age of about 12 yr.The two forms also differ by several weeks in spawn-
ing time. Electrophoretic studies revealed that the two forms differ in fre-
quencies of several genes and suggest that the two forms should be con-
sidered sibling species. In all likelihood, the dwarf form is derived from
the normal form, although whether differentiation occurred in the same
or different lakes is uncertain.
Although much of the differentiation of whitefish has occurred over
some unknown length of postglacial time, several cases of recent translo-
cations of these fish show that change can be very rapid (Lindsey 1981).
In northern Italy, for example, whitefish did not originally occur in Lake
Maggiore. In the late 1860s, two species of
Coregonus
from Lake Con-
stance were translocated to Lake Maggiore. These species differed in the
number of gill rakers, but in Lake Maggiore they apparently interbred
freely, yielding a form with an intermediate number of gill rakers. Much
