부산대학교 도서관

Cetaceans harbour a relatively diverse and highly specific fauna of trophicallytransmitted metazoan parasites including digeneans, cestodes, nematodes and acanthocephalans (Aznar et al., 2001; Raga et al., 2009). Due to the origin of host-parasite associations (see Section 1.1.2) and the phylogenetic distance between cetaceans and other marine vertebrates, most helminth species found in cetaceans are specific to them. Therefore, the composition and species richness of helminth faunas from any cetacean species in any locality (i.e., the helminth component community) is expected to be restricted by two factors. First, the local diversity of helminth species would largely result from an interplay between regional history and local ecology (Holmes et al., 1990). Second, cetaceans would behave as an isolated group for parasitic exchange, which would not share helminth species with other marine vertebrates and, therefore, the local community of cetacean species would be the most relevant factor setting the upper limits of diversity and composition of their helminth faunas (Aznar et al., 1998). At the level of individual hosts (infracommunity level), species richness and composition would primarily depend on the probability of encounter between hosts and parasites. It has been suggested that the infective stages of helminths in the marine realm are highly “diluted” (Hoberg, 1996, 2005; Raga et al., 2009; Valente et al., 2009; Santoro et al., 2010), thus the likelihood of contact between cetaceans and infected prey would be low. This could bring about two consequences. First, helminth infracommunities should be depauperate compared with the richness observed at component community; second, infracommunities should constitute random subsets of the pool of species of the component community. These predictions about the structure and composition of helminth communities of cetaceans have been tested only in a few species (Raga et al., 2009) and, therefore, much more data are required from other systems to confirm them. On the other hand, the peculiarities associated with trophically-transmitted parasites also make them ideal candidates to monitor not only trophic interactions, but also long-term changes in the ecosystem. However, the use of helminths as biological indicators of global change is far from understood and requires further investigation. To our knowledge, long-term (i.e., decadal) parasitological studies have never been carried out in the case of cetaceans due to the difficulties to obtain appropriate host sample sizes. Certainly, there are surveys collating parasitological data for long periods (e.g., Carvalho et al., 2010), but data have never analyzed using an explicit framework to analyze temporal changes. A key factor to interpret factors that drive structure and temporal dynamics of helminth communities is a good knowledge of life cycles. However, very little is known about the life-cycles of helminths from cetaceans due to the obvious difficulties associated with working on hosts living in the marine realm, especially those inhabiting oceanic waters (Aznar et al., 2001; Raga et al., 2009). In general, there are major gaps about the identity of intermediate and paratenic hosts (if any) of the taxa infecting cetaceans. In fact, the complete life-cycle of trophically-transmitted cetacean helminths has only been elucidated for a few species, i.e. Anisakis spp. The striped dolphin, Stenella coeruleoalba, is the most abundant cetacean in central Spanish Mediterranean waters (Gómez de Segura et al., 2006). Parasitological research on this species is scarce and based on small host sample sizes. The marine mammal stranding network of the Marine Zoology Unit, University of Valencia, has been collecting data from this species from western Mediterranean waters since the late 1980s. Animals were found stranded dead or alive along the coasts of the Valencian Community, although in 1990 additional samples were available from Catalonia and Murcia. This has provided an invaluable set of long-term parasitological data which makes the striped dolphin a good candidate to investigate the factors that determine the structure and composition of helminth communities from the digestive tract, to explore long-term changes in their structure and to elucidate the life cycles of its helminth species. The following objectives have been addressed: 1. To describe the helminth fauna infecting the digestive tract of the striped dolphin, Stenella coeruleoalba, from the western Mediterranean. 2. To investigate the factors which determine the structure and composition, including specificity, of the helminth communities. 3. To explore long-term changes in the structure of helminth communities. 4. To search cetacean helminth larvae down the trophic web aiming to elucidate lifecycles and routes of infection. The intestinal helminth community of the striped dolphin in western Mediterranean waters was investigated based on a sample of 52 animals stranded along the coasts of Catalonia, Valencian Community and Murcia in 1990. The community was composed of three species of tetrabothriid cestode, namely, Tetrabothrius forsteri, Trigonocotyle globicephalae and Strobilocephalus triangularis, and immature individuals of the acanthocephalan Bolbosoma vasculosum. All these species are specific to cetaceans but B. vasculosum probably reproduces in other cetaceans, perhaps sympatric fin whales. Infection levels were low except for T. forsteri. At infracommunity level, cooccurrence of helminth species exhibited just a slight excess of positive associations between species. Overall, these data could be interpreted as evidence that some helminth species (most likely tetrabothriid species) use the same