Biology Reference
In-Depth Information
camara
,a widely planted, exotic tetraploid species. Hybrids between the two
produce triploid progeny that spread vigorously through the natural habi-
tat of
L. depressa
(Sanders 1987). It appears likely that in time these hybrids
will completely displace diploid populations of
L. depressa
.
Several freshwater mussels and crayfish in North America are also suf-
fering from hybridization because of the introduction of species to waters
to which they are alien. Hybridization has been documented between
several species of the genus
Orconectes
, thus complicating the already seri-
ous problem of competitive displacement of locally endemic crayfish
described earlier (Perry et al. 2001b). Several cases of interspecific
hybridization between mussels also have been documented, and the
decline of two related species in the Susquehanna River has been attrib-
uted to hybridization (Perry et al. 2002). Hybridization is probably much
more widespread than yet recognized among freshwater invertebrates.
In New Zealand, introduction of the common blue butterfly (
Zizina
labradus
) from Australia has led to the genetic extinction of the southern
blue butterfly (
Z. oxyleyi
) by hybridization (Barlow and Goldson 2002).
About 38% of the fish extinctions in North America within recorded
history (Miller et al. 1989) were judged to have been caused in some
measure by introgressive hybridization. These include the extinctions of
at least two subspecies of cutthroat trout (
Oncorhynchus clarki
) and two
locally endemic species of
Gambusia
that hybridized with the widely
introduced
Gambusia affinis
.
Isolated populations of fish in springs and headwater streams are at high
risk from hybridization. Often these forms are closely related to more wide-
spread forms from which they have diverged because of their isolation. In
Texas, for example, the Leon Springs pupfish (
Cyprinodon bovinus
) occurs in
a small, spring-fed tributary of the Pecos River. Recent examination of
these field populations revealed that hybridization has occurred with the
more widespread sheepshead minnow (
Cyprinodon variegatus
), which was
introduced to its habitat (Echelle and Echelle 1997).All field populations of
this endemic are affected, but fortunately, a pure captive population exists.
In a related case, the Pecos pupfish (
Cyprinodon pecosensis
), which occupies
the main portion of the Pecos River in Texas and New Mexico, has suffered
introgression from the sheepshead minnow over about 300 km of the river
in Texas (Wilde and Echelle 1997). Both of these cases of introduction of
sheepshead minnows to waters with related species are probably the result
of the common use of sheepshead minnows as bait fish.
A special circumstance involves hatchery and wild salmon in the
North Atlantic and North Pacific. In Norway, for example, Atlantic
