9.8.1 Glochidiosis
Margaritifera margaritifera, the fresh water pearl mussel is
an aquatic bivalve resident in fast flowing cool waters low in
calcium, and their distribution is restricted and threatened in
Europe. Glochidia, the microscopic larval stage of the mussel,
are released from adult animals into the water and attach to
the gill lamellae of resident trout and salmon fingerlings (or
parr) for a period, before it detaches and establish in the
bottom substrate. On contact with the gills, the glochidia
clamps to the lamellae using sharp teeth and enclose a portion
of the lamellae in the mantle cavity. Both wild and farmed
stock including Atlantic salmon in Scotland and Norway, and
Chinook and coho parr in America have been affected by
glochidia (Fig. 9.57). In Japan, the glochidia of M. laevis have
occasionally been reported on salmonids. Following attachment to the gill epithelium, a localised hyperplasia and fusion
of lamellae occurs and may surround the developing larvae,
with the area becoming thinner as the glochidia develop.
Encysted parasites occur to a lesser degree in the gill rakers
and occasionally, the pseudobranch. When large glochidia are
located distally, this results in clubbing of the filaments
(Fig. 9.58). Stricture of gill capillaries and hyperplasia is
Fig. 9.56 Salmincola thymalli on the gills of wild grayling
Fig. 9.57 Glochidia of Margartifera margartifera attached to the gills
of wild brown trout
Fig. 9.55 Severe infestation of Salmincola sp. in the gills of wild
rainbow trout. Note necrotic gill tips
Fig. 9.58 Glochidia of Mytilus edulis (spat) on gills of Atlantic
salmon. Low power
9.8 Bilvalvia
147
Margaritifera margaritifera, the fresh water pearl mussel is
an aquatic bivalve resident in fast flowing cool waters low in
calcium, and their distribution is restricted and threatened in
Europe. Glochidia, the microscopic larval stage of the mussel,
are released from adult animals into the water and attach to
the gill lamellae of resident trout and salmon fingerlings (or
parr) for a period, before it detaches and establish in the
bottom substrate. On contact with the gills, the glochidia
clamps to the lamellae using sharp teeth and enclose a portion
of the lamellae in the mantle cavity. Both wild and farmed
stock including Atlantic salmon in Scotland and Norway, and
Chinook and coho parr in America have been affected by
glochidia (Fig. 9.57). In Japan, the glochidia of M. laevis have
occasionally been reported on salmonids. Following attachment to the gill epithelium, a localised hyperplasia and fusion
of lamellae occurs and may surround the developing larvae,
with the area becoming thinner as the glochidia develop.
Encysted parasites occur to a lesser degree in the gill rakers
and occasionally, the pseudobranch. When large glochidia are
located distally, this results in clubbing of the filaments
(Fig. 9.58). Stricture of gill capillaries and hyperplasia is
Fig. 9.56 Salmincola thymalli on the gills of wild grayling
Fig. 9.57 Glochidia of Margartifera margartifera attached to the gills
of wild brown trout
Fig. 9.55 Severe infestation of Salmincola sp. in the gills of wild
rainbow trout. Note necrotic gill tips
Fig. 9.58 Glochidia of Mytilus edulis (spat) on gills of Atlantic
salmon. Low power
9.8 Bilvalvia
147