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An optimised multi-host trematode life cycle: fishery discards enhance
trophic parasite transmission to scavenging birds
q
Ana Born-Torrijos
a,
⇑
, Robert Poulin
b
, Ana Pérez-del-Olmo
a
, Jacopo Culurgioni
c
, Juan Antonio Raga
a
,
Astrid Sibylle Holzer
d
a
Cavanilles Institute for Biodiversity and Evolutionary Biology, Science Park, University of Valencia, P.O. Box 22 085, 46071 Valencia, Spain
b
Department of Zoology, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand
c
Department of Life and Environmental Sciences, University of Cagliari, Via T. Fiorelli, 1, 09126 Cagliari, Italy
d
Institute of Parasitology, Biology Centre of the Academy of Sciences of the Czech Republic, Branišovská 31, 370 05 C
ˇeské Bude
ˇjovice, Czech Republic
article info
Article history:
Received 17 March 2016
Received in revised form 14 June 2016
Accepted 16 June 2016
Available online 1 August 2016
Keywords:
Cardiocephaloides longicollis
Fishery discards
Human-induced impact
Host specificity
Mediterranean
Trematodes
abstract
Overlapping distributions of hosts and parasites are critical for successful completion of multi-host par-
asite life cycles and even small environmental changes can impact on the parasite’s presence in a host or
habitat. The generalist Cardiocephaloides longicollis was used as a model for multi-host trematode life
cycles in marine habitats. This parasite was studied to quantify parasite dispersion and transmission
dynamics, effects of biological changes and anthropogenic impacts on life cycle completion. We compiled
the largest host dataset to date, by analysing 3351 molluscs (24 species), 2108 fish (25 species) and 154
birds (17 species) and analysed the resultant data based on a number of statistical models. We uncovered
extremely low host specificity at the second intermediate host level and a preference of the free-
swimming larvae for predominantly demersal but also benthic fish. The accumulation of encysted larvae
in the brain with increasing fish size demonstrates that parasite numbers level off in fish larger than
140 mm, consistent with parasite-induced mortality at these levels. The highest infection rates were
detected in host species and sizes representing the largest fraction of Mediterranean fishery discards
(up to 67% of the total catch), which are frequently consumed by seabirds. Significantly higher parasite
densities were found in areas with extensive fishing activity than in those with medium and low activity,
and in fish from shallow lagoons than in fish from other coastal areas. For the first time, C. longicollis was
also detected in farmed fish in netpens. Fishing generally drives declines in parasite abundance, however,
our study suggests an enhanced transmission of generalist parasites such as C. longicollis, an effect that is
further amplified by the parasite’s efficient host-finding mechanisms and its alteration of fish host beha-
viour by larvae encysted in the brain. The anthropogenic impact on the distribution of trophically-
transmitted, highly prevalent parasites likely results in a strong effect on food web structure, thus making
C. longicollis an ideal bioindicator to compare food webs in natural communities versus those impacted by
fisheries and aquaculture.
Ó2016 Australian Society for Parasitology. Published by Elsevier Ltd. All rights reserved.
1. Introduction
Trophically transmitted parasites are central elements in most
aquatic food webs (Lafferty et al., 2006, 2008) and exert strong
effects on host community and food web structures when infecting
a high number of host species (Thompson et al., 2005). Such para-
sites can manipulate the behaviour of intermediate hosts, funda-
mentally altering the patterns of contact between predator and
prey. In gammarids, infection by the acanthocephalan Echi-
norhynchus truttae alters both attack rates for predators and con-
sumption of prey, with a 30% increase in maximal predation
rates (Dick et al., 2010). Changes in predator–prey links can
strongly impact food web structure and dynamics, thereby altering
energy flow through the food chain (Marcogliese, 2004; Lagrue
et al., 2011). In multi-host parasites, host distribution strongly
affects parasite transmission and successful life cycle completion
(Esch and Fernandez, 1994; Fredensborg et al., 2006). Nowadays,
marine habitats are influenced considerably by human activities
such as fishing or aquaculture (e.g. Edgar et al., 2005; Grigorakis
and Rigos, 2011). The impact of fisheries on the distribution and
transmission of parasites that use fish as one of several hosts has
http://dx.doi.org/10.1016/j.ijpara.2016.06.005
0020-7519/Ó2016 Australian Society for Parasitology. Published by Elsevier Ltd. All rights reserved.
q
Nucleotide sequence data reported in this paper are available in GenBank
TM
under the accession number KT454991.
⇑
Corresponding author. Tel.: +34 96 354 36 85; fax: +34 96 354 37 33.
E-mail address: A.Isabel.Born@uv.es (A. Born-Torrijos).
International Journal for Parasitology 46 (2016) 745–753
Contents lists available at ScienceDirect
International Journal for Parasitology
journal homepage: www.elsevier.com/locate/ijpara
been little studied. Wood et al. (2010) hypothesised that fishing
would drive a long-term, global decline in fish parasites due to
the reduction of host density, food web complexity and thus para-
site transmission efficiency; however, the response of parasites to
fishing depends on their life cycle strategy and the fishing status of
the host (Wood and Lafferty, 2015). On the other hand, discards
from fishing, which are known to attract piscivorous birds and
other wild animals that serve as final hosts (Oro and Ruiz, 1997;
Morton and Yuen, 2000; Arcos et al., 2001; Bozzano and Sardà,
2002), may result in increased parasite transmission. However, in
the long run discards affect fish host distribution and availability
(e.g. Tasker et al., 2000; Coll et al., 2008).
Here, we investigate the case of the trematode Cardiocephaloides
longicollis (Rudolphi, 1819) Dubois, 1982 (Strigeidae), which is
widespread along the Mediterranean coast. Cardiocephaloides
longicollis has a complex life cycle. Adult parasites colonise the
digestive tract of their definitive hosts, predominantly seagulls.
Eggs are released via the host’s faeces and drop to the seafloor
where free-swimming miracidia hatch. Following its burrowing
entry into the first intermediate host, nassariid snails, C. longicollis
proliferates by massive asexual reproduction within sporocysts
and forms actively swimming cercariae as the next transmission
stage. Cercariae emerge from the snails and penetrate the skin of
their second intermediate host, fishes. Thereafter, cercariae
migrate to and encyst in the fish brain as metacercariae (Prévot
and Bartoli, 1980). Cardiocephaloides longicollis is trophically trans-
mitted to the definitive host when infected fish are consumed by
seabirds (Bartoli and Prévot, 1986; Fredensborg and Longoria,
2012).
