4. Discussion
Our results suggest the occurrence of both behavioural and immunological
changes as a consequence of a simulated infection in
O. vulgaris. Indeed, behavioural observations revealed that E. coli LPS injection
determines a significant decrease of animal activity in exploring
the surroundings by the typical crawling behaviour [29] and thereby
a significant increase in the percentage of time spent inside den in the
isolated condition, suggesting a general depression of behaviours consequent
to infection. Although we did not record a significant effect of the
treatment on the behaviour of sham-injected individuals, we are aware
that the unavoidable limits of our experimental procedure do not allow
us to completely exclude a certain susceptibility of animals to meremanipulation
(anaesthesia, haemolymph sampling and sham-injection).
As a matter of fact, the non random distribution of treated and control
animals (see Materials and methods section) could have masked an
effect of manipulation, because control individuals showed a low level
of activity and interaction with the conspecific also before injection. At
a first sight the increase in the latency to attack the crab after treatment
recorded in both LPS-treated and sham-injected individuals could be
interpreted as a consequence of manipulation. However we believe
that it could be the result of a habituation to stimulus (i.e. the crab),
facilitated by the fact that animals were not allowed to catch their
prey over a series of subsequent instances, rather than a generalized effect
ofmanipulation. Indeed, in subsequent trials, individualswere often
observed to abandon the quick attack strategy in favour of a very slow
and careful approach, while trying to find alternative way of catching,
e.g. reducing little by little the distance to the prey before launching
the attack.
Even considering constraints in interpreting some results the differential
behavioural changes in LPS-treated animals in the two conditions,
i.e. isolated and sight-allowed, still represent a notably intriguing
finding. Indeed, in the sight-allowed condition, when individual's
activity is certainly more stimulated by the presence of a conspecific
(as documented by the higher activity pattern, compared to isolation
condition, exhibited by all animals before treatment), LPS-treated