intermediate/paratenic hosts, or simply as evidence that some species tend to co-occur simply because one of them (T. forsteri) is highly prevalent. Significant effects of host body length, age or sex on the abundance of any of the helminth species, or on infracommunity descriptors (total abundance, species richness and Brillouin’s diversity index) were not detected. Note, however, that the host sample was mostly composed of adult animals and, therefore, ontogenetic effects on infection levels cannot be ruled out (see also below). Overall, the observation of depauperate, largely unpredictable helminth infracommunities agrees with the hypothesis that large and vagile oceanic predators have few contacts with infective stages of parasites. Two species of the family Brachycladiidae, namely, Brachycladium atlanticum and Oschmarinella rochebruni, were found infecting the hepato-pancreatic ducts of striped dolphins (n= 103). A parasitological survey of four additional sympatric cetacean species, i.e., Risso’s dolphin (n= 18), bottlenose dolphin (n= 14), common dolphin (n= 8) and long-finned pilot whale (n= 5) indicated that O. rochebruni was restricted to the striped dolphin, whereas B. atlanticum was found in both striped dolphins and common dolphins. Infection parameters were not significantly different between both dolphin species. In the striped dolphin, host individual exerted significant effects upon the morphology and fecundity of B. atlanticum but these effects did not result from densitydependence (i.e., crowding effects). However, a question that would deserve further investigation is to ascertain the proportion of worms from the same individual dolphin that is genetically identical. Digeneans multiply asexually within the first intermediate (mollusc) host and, therefore, worms might be recruited as clonal packets through the food web up to the cetacean definitive host. If so, intrahost variability in individual cetaceans might be lower than interhost variability, such as we observed. Interhost variability of morphological traits related to host individual had previously been reported in other brachycladiids. Our study therefore support the claim by other authors that diagnosis at the species level based on morphometry or morphometrical ratios might be prone to error. Individuals of B. atlanticum collected from common dolphins had a significantly smaller body size and fecundity than those from striped dolphins. Our analyses also suggested that the effects upon fecundity were a mere consequence of having a smaller body size. Brachycladium atlanticum is therefore able to reproduce in common dolphins but its fitness is arguably lower than that in the striped dolphin. In summary, specificity of O. rochebruni for striped dolphins could be defined by a contact filter, whereas that of B. atlanticum could result from a combination of both contact and compatibility filters. The gastric helminth fauna of the striped dolphin in western Mediterranean waters was composed of one digenean species, Pholeter gastrophilus, and one nematode species, Anisakis pegreffii. All stomach chambers were infected with P. gastrophilus, but the bulk of fibrotic nodules was found in the fundic stomach. The number of nodules of P. gastrophilus did not correlate significantly with the length or age of the dolphins, and there were not significant differences between sexes. The identity of A. pegreffii was ascertained based on molecular analysis of both larval and adult individuals. Most anisakid individuals were collected in the pyloric stomach, and some in the forestomach; no worms were detected within the fundic stomach. Anisakis pegreffii showed low infection levels in Mediterranean striped dolphins compared with infection of Anisakis spp. in other cetaceans. As noted in Chapter 4, the probability of transmission is especially low for trophically-transmitted helmiths in the oceanic realm because infective stages are highly “diluted” (Valente et al., 2009 and Santoro et al., 2010 for marine turtles; Hoberg, 1996, 2005 for marine birds; and Raga et al., 2009 for marine mammals). Mateu et al. (2015) reported a prevalence of 1.4% of A. pegreffii in N. elongatus (see Chapter 8), and the present study could support the idea that, in western Mediterranean waters, striped dolphins become infected with A. pegreffii by the ingestion of, inter alia, parasitized myctophid species. During the period 1990-2010, two events could have impacted recruitment of trophically-transmitted helminths of striped dolphins. First, the western Mediterranean population of striped dolphin suffered two outbreaks of mortality (in 1990 and 2007) caused by the dolphin morbillivirus (Raga et al. 2008). Although the total number of individuals killed could not be calculated, Aguilar and Raga (1993) suggested that several thousands individuals could have died. Second, there is correlational evidence that overfishing of sardine, Sardina pilchardus, one of the putative main prey species of striped dolphins in the area, may have caused a significant dietary shift towards demersal prey, particularly hake, Merluccius merluccius (Gómez-Campos et al., 2011). Therefore, we investigated the long-term dynamics of the intestinal helminth community of striped dolphin based on a sample of 128 animals collected during the period 1990-2010. Two sets of data were defined: (i) dolphins which died from DMV outbreaks during 1990 and 2007 (Epizootic sample) (n= 66), and (ii) dolphins which stranded from unknown causes during other years (Non-epizootic sample) (n= 62). In addition, the BIO-ENV procedure of PRIMER (Clarke and Gorley, 2006) was used to explore whether there was a significant relationship between the structure of the intestinal helminth community and the