Parasites use a variety of transmission strategies to ensure the
successful completion of their life cycle. Cercariae of C. longicollis
overcome the distance between their benthic snail hosts and their
target fish hosts, and disperse by following a strategy of alternating
swimming and resting periods, using positive phototaxis for orien-
tation (Bartoli and Prévot, 1986; Combes et al., 1994). Metacer-
cariae of C. longicollis infect the optic lobes of the fish’s
mesencephalon (Prévot and Bartoli, 1980; Osset et al., 2005;
Bartoli and Boudouresque, 2007), causing a fundamental change
in host behaviour due to impaired vision and motor control
(Prévot, 1974. Recherches sur les cycles biologiques et l’écologie
de quelques trématodes nouveaux parasites de Larus argentatus
michaelis Naumann dans le midi de la France. Thèse Université
d’Aix-Marseille, France; Barber and Crompton, 1997). Infected fish
dwell in the upper part of the water column and show body oscil-
lations that make their bright flanks visible to flying birds (Osset
et al., 2005; Bartoli and Boudouresque, 2007), and are thus more
prone to predation (Poulin, 2001; Seppälä et al., 2004; Osset
et al., 2005; Fredensborg and Longoria, 2012). In other trematode
species, the number of metacercariae located in the eye or the
brain of fish has been directly related to the degree of conspicuous
behaviour induced (Barber and Crompton, 1997; Fredensborg and
Longoria, 2012). In this scenario, anthropogenic influences such
as aquaculture and fisheries are of special importance as they
may further enhance the transmission of the parasite to their final
hosts. Osset et al. (2005) first hypothesised that an increased trans-
mission of C. longicollis could occur in areas of intensive commer-
cial fishing, since discards are frequently consumed by seabirds
(Witt et al., 1981; Christel et al., 2012). Subsequently this would
result in increased egg release from final hosts and, long term, in
a general increase in parasite biomass in the ecosystem.
We focused on C. longicollis as a model for studying the
influence of biological and anthropogenic factors on generalist
multi-host life cycles in coastal areas. Our aim was to provide a
comprehensive survey of the occurrence of C. longicollis along
the Mediterranean coastline, a better understanding of its life cycle
strategies and the impact of anthropogenic activities on trematode
transmission and abundance. First, the host spectrum used by C.
longicollis was determined by assessing its occurrence and degree
of specialisation at all levels, i.e. in snail, fish and bird hosts.
Secondly, the marine microhabitat targeted by cercariae was deter-
mined by comparing infection levels in benthic, demersal and pela-
gic fish species. Thirdly, the effect of fish size on the accumulation
of C. longicollis metacercariae was analysed, evaluating the possible
implications of different parasite loads. Finally, it was tested
whether increased transmission of C. longicollis occurs in areas of
intensive commercial fishing as well as in aquaculture installa-
tions, and the impact of these human activities on trophically
transmitted parasites, their hosts and the food web, was discussed.
2. Materials and methods
2.1. Sampling sites and habitat definition
An opportunistic sampling strategy was adopted for this study,
focussing on the examination of a diverse range of hosts suspected
to be part of the life cycle of C. longicollis. All sampling sites are
indicated in Fig. 1 and Supplementary Tables S1–S3. Molluscs were
hand-collected in shallow areas that are likely important for C.
longicollis transmission but differ in the intensity of local anthro-
pogenic activities. Fish species were sampled in 14 localities along
the western Mediterranean coast (Spain) and in three lagoons in
southern Sardinia (Italy) (see Fig. 1), including one Sparus aurata
aquaculture site (netpens). The largest number of fish individuals
and species were collected in Carboneras and Santa Pola (Spain),
areas of intensive fishing and aquaculture activities. Demersal,
benthic and pelagic fish (caught by trawling, netting and trapping)
were obtained from commercial fishers or fish markets. Birds were
obtained from bird sanctuaries, one in the Ebro Delta and one in
Valencia (Spain) (approximately 200 km apart). Birds move over
large distances so that their site of capture is not indicative of their
distribution and feeding sites. No formal ethics approvals were
required for this as both fish and bird hosts were obtained post-
mortem.
2.2. Host taxa and screening methodology
A total of 3351 molluscs belonging to 24 species and 18 families
(Supplementary Table S1) were collected and screened for sporo-
cysts and mature cercarial infections using standardised methods
(see Born-Torrijos et al., 2014). A total of 2108 fish belonging to
25 species and 10 families was examined for the presence and
quantity of metacercariae of C. longicollis in the brain (Supplemen-
tary Table S2). Fish were measured and weighed before dissection.
Two samples (see Supplementary Table S2) were examined as a
pool, for which the total number of parasites was determined after
chloropeptic tissue digestion. The bird sanctuaries provided 154
birds belonging to 17 species and four families (Supplementary
Table S3), which were frozen until necropsy. The alimentary tract
(from oesophagus to cloaca) was removed and examined for adult
specimens of C. longicollis. Given the opportunistic nature of sam-
pling, sample sizes of some species are low.
2.3. Parasite identification
All life cycle stages of C. longicollis, i.e. adults as well as sporo-
cysts, cercariae and metacercariae, were identified following the
descriptions by Dubois (1968) and Prévot and Bartoli (1980), and
preserved in 100% ethanol for molecular analyses. Eight random
samples of metacercariae from Dentex dentex,Chromis chromis,
Diplodus puntazzo and Lithognathus mormyrus, cercariae from
Nassarius reticulatus and Cyclope neritea, and adults from Larus
746 A. Born-Torrijos et al. / International Journal for Parasitology 46 (2016) 745–753
michahellis were analysed by rDNA sequencing, to confirm micro-
scopic parasite identification. DNA extraction, 28S rDNA amplifica-
tion and sequencing was performed as described in Born-Torrijos
et al. (2012). For additional internal transcribed spacer region 2
(ITS2) rDNA amplification and sequencing of the adults, primers
3S (forward 5
0
-GGT ACC GGT GGA TCA CGT GGC TAG TG-3
0
)
(Morgan and Blair, 1995) and ITS2.2 (reverse 5
0
-CCT GGT TAG
TTT CTT TTC CTC CGC-3
0
)(Cribb et al., 1998) were used with an
annealing temperature of 54 °C for 50 s.
