octopuses showed a significant decrease in the time spent interacting
with the conspecific, while no significant effect was observed with regard
to the other activities. This suggests that avoidance of interactions
may represent a more specific adaptive response rather than a simple
consequence of a general trade-off between behavioural and physiological
responses to infection that are expected to entail a general depression
of all behavioural activities [2]. The influence of health status on
interaction abilities is important in social species [4], but it may play a
relevant adaptive role also in species with mostly solitary habits, such
as O. vulgaris [34]. Indeed, competitive interaction might occur in different
cephalopod species that permanently occupy dens and actively
defend their dens from conspecifics (reviewed in [35]). Moreover different
species have been documented to formand maintain hierarchies
in captivity [35,36]. Our results, highlighting for the first time the behavioural
responses to both infection and mere manipulation in this
species, can set the basis for further studies to exhaustively determine
how health status can influence the outcome of reciprocal competitive
interaction.
With respect to the effect of immune-challenge on immunological
parameters, LPS injection rapidly activates immune defences in response
to the treatment and this effect seems to start vanishing 24 h
after injection. Indeed, the circulating haemocytes increased, following
the LPS injection, to successively decrease and reach, 24 h after the
treatment, a level similar to that of sham-injected specimens. A rapid
pattern of immune response has been observed in other cephalopods
where bacteria were cleared from the circulation in less than 4 h
[18,37]. Haemocytes represent a key component