Biology Reference
In-Depth Information
As soon as fi sh larvae start to capture and ingest live prey, the numbers
of bacteria in the larvae increase exponentially up to values as high as
10
5
per larva within a few days (Munro et al., 1994; Muroga et al., 1987).
Members of α- and γ-proteobacteria are quite common in fi sh larvae
(Nakase et al., 2007; Schulze et al., 2006). These bacteria are located mainly
in the larval gut, where as has been proved disinfection of the surface
of the larvae does not decrease the numbers of culturable bacteria to a
noticeable degree. However, the biodiversity of microbiota associated
with the gut is quite low, as only a few species dominate.
Vibrio
species are
the main component of the gut microbiota in rearing conditions and are
mainly derived from bacteria associated with live prey (Fjellheim et al.,
2007; Skjermo and Vadstein, 1993; Verschuere et al., 1997).
After the mouth opening and the onset of exogenous feeding, it is
relatively easy to manipulate the bacterial diversity in the gut by using
bacteria bioencapsulated in the prey, rotifers or
Artemia
metanauplii
(Makridis et al., 2000a). This phase is critical in many aspects for the
successful rearing of marine fi sh larvae as pioneer species, i.e., bacterial
populations that fi rstly colonize the gut, have a competitive advantage
compared with bacteria that reach the gut at a later stage, when available
adhesion sites have been reduced and environmental conditions are
infl uenced by the bacterial populations already established (Hansen and
Olafsen, 1999; Skjermo and Vadstein, 1999).
A main characteristic of the type of colonization in the gut of larvae
is that species established during this period are mainly opportunistic,
taking advantage of the input of nutrients in a “new” environment (larval
gut). The establishment of excessive numbers of bacteria in weakened
larvae may result in high mortalities, but mortalities in healthy larvae, not
exposed to environmental stress and fed on high quality prey, are low and
not dependent on the numbers of bacteria present in the gut (Makridis et
al., 2008; Salvesen et al., 1999).
At the stage of weaning, the numbers of bacteria decrease, and
relatively few bacteria are measured in the fi sh gut per unit of gut weight
compared with earlier stages (Makridis et al., 2008). The development
of the stomach and its acid barrier makes direct infl uence of species
established in the intestinal microbiota by addition of probiotics in the
feed more complicated.
NEW STRATEGIES FOR BACTERIAL INFECTIONS
CONTROL IN MARINE FISH LARVAE
The development of new strategies is based mostly on the study and
knowledge of microbial diversity and ecology in larviculture systems (
Fig.