2.4. Statistical analyses
In all hosts, infection prevalence (percentage of infected hosts in a
sample), mean intensity and range (number of parasite individuals
in an infected host), as well as mean abundance (±S.D.) (average
number of parasite individuals per host in a host population) were
calculated (Bush et al., 1997). The data acquired in the present study
were combined with published data from existing records, obtained
through a search of the Web of Science (ThomsonReuters, https://
webofknowledge.com/) (search string: Cardiocephal⁄), expanded
by a manual bibliographic search of references in publications from
the primary search. For statistical analyses, only true zero preva-
lences were included, i.e. the local prevalence is zero but there are
existing records that show this host species can actually be infected
by the studied parasite. Nevertheless, host species examined in this
study or in the existing literature found to be uninfected were added
to Supplementary Table S2, to indicate host taxa where C. longicollis
has never been found. In order to estimate whether fish habitat (ben-
thic, demersal and pelagic; information based on FishBase (Froese
and Pauly, 2015. FishBase. World Wide Web electronic publication.
www.fishbase.org, version (02/2015)) is related to changes in preva-
lence of C. longicollis, the data were analysed using generalised linear
mixed models (GLMMs). The effect of season of fish capture was also
investigated, but not explored here (see Supplementary Data S1,
Supplementary Fig. S1). To check for congruent patterns in preva-
lence or abundance depending on fish habitat, our data combined
with previously published records were analysed with different lin-
ear models (LMs). To examine possible differences in C. longicollis
prevalence in fish in relation to varying fishing pressures in the
Mediterranean, GLMMs were performed. The fishing activity of the
sampling areas was estimated on the basis of the hold capacity of
vessels from the nearest harbour, i.e. the fishing fleet size between
2010 and 2014, as presented in the European Atlas of the Seas (Direc-
torate General for Maritime Affairs and Fisheries of the European
Commission, DG MARE, http://ec.europa.eu/maritimeaffairs/at-
las/maritime_atlas/#lang=EN;bkgd=6:0.5;mode=1;pos=11.535:51.
462:5;theme=63:0.9:1;time=2014; last accessed 17/03/2016), mea-
sured in gross tonnage (GT, i.e. as an indicator of the fishing fleet
capacity or the fleet’s overall internal volume for fish captured). This
is the only measure of total fishing activity available across all ports
(different countries and reporting regulations) that is reported to the
European Commission (EC). Three categories of GT were used, i.e.
low hold capacity, <3000 GT; medium hold capacity, 3000–12,000
GT; high hold capacity, >12,000 GT. To study the influence of fish-
eries on parasite transmission in more detail, we aimed to compare
discard data (host species, size, etc.) from different ports with the
parasite data from the present study. However, while the Secretaría
General de Pesca (Spanish Ministry of Agriculture, Food and Environ-
ment) kindly provided data on landings, discards are not officially
recorded by either national or international organisations such as
the Food and Agriculture Organization of the United Nations (FAO)
but are only reported occasionally in individual local studies
(Pauly et al., 2014). The latter demonstrate that the amount of dis-
cards shows great variability in terms of weight and species compo-
sition (e.g. Sánchez et al., 2004), or even depending on the season of
capture, fishing strategy and number of vessels (e.g. Arcos et al.,
2001; Tzanatos et al., 2007; Coll et al., 2014), and it is often not pro-
portional to the landed tonnage. As a concrete example for more in-
depth analysis we selected local discard data from a heavily fished
area that overlaps geographically with our dataset (García-Rivera
et al., 2015), to infer the amount of C. longicollis metacercariae that
may be discarded (see Discussion).
Fish infection data were also analysed for differences between
shallow lagoons and deeper coastal waters, using generalised lin-
ear models (GLMs). The correlation between the prevalence and
mean intensity of C. longicollis infection was calculated for all fish
samples, using a Spearman rank correlation. To investigate the
relationship between fish length and C. longicollis abundance, two
subsets of 198 Diplodus vulgaris and 196 Diplodus annularis were
used. Fish size data were stratified into size classes and the effect
of size on the number of metacercariae per individual fish was
analysed using a negative binomial GLM. Thereafter, to detect
potential parasite-induced host mortality, the degree of aggrega-
tion was calculated for each host size class using the index of dis-
crepancy (Poulin, 1993), using the software package Quantitative
Fig. 1. Map showing the geographical distribution of Cardiocephaloides longicollis in Mediterranean and Black Seas and reports of it in different hosts, including published data
and those of the present study. The silhouettes represent reports of infection in fish, seagulls and gastropods, and numbers indicate the numbers of infected host species that
have been found in the sampling localities.
A. Born-Torrijos et al./ International Journal for Parasitology 46 (2016) 745–753 747
Parasitology (QP web, powered by R, Version 1.0.9) (Reiczigel et al.,
2013, http://www2.univet.hu/qpweb, last accessed 17/03/2016).
Similar to fish data, bird data were analysed for the correlation
between parasite prevalence and intensity. All analyses were con-
ducted in R (R Core Team, version 3.0.1: R: A Language and Envi-
ronment for Statistical Computing. Vienna, Austria: R Foundation
for Statistical Computing, http://www.R-project.org, last accessed
17/03/2016) and detailed information on all models and conditions
can be found in Supplementary Data S1.
3. Results
3.1. Cardiocephaloides longicollis hosts
In molluscs, the occurrence of C. longicollis was restricted to nas-
sariid snails, with only one new host record, Cyclope neritea, added
to the previously recognised host species, Nassarius corniculum and
N. reticulatus (Fig. 2). This indicates a high degree of host specificity
at this level. Prevalence was relatively low in C. neritea (0.65% in
the Ebro Delta, and one of two snails in Carboneras), whereas in
the more abundant N. reticulatus, the prevalence was higher with
8.3%, exceeding all previous records of C. longicollis in mollusc
hosts. Our data demonstrate that C. longicollis has a much wider
host spectrum in the second intermediate host than previously
reported, by adding 12 new host records: Diplodus sargus,D. dentex,
Spicara maena,Spondyliosoma cantharus,Pagellus acarne,Pagellus
erythrinus,Pagellus bogaraveo,Oblada melanura,Zosterisessor ophio-
cephalus,Coris julis,C. chromis,Serranus scriba. This includes the
Sparidae, Gobiidae and Labriidae as previously recorded host fam-
ilies and the Pomacentridae and Serranidae as new host families,
resulting in a total of 31 fish host species from nine fish families
(see Supplementary Table S2 and references therein). For the first
time, C. longicollis was detected in S. aurata from aquaculture net-
pen facilities, located in the southwestern Mediterranean region,
with 53.9% prevalence. Wild S. aurata from lagoons in Italy showed
similar infection prevalence (40–62.5%). With regard to final hosts,
the infection levels found in three larid species, i.e. within the gull
family, were high with P50% prevalence and thus up to 22 times
higher than in previous reports (Supplementary Table S3). How-
ever, C. longicollis was absent from terns and smaller larid species
in the present study, although these had been reported as hosts
in previous studies (see Supplementary Table S3).
