Geoscience Reference
In-Depth Information
Adult and juvenile pearl mussels tend to have
similar habitat 'preferences', although adults are
found over a wider range of physical conditions
and juveniles appear to be more exacting in
their requirements and have a sensitivity to
environmental disturbance (Hastie et al. , 2000a).
During the summer, after an incubation period
of several weeks, female mussels discharge their
larvae (glochidia) into the river. The glochidia
resemble microscopic snapping clams and these
drift downstream and, if successful, snap shut
and encyst on the gills of a suitable host fish.
M. margaritifera glochidia are very host-specific
and can only complete their development on
Atlantic salmon ( Salmo salar )orsea/browntrout
( Salmo trutta ). Usually juvenile fish (fry and parr)
are selected (Young and Williams 1984). The
presence of freshwater pearl mussels in any river
therefore depends on host salmonid availability.
The encystment phase lasts for several months
before the glochidia metamorphose into tiny
mussels, drop off the fish and settle into the river
bed (Young and Williams, 1984). A diagram of the
life cycle is provided in Figure 10.1.
Status
Since the 19th century, when pearl mussels were
numerous across most of Scotland, the number
of Scottish rivers with healthy mussel populations
has declined dramatically. The results of a national
survey were published in 2000 (Cosgrove et al.,
2000b) and this, along with additional recent data
from a Scottish Natural Heritage (SNH) confidential
Attachment & encystment
glochidia overwinter on gills
of host salmonid fish
Glochidia release
2-3 x 10 6 glochidia released in summer
(mortality >99.99%)
Settlement
mussel seed (0.4mm) drop off
gills in spring/early summer
Fertilisation
fertilised eggs develop within female
Maturation
12-20 years
Juveniles (<65mm)
Burrow into clean stable riverbed sediments
Adult mussel (5-150mm)
Mature from ca. 12-80 years
Figure 10.1 The freshwater pearl mussel life cycle.
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