diet of the striped dolphin. This analysis was carried out based on a sample of 71 dolphins from which both parasitological and dietary data were available. Out of the 128 dolphins examined, only 4 were uninfected. The very same 4 helminth species, i.e. Tetrabothrius forsteri, Trigonocotyle globicephalae, Strobilocephalus triangularis and Bolbosoma vasculosum were found in this enlarged sample of striped dolphin (see Chapter 4). This strongly suggests that specificity likely prevents the contact and/or establishment of other helminths (Mateu et al., 2011), i.e., there is a ‘pool exhaustion’ of all potential local immigrants to the community. A PERMANOVA analysis indicated that there were significant differences in the structure of the helminth community between the ‘epizootic’ samples. In particular, B. vasculosum was fairly frequent in 1990 (prevalence: 51.9%) and did not appear in 2007; differences in abundance were highly significant. In contrast, differences in community structure were not significant in the comparison between non-epizootic samples. Furthermore, we did not find a significant relationship between diet and community structure. Thus, the question remains whether we are unable to detect clear long-term changes in the helminth fauna of S. coeruleoalba because we analysed only heavily parasitized animals (i.e., a non-random sample) or because there were really no changes to be detected. In any event, it seems clear that infection levels of B. vasculosum were significantly higher in 1990. There are no specific changes in diet to blame because the identity of potential paratenic hosts for B. vasculosum could not be ascertained through dietary analysis. Whether 1990 was an exceptional year is therefore an open question. According to the above results, the description of larval stages of helminths infecting striped dolphins and the identification of intermediate and/or paratenic hosts of these parasites became an important task to assist interpretation of patterns in definitive hosts. Thus, we selected species reported as important prey for striped dolphins and other oceanic cetacean species, i.e. mesopelagic fish and cephalopods, and examined them for infective stages of cetacean helminths. In particular, we analyzed 1012 individuals of eight myctophid species (Ceratoscopelus maderensis, Lampanyctus crocodilus, Notoscopelus elongatus, Benthosema glaciale, Myctophum punctatum, Lobianchia dofleini, Diaphus holti and Hygophum benoiti) and 792 individuals of two cephalopod species (Alloteuthis media and Sepietta oweniana). Hosts were collected during 2010-2012 from the Gulf of Valencia and Alboran Sea (Spanish Mediterranean), which include localities proposed as Protected Areas of Mediterranean Importance due to the high cetacean diversity and abundance. Five helminth taxa were found in myctophiids, and none in cephalopods. Only the nematodes Anisakis pegreffii and A. physeteris were identified as larvae of species infecting cetaceans, and were found only in N. elongatus and C. maderensis with very low prevalence (overall prevalence for Anisakis: 8.1% and 0.5%, respectively). Their prevalence in N. elongatus was significantly higher than that from the other three myctophid species with n>50 individuals. Our study suggests, for the first time, that myctophids could play a role as paratenic hosts in the oceanic life-cycle of A. pegreffii and A. physeteris in the western Mediterranean. None of the other larvae identified at least to family level infect cetaceans, but some of them can be transmitted to large predatory fish. The extreme scarcity of such cetacean parasites in this, and previous parasitological surveys of mesopelagic fish and cephalopods is at odds with the key role of these preys in the diet of oceanic cetaceans. Infective stages of trophically-transmitted helminth circulate through oceanic food webs and, therefore, they can be detected in many fish and cephalopod species even though many of them are not prey of cetaceans. For this reason, we investigated the helminth fauna of Bathypterois mediterraneus, a demersal deep-sea that is not consumed by striped dolphins, but (i) it is the most common fish below 1500m in western Mediterranean waters (Carrasson and Matallanas, 2001); (ii) it occurs on the continental slope, where striped dolphins are frequently found (Gómez de Segura et al., 2008) and (iii) the most important food items of its diet are benthopelagic planktonic calanoid copepods (Carrasson and Matallanas, 2001) which may act as intermediate and/or paratenic hosts for cetacean parasites. In July 2010, 170 specimens of B. mediterraneus were captured from the continental slope in Western Mediterranean waters. Samples were obtained from the continental slope of two different areas, namely, the coast of Catalonia (off Barcelona) and the Balearic Islands in three different bathymetric strata at depths between 1000 and 2200m. The parasite fauna of B. mediterraneus included a narrow range of species: Steringophorus cf. dorsolineatum, Scolex pleuronectis, Hysterothylacium aduncum, Anisakis sp. larva 3 type II and Sarcotretes sp. Steringophorus cf. dorsolineatum and H. aduncum were the most predominant parasites. Hysterothylacium aduncum showed significant differences in abundance between depths of 2000-2200m with 1000-1400m and 1400-2000m, irrespective of locality, whereas S. cf. dorsolineatum showed significant differences between the two localities at all depths except for 2000-2200m. We suggest the possible usefulness of these two parasites as geographical indicators for discriminating discrete stocks of B. mediterraneus in western Mediterranean waters. Only 4 specimens of Anisakis sp. were identified as infective stages of helminths infecting cetaceans.