Sequence identity of 28S rDNA sequences of C. longicollis with a
previously published record (GenBank accession number
AY222171) was 100% (bp compared: 1307 bp of D. dentex,
1127 bp of C. chromis, 437 bp of D. puntazzo, 607 bp of L. mormyrus,
1216 bp of N. reticulatus and 1097 bp of C. neritea). An ITS rDNA
sequence of adults was deposited in GenBank (accession number
KT454991).
Fig. 2. Complex life cycle of Cardiocephaloides longicollis including the main factors that impact parasite transmission, as revealed by the present study.
748 A. Born-Torrijos et al. / International Journal for Parasitology 46 (2016) 745–753
3.2. Effects of habitat on infection levels in fish hosts
Using different statistical models, relationships between C.
longicollis infection levels and biological as well as anthropogenic
factors were estimated. Both prevalence and intensity of infection
were used as indices for parasite quantity and we detected a strong
positive correlation between these two parameters across all sam-
ples in fish (Supplementary Fig. S2), irrespective of whether data
from the literature was included (r
s
= 0.69, P< 0.001, n= 51) or
not (r
s
= 0.76, P< 0.001, n= 40), thus reflecting the consistency
between indices across samples. When analysing this correlation
with regard to habitat type, we found that parasite prevalence
and intensity showed high variability in demersal fish, in contrast
to pelagic and benthic hosts, with a medium prevalence in pelagic
fishes and a high prevalence in benthic ones. Thus, overall, demer-
sal samples showed the highest and most diverse infection levels.
No significant differences in parasite prevalences between habi-
tats were found (GLMM, benthic P= 0.1959, demersal P= 0.2245
when compared with pelagic), possibly due to the heterogeneous
infection levels in demersal and pelagic habitats, and due to a sin-
gle benthic sample included in the GLMM, i.e. Z. ophiocephalus
(GLMM, proportion of variance explained by random effect ‘fish
species’ = 72%, Supplementary Table S4). However, demersal and
benthic habitats showed higher parasite prevalence and mean
abundance when ignoring the fish species effect (Table 1,Supple-
mentary Fig. S3), with no significant differences between these
two habitats. Including both new and previously published data,
the mean abundance was approximately four times greater in
demersal and approximately 23 times greater in benthic fish than
in pelagic fish (Table 1). The difference in prevalence, although not
so great, is still considerable, being 38.3% and 131.6% greater in the
demersal and benthic habitats, respectively, compared with the
pelagic one. Generally, pairwise comparisons were concordant
with linear model results.
3.3. Factors causing increased parasite occurrence in fish
Cardiocephaloides longicollis infection levels did not differ sub-
stantially between sites, when comparing data on the same fish
species captured at different sites with similar rates of fishing
activity (D. vulgaris,S. aurata and L. mormyrus;Supplementary
Table S2). Lithognathus mormyrus generally showed high infection
rates, however the maximum of 100% prevalence was reached only
in an area characterised by aquaculture netpens. Areas with inten-
sive fishing and aquaculture activities such as Santa Pola and Car-
boneras showed high mean intensities of infection (up to 26
metacercariae per fish). Overall, the highest prevalences of C. longi-
collis occurred in fish captured close to harbours frequented by ves-
sels with high fish hold capacities, being significantly higher than
in harbours with low and medium fish hold capacity (74.7% in both
cases; GLMM and pairwise comparisons, P< 0.001; Fig. 3A, Supple-
mentary Table S5), which show no significant differences from
each other (P= 0.995, Supplementary Table S5). Furthermore, shal-
low areas (lagoons) were identified as sites with high parasite
numbers in fish, with mean infection intensities of 19.8–49.9 in
the Italian lagoons. The highest C. longicollis infection rates were
detected in snails from the Ebro Delta lagoon (up to 8.3%) and were
found to be significantly higher in fish (40% to 100%) from lagoons
than in fish from other coastal areas (GLM, P= 0.014, Fig. 3B, Sup-
plementary Table S5).
3.4. Accumulation of C. longicollis metacercariae with host size
In D. vulgaris, both parasite intensity and abundance showed a
tendency to increase with host size, but reached their highest val-
ues in intermediate size classes 4–6 (120–148 mm), where the
highest numbers of metacercariae were recovered (average of
33–38 individuals per fish) (Fig. 4,Supplementary Table S6). Preva-
lence showed a steady increase with D. vulgaris size, however in
the two largest size classes, although prevalence was the highest
with 93–100% of fish infected, the abundance and the intensity
of C. longicollis decreased relative to medium-sized classes (Fig. 4,
Supplementary Table S6). The aggregation level, calculated by the
discrepancy index, was similar in all size classes, except for the lar-
gest size class where it decreased markedly (Supplementary
Table S6). In D. annularis, the mean intensity and abundance
showed the same tendency to increase with host size, with the
highest values obtained in the two largest size classes (140–
162 mm) where, once again, the aggregation of metacercariae
was the lowest (Fig. 4,Supplementary Table S7). Size class 8 con-
tained the highest number of metacercariae with an average of
73 parasites per fish brain. Infection prevalence showed a steep
increase, from 46.2 to 79.0%, in size classes 1–2. In the four largest
size classes it was greater than 95%, indicating that from approxi-
mately 120 mm total length, almost every fish is infected. The
highest mean abundance and mean intensity occurred in both spe-
cies at standard lengths of approximately 140–150 mm. The num-
ber of metacercariae in D. vulgaris was significantly higher in size
classes 3–8 compared with the lowest size class, i.e. 1 (P< 0.01),
and in D. annularis, higher in all other size classes compared with
size class 1 (P< 0.01) (negative binomial GLM).
Table 1
Results from statistical analyses on the fish habitat targeted by cercariae of Cardiocephaloides longicollis. Results evaluating the effect of the fish habitat on the response variable
(i.e. prevalence or mean abundance of C. longicollis), obtained with different datasets and not controlling for fish species. Results of linear models (LM) (Response variable fish
habitat) are indicated in italics, followed by the pairwise comparison results in square brackets. The response variables were arcsin-transformed. The percentage indicates the
increase of the response variable in the specified habitat, compared with the pelagic habitat. Only significant results (at
a
= 0.050) are presented.
