Basic HTML Version
International Journal of Marine Science 2013, Vol.3, No.40, 319-332
http://ijms.sophiapublisher.com
325
was complete, the TVLO abundance decreased to
match that of the source water.
The TVLO in shrimp hepatopancreas were significantly
higher than in rearing water and pond sediment (
P
<
0.05,
t
-test) during the corresponding period. This may
be because of their known close association with the
digestive tract. Lalitha and Surendran (2004) have also
recovered intestinal bacterial density much higher (>
3.5 log CFU g
-1
) than those in rearing waters growing
Macrobrachium
rosenbergii
. The abundance and
composition of bacteria associated with intestinal tract
is related to the environment as well as to the food the
shrimps consume. It has been reported that the food
materials concentrated in the intestine of marine
animals can provide a favorable ecological niche for
symbiotic heterotrophic populations, and these can be
of nutritional importance to the host organisms by
producing enzymes that help in digestion or by
supplying part of its nutrients (Cahill, 1990). Some
authors have claimed that extensive bacterial flora is
not commonly found in the hepatopancreas because
they are prevented from entering by the gastric sieve
(Hopkin and Nott, 1980) in combination with
digestive enzymes (Raissy et al., 2011). Damage to
gastric sieve by artificial pellet-formed foods has been
suggested to occur in cultivated animals (Gomez-Gil
et al., 1998) leading to the colonization of bacteria in
the internal organs (Raissy et al., 2011). Modified
extensive culture method followed in the present study
relies on natural productivity to some extent at least in
the initial days of culture in contrast to semi-intensive
and intensive culture methods. Additional work is
required to determine whether the differences in
colonization are in fact, due to the occurrence of
particular lining and/or digestive mechanisms in
different culture systems. A wide variation in the
weight of hepatopancreas of shrimps was also
observed during analysis (data not shown) which
could be attributed to the differential amounts of
ingesta in shrimps sampled in spite of our best efforts
to collect specimen with full stomach. Further,
insignificant correlation between the numbers of
bacteria retrieved and weight of the hepatopancreas
indicates that the majority of the bacteria were
associated with the digestive tract itself rather than
with the diet. This is in accordance with studies by
Gomez-Gil et al. (1998). It has also been documented
that shedding of the chitinous lining of the hindgut
due to frequent moulting in shrimps during a rapid
growth phase can greatly influence the bacterial
number and type of bacteria in the digestive system of
shrimps (Dempsey et al., 1989).
Representative
Vibrio
isolates collected from different
shrimp pond components in this study were identified
using standard biochemical tests. It may be noted that
molecular methods for taxonomic identification of
Vibrio
spp. have not been used in this study as they
would be useful at a secondary level when there are
indications of disease outbreaks or the number of
vibrios are above threshold limits. The rearing waters
and pond sediments had a fairly diverse
Vibrio
community with a larger prevalence of species such as
V. metschnikovii
,
V. fluvialis
and those belonging to
the genus
Aeromonas
. These species were also found
in the shrimp hepatopancreas suggesting that the
bacterial flora colonizing the shrimp digestive system
is normally drawn from the aquatic system either in
the rearing water or the pond sediment. Some
Vibrio
species recorded solely from the shrimp
hepatopancreas included
V. logei
,
V. marinus
and
V.
salmonicida
. The halophilic and psychrophilic
bacterium
V. salmonicida
(
Aliivibrio salmonicida
) is
closely related to the luminous bacteria
V. fischeri
and
V. logei
(Fidopiastis et al., 1999). It is known to be a
causative agent of cold water vibriosis in marine
aquaculture causing tissue degradation, hemolysis and
sepsis
in vivo
(Hjerde et al., 2008). In the present
study, the average abundance of
V. salmonicida
in
shrimp hepatopancreas was about 17% through the
culture period. It is unlikely that they would have
reached a threshold level to affect the shrimps. The
presence of
V. gazogenes
was also recorded in shrimp
hepatopancreas. This strain is generally found in the
mid and hind gut of the shrimp and it’s presence could
be due to cross contamination with other parts of
shrimp digestive system while removing the
hepatopancreas. Reports of infection by particular
members of the genus
Vibrio
within cultured penaeid
shrimp populations leading to disease are numerous
(Lightner, 1988; Lavilla-Pitogo et al., 1990; Ruangpan
and Kitao, 1991; Sung et al., 1999; Liu et al., 1996).
Sung et al (2001) have even reported growth
retardation.
V. gazogenes
is known to have
antagonistic activity towards a panel of shrimp
pathogenic vibrios causing a significant decline in the
number of
Vibrio
-like bacteria in the fore and hind gut