function of the spatial localization of the second intermediate host.
The cercariae ofCardiocephalus longicollis(Strigeidae) produced by a
benthic prosobranch (Amyclina corniculum) show positive phototactism
associated with negative geotactism. The resulting swimming behaviour
locates the cercariae approximately halfway between the bottom and
water surface, where the second intermediate hosts (fishes of the family
Sparidae) usually reside. In contrast, cercariae ofDeropristis inflata
(Acanthocolpidae), also produced by a benthic prosobranch (Hydrobia
acuta), show negative phototactism associated with positive geotactism
and remain on the substrate, where their second intermediate hosts,
which are annelids (Nereis diversicolor), are present.Information originating in the DSH received by the cercaria
Cercariae can be sensitive to physical (shadow, turbulence, contact) or
chemical signals (excreted, secreted substances) produced by the host
(Rea and Irwin, 1995; Haberlet al., 2000). Such stimuli are more or less
specific and generally act at a short distance between the parasite
infective stage and the host. Cotylomicrocercous cercariae (cercariae with
a short tail transformed into a sticky sucker) ofOpecoeloides columbella
emerge just after sunrise from a prosobranch snail (Columbella rustica)
that lives among photophilic algae. The cercariae remain immobile and
attached to the rocky substrate by their ventral sucker. When turbulence is
produced by a potential host passing by, the cercaria immediately
becomes erect, attached by the tip of its sticky tail, and awaits host contact
(Jousson and Bartoli, 2000).Information originating in the cercaria received by the DSH
Cercariae are able to produce signals that can be perceived by the DSH.
This is the case for two species of trematodes (Knipowitschiatrema nicolai
andCondylocotyla pilodorus) whose macrocercous cercariae (cercariae
with an unusually long and large tail), emitted by a benthic mollusc
(Cerithium vulgatum), mimic a polychaete annelid and are usually con-
sumed by the fish intermediate host (Belone belone). The cercariae mimic
both morphological and behavioural characteristics of intermediate hosts’
prey items: thanks to their large tail, they exhibit the swimming move-
ment and the violet colour that characterize planktonic organisms (Prévot,
1974; Pearson and Prévot, 1985). Thus, shape, movement and colour all
seem to be involved in the information diffused by the cercaria. Bartoli
and Gibson (1998) described an unusual visual signal emitted by the
cercariae ofCantharus dorbignyi. The body of the cercaria resembles a
flattened disc, which moves and turns in such a way that it makes the
cercaria alternately very visible and practically invisible. During thisTrematode Transmission Strategies 9