Response variable
Prevalence Mean abundance
(a) Other
together with
present study
(i) True zero data
included (n= 86)
n.s. [n.s.] Demersal (t = 2.495, P = 0.015) 488.7%, benthic (t = 2.426,
P = 0.018) 731.4% [dem > pel; bent > pel](t = 2.495, P= 0.039;
t = 2.426, P= 0.046)
(ii) True zero data
not included
(n= 77)
Demersal (t = 2.559, P = 0.013) 38.3%[dem > pel](t = 2.559,
P= 0.033)
Demersal (t = 2.994, P = 0.004) 697.7%, benthic (t = 2.699,
P = 0.009) 906% [dem > pel; bent > pel](t = 2.994, P= 0.011;
t = 2.699,=0.024)
(b) Data present
study
(i) True zero data
included (n= 58)
Benthic (t = 2.154, P = 0.036) 74.5% [n.s. ] Demersal (t = 2.318, P = 0.024) 407.1%, benthic (t = 3.176,
P = 0.002) 1641.4%[dem > pel](t = 3.176, P= 0.007)
(ii) True zero data
not included
(n= 52)
Demersal (t = 3.314, P = 0.002) 55.8%, benthic (t = 3.485,
P = 0.001) 131.6%[dem > pel; bent > pel](t = 3.314, P= 0.005;
t = 3.485, P= 0.003)
Demersal (t = 2.946, P = 0.005) 618%, benthic (t = 3.828,
P < 0.001) 2373.3%[dem > pel; bent > pel](t = 2.946, P= 0.013;
t = 3.828, P= 0.001)
Bent, benthic; Dem, demersal; Pelag, pelagic.
A. Born-Torrijos et al. / International Journal for Parasitology 46 (2016) 745–753 749
3.5. Bird infection data
Among the bird species examined in this study, only larids were
infected with C. longicollis, showing 27–69% prevalence. The spe-
cies L. michahellis,L. argentatus and L. audouinii showed much
higher prevalences than previously reported, with 69.2%, 66.7%
and 50% prevalence of infection, respectively (Supplementary
Table S3). Chroicocephalus ridibundus and terns (19 specimens)
were not infected, although six previous records exist for these
hosts (see Supplementary Table S3 for references). Our data,
together with that in the published literature, showed no signifi-
cant relationship between prevalence and mean intensity across
samples (P= 0.1302), possibly due to the low number of samples
(n= 12).
4. Discussion
Parasites, and more so generalists with a complex life cycle, can
have strong impacts on ecosystems and food webs (Thompson
et al., 2005), especially if their biomass is substantial such as that
of trematodes in intertidal areas and estuaries (Kuris et al.,
2008). In the present study, we investigated the distribution, life
cycle strategies and successful transmission dynamics of the
multi-host generalist parasite C. longicollis and we were able to
demonstrate that shallow waters as well as fishing discards
strongly impact its life cycle, leading to a considerable increase
in parasite numbers in the ecosystem (results summarised in
Fig. 2).
We determined that C. longicollis has an extremely wide host
spectrum at the level of the second intermediate host, as it infects
a wide range of phylogenetically distantly related fish species (see
Supplementary Table S2 and references therein). Furthermore, we
show that the mean abundance of C. longicollis in demersal and
benthic fish species is four and 23 times higher, respectively, than
in pelagic fish; thus cercariae appear to be more specific to the
host’s environment than to the host itself (Combes et al., 1994).
The strong preference for the benthic habitat demonstrated in
our study is possibly biased as only two benthic fish species were
found to be infected in our analysis (only true zero values were
included, see Section 2.4), i.e. Z. ophiocephalus (75.6% prevalence)
and S. scriba (75%). Previous reports generally showed extremely
low infection levels in bottom-dwelling fish (prevalences <2.9%,
Supplementary Table S2), with the exception of Neogobius melanos-
tomus (58%) and Zoarces viviparus (11%). We believe that these spe-
cies, and probably some others, act as biological sinks for C.
longicollis as they are unlikely prey items for birds, thus preventing
substantial parasite numbers from completing their life cycle. The
only way these fish are made available to seabirds is as discards of
Fig. 3. Impact of fisheries and water depth on Cardiocephaloides longicollis numbers in fish. Parasite prevalence in relation to (A) fishing activity in the sampling area,
estimated on the basis of the hold capacity of vessels from the nearest harbour and measured in gross tonnage (three categories), and to (B) habitat type, i.e. shallow water
(lagoons) and other coastal areas (non-lagoons). Data obtained for the present study and true zeros are included. Statistically significant differences between levels (at
a
= 0.050) are indicated with asterisks. The bottom and top of the box show the first and third quartiles and the whiskers indicate the maximum and the minimum values.
Fig. 4. Accumulation of Cardiocephaloides longicollis metacercariae with increasing
fish size. The degree of aggregation of the metacercariae was calculated for each
host size class as the index of discrepancy (D index). The number of metacercariae
per size class of (A) Diplodus vulgaris (size class, standard length (mm)) (1, 66–98;
2,100–110; 3,111–119; 4,120–129; 5,130–139; 6, 140–148; 7,151–159; 8,160–220)
and (B) Diplodus annularis (1,70–79; 2,80–89; 3, 90–99; 4, 100–109; 5, 100–119; 6,
120–129; 7, 130–139; 8, 140–149; 9, 150–162) are shown. Lines represent infection
prevalence. The bottom and top of the box show the first and third quartiles, the
circles show the outliers and the whiskers indicate the maximum and the minimum
values after excluding the outliers.
750 A. Born-Torrijos et al. / International Journal for Parasitology 46 (2016) 745–753
trawling fisheries (e.g. Oro and Ruiz, 1997; Massutí and Reñones,
2005). Overall, demersal fish clearly showed the highest infection
levels, reaching 100% prevalence in D. vulgaris and L. mormyrus,
and a maximum of 220 parasites in a single brain (D. annularis).
Thus, the swimming behaviour of the cercariae increases their
exposure to demersal fish, as previously hypothesised by Combes
et al. (1994) and Osset et al. (2005), and thus pays off as the bulk
of infected fish species come to visit surface waters and enable
trophic transmission to bird hosts.
In D. annularis and D. vulgaris, an accumulative effect of metac-
ercariae with host size was observed, similar to other trematode
species (e.g. Anderson and Gordon, 1982; Thomas et al., 1995).
The highest abundance and intensity occur in hosts approximately
140–150 mm standard length, a smaller size than previously deter-
mined (Osset et al., 2005). Differences in the size-related aggrega-
tion of metacercariae between different hosts can be explained by
the different spatial distribution of these hosts and thus different
exposure to cercariae (Poulin, 2001). However, parasite aggrega-
tion decreases in the largest size classes of both species, which is
consistent with parasite-induced host mortality, i.e. heavily
infected individuals are removed from the population either by
death or by being preyed on more intensively (Anderson and
Gordon, 1982; Poulin, 2001). The accumulation of metacercariae
leads to decreasing visual capacity and increasing loss of motor
control and subsequent surface ‘‘flashing” (Osset et al., 2005;
Bartoli and Boudouresque, 2007), which may result in the removal
of the fish with the highest infection levels by predatory seabirds,
hence lowering average infection levels in the remaining fish that
survive and grow into a larger size class.
