Ecological and pathological studies of gill monogeneans in floating cage cultured seabass lates calcarifer (bloch)
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Date
1994-06
Authors
Balasuriya, L.K.S.W.
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Abstract
Gill monogeneans were observed to be the most serious parasitic disease
problem in floating cage cultured seabass in Malaysia. It appeared that they were
the primary cause for bacterial infections. which often caused severe mortalities.
Three species of diplectanid monogeneans. Pseudorhabdosynochus latesi
(Tripathi 1955). P. monosquamodiscusi (Balasuriya and Leong 1994) and
Diplectanum penangi (Liang and Leong 1991) occurred on the gills, the former
two often with a high prevalence and mean intensity.
Out of the two congeneric species, P. latesi was the dominant parasite. The
intensity of P. latesi infection continuously increased with the growth of the
fish. In contrast. the intensity of P. monosquamod.iscusi were found to be highest
in middle size fish (10-30 em) and reduced in large size fish (>30 em). It appeared
that when the intensity of P. latesi increased. it competitively excluded P.
monosquamodiscusi.
P. monosquamodist:usi differed from the other diplectanid monogeneans
described from seabass. by having only one squamodisc on the ventral side of the
opisthaptor. Internally, the morphology of the sclerotized vagina was of
hourglass shape. The penis bulb of this species consisted of four concentric layers
of apparent muscular origin.
Monogeneans appeared on the gills of the newly introduced fish. two weeks
after being placed in the cages and the intensity of the infection increased rapidly
after four weeks. A close relationship was observed between the stocking density
of seabass in cages and gill monogenean population.
The monogeneans showed definite and different distribution patterns on
the gill arches. The distribution among the left and U1e right gill arches were
random. The distribution on various gill arches and their microhabitats varied
with the intensity of the infection.
The life cycles of P. monosquamodiscusi and P. latesi were similar to each
other. Both species laid one egg at a time. averaging one egg/hour in captivity.
However. only the first two to three eggs were viable. The eggs have basic
tetrahedral shape with a long filament attached to one comer. The eggs developed
to oncomiracidium larvae within two to four days at 30'C and 32 ppt salinity.
The eggs could tolerate a wide range of salinity ( 10-40 ppt). The rate of hatching
was faster in low salinities (l0-15 ppt) compared to high salinities (30-35 ppt).
The life span of the fast swimming oncomiracidium larvae was eight to ten
hours. The larvae first attached to the skin and later migrated to the gills. The
larvae developed to mature parasites within 10 to 14 days.
The histopathology of infected gills showed that P. monosquamodiscusi
and P. latesi are less pathogenic than D. penangL No host tissue reaction
developed at the site of attachment of these two species probably due to their
migratory behavior on the gill arches. In contrast. prominent host-tissue
• reaction developed at the site of attachment of D. penangL The histopathological
changes in the gills of severely infected fish included hypertropy. hyperplasia,
excess number of mucus and chloride cells and fusion of secondary lamella.
A freshwater bath for about 15 -30 minutes was the best treatment for this
monogenean infections. Proper husbCL."1dry practices could significantly reduce
this parasitic infection in cages.
Description
Keywords
Pathological studies , Gill monogeneans , Floating cage cultured seabass , Calcarifer (bloch)