In the present study, infection rates in snails and definitive bird
hosts were much higher than previously reported and we were
able to relate enhanced parasite transmission to two important
factors: (i) close proximity of all hosts used by C. longicollis during
its life cycle and (ii) the strong impact of fisheries on trophic trans-
mission to definitive bird hosts.
Despite a few parasitological studies on gull species (references
in Supplementary Table S2), information on C. longicollis distribu-
tion in final bird hosts is scarce. Following encounter and host
compatibility/suitability filters (Combes, 2001), differences in bird
feeding habits may explain why L. michahellis in the present study
showed much higher parasite loads than Larus audouinii. The for-
mer is larger and consumes mostly fish of 150–250+ mm body
length (versus <50–140 mm in L. audouinii,Oro and Ruiz, 1997;
Arcos et al., 2001). Larus michahellis’ prey size range encompasses
the highest abundance and intensity of C. longicollis infection in
D. vulgaris and D. annularis, with 100% infection prevalence in the
latter species, which maximises parasite transmission. In contrast,
smaller bird hosts such as Chroicocephalus ridibundus or terns
include more than 50% small fish in their diet (20–100 mm, Oro
and Ruiz, 1997), which transmit lower parasite numbers, thus
resulting in lower infection levels or no infection, as in the present
study. Moreover, L. audouinii and L. michahellis exhibit behavioural
plasticity depending on food availability, both showing a clear ten-
dency to use food resources of anthropogenic origin (Witt et al.,
1981; Bartoli, 1989; Ramos et al., 2009), often fishery discards
(Arcos et al., 2001; Christel et al., 2012, and references therein).
Approximately 39–48% of the diet of adult breeding Audouin’s
gulls comes from discarded demersal fish (Navarro et al., 2010),
hence supporting discards as a source of easily available food for
opportunistic scavengers (e.g. Oro and Ruiz, 1997; Sánchez et al.,
2004). The highest infection levels in L. michahellis (prevalences
of 61.5 and 69.2%, Supplementary Table S3) were detected in areas
of the Mediterranean region characterised by intensive fishery
activities, with parasite levels 22 times higher than previously
reported and in clear contrast to areas with low fishing activity
(prevalence of 10.4% in the same species). Bycatch constitutes an
important but highly variable fraction of the total catch (13–67%
in bottom trawl fisheries in the western Mediterranean region;
Tudela, 2004)(Sánchez et al., 2004; Tzanatos et al., 2007). In the
Ebro Delta, which has an important commercial fishery, discards
are estimated to be approximately 41% of landed fish (Oro and
Ruiz, 1997; Coll et al., 2008). Discarded fish species and sizes vary
somewhat (Oro and Ruiz, 1997; Machias et al., 2004; Sánchez et al.,
2004; Tzanatos et al., 2007), but clearly overlap with those show-
ing the highest C. longicollis infection rates (see Supplementary
Table S2). A link between increased transmission and fishery activ-
ities thus becomes obvious, and was confirmed by prevalences of C.
longicollis being approximately 75% higher in fish captured close to
harbours with high fish hold capacity compared with harbours of
medium or low fish hold capacity (Fig. 3A). As explained earlier,
no official statistics exist on discards, with the only exception
being individual local studies that give examples of the parasite
biomass that is discarded into the sea. We inferred the amount
of discard-associated C. longicollis metacercariae, combining dis-
card data in a heavily fished area that overlaps geographically with
our dataset, i.e. Santa Pola (García-Rivera et al., 2015), by analysing
the four most discarded fish hosts in this area, i.e. Boops boops,Pag-
ellus acarne,Pagellus erythrinus and Diplodus annularis. The number
of C. longicollis metacercariae discarded at sea near Santa Pola
every year is estimated to be over 100,000 individuals, i.e. eight
times higher than all metacercariae collected in this study.
We can furthermore infer a strong enhancing effect of shallow
water areas on the infection rates of C. longicollis at all host levels.
Coincident with the highest prevalences of C. longicollis in fish (40–
100%) from lagoons, the cercarial prevalence in N. reticulatus (8.3%)
also peaked in the Ebro Delta lagoon, exceeding all previous
records (Supplementary Table S1). Confined habitats are most
favourable for C. longicollis as snail and fish hosts are concentrated
in a small water volume, facilitating the parasite’s transmission.
Fish showed significantly higher infection levels in lagoons than
in other coastal areas, however, fish are mobile and although cap-
tured in non-lagoon areas, they may have visited shallow water
habitats temporarily, hence explaining the large range of infection
levels observed outside lagoons (Fig. 3B). Given their sessile nature,
snails are possibly the best indicator for locally variable parasite
numbers. Due to insufficient infection data in non-lagoon areas,
statistical comparison of snail data from different habitats was
not possible; however, this should be included in future studies
to confirm our preliminary results. Similar to lagoons, aquaculture
installations may act as enhancers of C. longicollis transmission. The
organic enrichment in areas close to farmed facilities and frequent
mortalities in sea cages attract scavengers such as gastropods (e.g.
nassariid snails, Morton and Yuen, 2000; Edgar et al., 2005) and
piscivorous birds (Oro and Ruiz, 1997; Christel et al., 2012; see
above). To the best of our knowledge, we report for the first time,
the presence of metacercariae of C. longicollis in S. aurata cultured
in netpens. High parasite prevalence in these installations (53.9%)
was similar to that in wild S. aurata from Italian lagoons, where
parasite transmission is increased. The environmental impact of
increasing fishery and aquaculture activities in the Mediterranean
region makes it essential to study their effects on parasites
since they affect a wide range of cultured and wild host species
and play an important role in food webs (Lafferty et al., 2008).
We expect that, in areas of enhanced parasite transmission,
trophically-transmitted generalist parasites with a complex life
cycle and efficient host-finding and manipulation mechanisms will
be able to substantially increase overall abundance. As a result,
such parasites are expected to considerably alter the energy flow
through the food web. Cardiocephaloides longicollis presents itself
as an ideal future bioindicator to compare food web structures in
natural communities with those impacted by fisheries and
aquaculture.
A. Born-Torrijos et al. / International Journal for Parasitology 46 (2016) 745–753 751
Acknowledgments
This research was supported by projects Prometeo/2015/018,
AGL2015-68405-R (MINECO/FEDER, UE) and Revidpaqua
ISIC/2012/003 of the Valencian Regional Government (JAR, APO),
Fundación Biodiversidad 2015 (APO) and by the European Centre
of Ichtyoparasitology (Czech Science Foundation, grant 505/12/
G112, Center of Excellence) (ASH). AB-T is the recipient of a doc-
toral fellowship (Ministry of Education and Science, Spain; grant
AP2009–2560) and a visiting studentship to the Department of
Zoology of the University of Otago in New Zealand (Ministry of
Education, Culture and Sports, Spain; grant EST12/00528). The
authors gratefully acknowledge Gabrielle S. van Beest and Lorenzo
Miquel-Mazzetti (University of Valencia, Spain) for their help with
fish examination, and the personnel of the Natural Park ‘‘Sierra
Helada” (Generalitat Valenciana) and the birds sanctuaries ‘‘Centre
de Recuperació de Fauna Salvatge de Torreferrussa” (Generalitat de
Catalunya) and ‘‘Centre de Recuperació La Granja” (Generalitat
Valenciana) (Spain). We would also like to thank anonymous refer-
ees for their helpful comments on the manuscript.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ijpara.2016.06.
005.
References
Anderson, R.M., Gordon, D.M., 1982. Processes influencing the distribution of
parasite numbers within host populations with special emphasis on parasite-
induced host mortalities. Parasitology 85, 373–398.
Arcos, J.M., Oro, D., Sol, D., 2001. Competition between the yellow-legged gull Larus
cachinnans and Audouin’s gull Larus audouinii associated with commercial
fishing vessels: the influence of season and fishing fleet. Mar. Biol. 139, 807–
816.
Barber, I., Crompton, D.W.T., 1997. The distribution of Diplostomum phoxini
metacercariae (Trematoda) in the brain of minnows (Phoxinus phoxinus). Folia
Parasites 44, 19–25.
Bartoli, P., 1989. Les trematodes digénétiques, parasites marqueurs de la biologie
des Goélands leucophées Larus cachinnans michaellis en Corse (Mediterranée
occidentale). Vie Marine, Marseille 10, 17–26.
Bartoli, P., Boudouresque, C.F., 2007. Effect of the digenean parasites of fish on the
fauna of Mediterranean lagoons. Parassitologia 49, 111–117.
Bartoli, P., Prévot, G., 1986. Stratégies d’infestation des hotes cibles chez les
trématodes marins parasites de Larus cachinnans michaellis de Provence. Ann.
Parasitol. Hum. Comp. 61, 533–552.
Born-Torrijos, A., Kostadinova, A., Raga, J.A., Holzer, A.S., 2012. Molecular and
morphological identification of larval opecoelids (Digenea: Opecoelidae)
parasitising prosobranch snails in a Western Mediterranean lagoon. Parasitol.
Int. 61, 450–460.
Born-Torrijos, A., Poulin, R., Raga, J.A., Holzer, A.S., 2014. Estimating trematode
prevalence in snail hosts using a single-step duplex PCR: how badly does
cercarial shedding underestimate infection rates? Parasites Vectors 7, 243.
Bozzano, A., Sardà, F., 2002. Fishery discard consumption rate and scavenging
activity in the northwestern Mediterranean Sea. ICES J. Mar. Sci. 59, 15–28.
Bush, A.O., Lafferty, K.D., Lotz, J.M., Shostak, A.W., et al., 1997. Parasitology meets
ecology in its own terms: Margolis et al.revisited. J. Parasitol. 83, 575–583.
Christel, I., Navarro, J., del Castillo, M., Cama, A., Ferrer, X., 2012. Foraging
movements of Audouin’s gull (Larus audouinii) in the Ebro Delta, NW
Mediterranean: A preliminary satellite-tracking study. Estuarine Coastal Shelf
Sci. 96, 257–261.
Coll, M., Palomera, I., Tudela, S., 2008. Decadal changes in a NW Mediterranean sea
food web in relation to fishing exploitation. Ecol. Model. 220, 2088–2102.
Coll, M., Carreras, M., Cornax, M.J., Massutí, E., Morote, E., Pastor, X., Quetglas, A.,
Sáez, R., Silva, L., Sobrino, I., Torres, M.A., Tudela, S., Harper, S., Zeller, D., Pauly,
D., 2014. Closer to reality: Reconstructing total removals in mixed fisheries
from Southern Europe. Fish. Res. 154, 179–194.
Combes, C., 2001. Parasitism: The Ecology and Evolution of Intimate Interactions.
University of Chicago Press, Chicago, USA and London, UK.
Combes, C., Fournier, A., Moné, H., Théron, A., 1994. Behaviours in trematode
cercariae that enhance parasite transmission: patterns and processes.
Parasitology 109 (Suppl.), S3–S13.
Cribb, T.H., Anderson, G.R., Adlard, R.D., Bray, R.A., 1998. A DNA-based
demonstration of a three-host life-cycle for the Bivesiculidae
(Platyhelminthes: Digenea). Int. J. Parasitol. 28, 1791–1795.
Dick, J.T.A., Armstrong, M., Clarke, H.C., Farnsworth, K.D., Hatcher, M.J., Ennis, M.,
Kelly, A., Dunn, A.M., 2010. Parasitism may enhance rather than reduce the
predatory impact of an invader. Biol. Lett. 6, 636–638.
Dubois, G., 1968. Synospis des Strigeidae et des Diplostomatidae (Trematoda). Mem.
Soc. R. Sci. 10, 1–258.
Edgar, G.J., Macleod, C.K., Mawbey, R.B., Shields, D., 2005. Broad-scale effects of
marine salmonid aquaculture on macrobenthos and the sediment environment
in southeastern Tasmania. J. Exp. Mar. Biol. Ecol. 327, 70–90.
Esch, G.W., Fernandez, J., 1994. Snail–trematode interactions and parasite
community dynamics in aquatic systems: A review. Am. Midland Nat. 131,
209–237.
Fredensborg, B.L., Longoria, A.N., 2012. Increased surfacing behavior in longnose
killifish infected by brain-encysting trematode. J. Parasitol. 98, 899–903.
Fredensborg, B.L., Mouritsen, K.N., Poulin, R., 2006. Relating bird host distribution
and spatial heterogeneity in trematode infections in an intertidal snail—from
small to large scale. Mar. Biol. 149, 275–283.
García-Rivera, S., Sánchez Lizaso, J.L., Bellido Millán, J.M., 2015. A quantitative and
qualitative assessment of the discard ban in European Mediterranean waters.
Mar. Policy 53, 149–158.
Grigorakis, K., Rigos, G., 2011. Aquaculture effects on environmental and public
welfare – The case of Mediterranean mariculture. Chemosphere 855,
899–919.
Kuris, A.M., Hechinger, R.F., Shaw, J.C., Whitney, K.L., Aguirre-Macedo, L., Boch, C.A.,
Dobson, A.P., Dunham, E.J., Fredensborg, B.L., Huspeni, T.C., Lorda, J., Mababa, L.,
Mancini, F.T., Mora, A.B., Pickering, M., Talhouk, N.L., Torchin, M.E., Lafferty, K.D.,
2008. Ecosystem energetic implications of parasite and free-living biomass in
three estuaries. Nature 54, 515–518.
Lafferty, K.D., Dobson, A.P., Kuris, A.M., 2006. Parasites dominate food web links.
Proc. Natl. Acad. Sci. U.S.A. 103, 11211–11216.
Lafferty, K.D., Allesina, S., Arim, M., Briggs, C.J., De Leo, G.A., Dobson, A.P., Dunne, J.A.,
Johnson, P.T.J., Kuris, A.M., Marcogliese, D.J., Martinez, N.D., Memmott, J.,
Marquet, P.A., McLaughlin, J.P., Mordecai, E.A., Pascual, M., Poulin, R., Thieltges,
D.W., 2008. Parasites in food webs: the ultimate missing links. Ecol. Lett. 11,
533–546.
Lagrue, C., Kelly, D.W., Hicks, A., Poulin, R., 2011. Factors influencing infection
patterns of trophically transmitted parasites among a fish community: host
diet, host–parasite compatibility or both? J. Fish Biol. 79, 466–485.
Machias, A., Maiorano, P., Vassilopoulou, V., Papaconstantinou, C., Tursi, A.,
Tsimenides, N., 2004. Sizes of discarded commercial species in the eastern–
central Mediterranean Sea. Fish. Res. 66, 213–222.
Marcogliese, D.J., 2004. Parasites: small players with crucial roles in the ecological
theater. Ecohealth 1, 151–164.
Massutí, E., Reñones, O., 2005. Demersal resource assemblages in the trawl fishing
grounds off the Balearic Islands (western Mediterranean). Sci. Mar. 69, 167–
181.
Morgan, J.A., Blair, D., 1995. Nuclear rDNA ITS sequence variation in the trematode
genus Echinostoma: an aid to establishing relationships within the 37-collar-
spine group. Parasitology 111, 609–615.
Morton, B., Yuen, W.Y., 2000. The feeding behaviour and competition for carrion
between two sympatric scavengers on a sandy shore in Hong Kong: the
gastropod, Nassarius festivus (Powys) and the hermit crab, Diogenes edwarsii (De
Haan). J. Exp. Mar. Biol. Ecol. 246, 1–29.
Navarro, J., Oro, D., Bertolero, A., Genovart, M., Delgado, A., Forero, M.G., 2010. Age
and sexual differences in the exploitation of two anthropogenic food resources
for an opportunistic seabird. Mar. Biol. 157, 2453–2459.
Oro, D., Ruiz, X., 1997. Exploitation of trawler discards by breeding seabirds in the
north-western Mediterranean: differences between the Ebro Delta and the
Balearic Islands areas. ICES J. Mar. Sci. 54, 695–707.
Osset, E.A., Fernández, M., Raga, J.A., Kostadinova, A., 2005. Mediterranean Diplodus
annularis (Teleostei: Sparidae) and its brain parasite: Unforeseen outcome.
Parasitol. Int. 54, 201–206.
Pauly, D., Ulman, A., Piroddi, C., Bultel, E., Coll, M., 2014. ‘Reported’ versus ‘likely’
fisheries catches of four Mediterranean countries. In: Lleonart J., Maynou F.
(eds), The Ecosystem Approach to Fisheries in the Mediterranean and Black
Seas. Sci. Mar. 78S1, 11–17.
Poulin, R., 1993. The disparity between observed and uniform distributions: a new
look at parasite aggregation. Int. J. Parasitol. 23, 937–944.
Poulin, R., 2001. Interactions between species and the structure of helminth
communities. Parasitology 122 (Suppl.), S3–S11.
Prévot, G., Bartoli, P., 1980. Démonstration de l’existence d’un cycle marin chez les
Strigeides: Cardiocephalus longicollis Szidat, 1928 (Trematoda: Strigeidae). Ann.
Parasitol. Hum. Comp.55, 407–425.
Ramos, R., Ramírez, F., Sanpera, C., Jover, L., Ruiz, X., 2009. Diet of Yellow-legged Gull
(Larus michahellis) chicks along the Spanish Western Mediterranean coast: the
relevance of refuse dumps. J. Ornithol. 150, 265–272.
Sánchez, P., Demestre, M., Martín, P., 2004. Characterisation of the discards
generated by bottom trawling in the northwestern Mediterranean. Fish. Res.
67, 71–80.
Seppälä, O., Karvonen, A., Valtonen, E.T., 2004. Parasite-induced change in host
behaviour and susceptibility to predation in an eye fluke-fish interaction. Anim.
Behav. 68, 257–263.
752 A. Born-Torrijos et al. / International Journal for Parasitology 46 (2016) 745–753
Tasker, M.L., Camphuysen, C.J., Cooper, J., Garthe, S., Montevecchi, W.A., Blaber, S.J.
M., 2000. The impacts of fishing on marine birds. ICES J. Mar. Sci. 57,
531–547.
Thomas, F., Renaud, F., Rousset, F., Cezilly, F., De Meeûs, T., 1995. Differential
mortality of two closely related host species induced by one parasite. Proc. R.
Soc. London, Ser. B 260, 349–352.
Thompson, R.M., Mouritsen, K.N., Poulin, R., 2005. Importance of parasites and their
life cycle characteristics in determining the structure of a large marine food
web. J. Anim. Ecol. 74, 77–85.
Tudela, S., 2004. Ecosystem effects of fishing in the Mediterranean: an analysis of
the major threats of fishing gear and practices to biodiversity and marine
habitats. In: Studies and Reviews. General Fisheries Commission for the
Mediterranean, No. 74. Food and Agriculture Organization of the United
Nations (FAO), Rome, Italy.
Tzanatos, E., Somarakis, S., Tserpes, G., Koutsikopoulos, C., 2007. Discarding
practices in a Mediterranean small-scale fishing fleet (Patraikos Gulf, Greece).
Fish. Manage. Ecol. 14, 277–285.
Witt, H.H., Crespo, J., De Juana, E., Varela, J., 1981. Comparative feeding ecology of
audouin’s gull Larus audouinii and the herring gull L. argentatus in the
Mediterranean. Ibis 4, 519–526.
Wood, C.L., Lafferty, K.D., 2015. How have fisheries affected parasite communities?
Parasitology 142, 134–144.
Wood, C.L., Lafferty, K.D., Micheli, F., 2010. Fishing out marine parasites? Impacts of
fishing on rates of parasitism in the ocean. Ecol. Lett. 13, 761–775.
A. Born-Torrijos et al. / International Journal for Parasitology 46 (2016) 745–753 753
