DOI: 10.2478/s11686-010-0011-9 © 2010 W. Stefański Institute of Parasitology, PAS Acta Parasitologica, 2010, 55(1), 29–38; ISSN 1230-2821 Digenean trematodes of Seriolella porosa (Pisces, Centrolophidae) in San Matías Gulf, Argentina Silvia E. Guagliardo1, Carla B. Schwerdt1, Sergio R. Martorelli2, Noelia A. Galeano1 and Ruben D. Tanzola1* 1Universidad Nacional del Sur, San Juan 670 (8000) Bahía Blanca; 2CEPAVE (CONICET-U.N.L.P) 2 Nº 584 (1900) La Plata; Argentina Abstract Seriolella porosa (Silver warehou) is an important fish resource in the Argentine Sea. No previous studies have been carried out to date on its parasite fauna. The present study aims to (i) identify the digenean trematodes that parasitize Silver warehou, (ii) estimate parasite frequency and abundance, and (iii) establish their relationship with fish size and sex. A total of 100 spec- imens of S. porosa from San Matías Gulf, Argentina were examined between September and October 2006. Host size and sex were determined. Parasites extracted from the stomach lumen were fixed, conserved in 5% formalin, and stained with Langeron’s hydrochloric carmine. The following parameters were also determined: dispersion coefficient, prevalence, intensity, and para- site abundance. Differences in parasitism were assessed using non-parametric tests. Four adult digenean species were found in the stomach, namely Lecithocladium cristatum, Aponurus laguncula, Elytrophalloides oatesi and Gonocerca cf. phycidis. L. cristatum was not only dominant but also the most prevalent and abundant species. None of the digeneans showed inter- specific association with each other and therefore they might follow different patterns of colonization (i.e., different interme- diate hosts, seasonal and/or spatial distribution of the infective stages). Significant differences were observed among the digeneans analyzed in respect to the sex of the host. Results from this study show that Silver warehou plays a key role as a final host of a significant number of digeneans in the ecosystem. This study is the first step in an attempt to further determine the larval stages in intermediate hosts in San Matías Gulf. Keywords Digenea, fish, Seriolella porosa, Argentina Introduction The Silver warehou, Seriolella porosa Guichenot, 1848 is a benthopelagic fish species that lives in the southwest Atlantic Ocean between parallels 35° and 55°S (Garciarena and Perrotta 2002). It is seasonally present in San Matías Gulf, Argentina, the site of reproductive aggregation and spawning of this species (Forciniti and Pérez Macri 1992, Perier and Di Giá- como 2002). In the Argentine Sea, the highest landings are in Mar del Plata port (47%), followed by San Antonio Oeste (25%). Catches have shown an increasing trend since 1972 and have been exported as frozen fish and, to a lesser extent, sold as fresh fish in the local market (Cousseau and Perrotta 1998). At present, there are no previous studies in Argentina on the systematics of helminth parasites of Silver warehou. Research on this host species contributes to knowledge about the sani- tary state of Silver warehou as they have a relevant role in the transmission of infective stages, as well as interpreting the trophic and ecological relationships among the different com- ponents of the ecosystem. In view of the above, the purpose of this study was to char- acterize the composition of digenean species hosted by S. porosa in San Matías Gulf and to evaluate some of the eco- logical implications for its parasite community. Materials and methods A total of 100 specimens of S. porosa (68 males and 32 females) were examined between September and October 2006. The area of capture was the northern sector of San Matías Gulf (41°00´S, 64°00´W), Argentina. The fish were caught by the commercial fleet trawlers from San Antonio Oeste port. Host size and sex were determined. Parasites were extracted from the digestive lumen and were subsequently relaxed in hot saline so- lution. They were fixed, conserved in 5% formalin and stored in *Corresponding author: rtanzola@uns.edu.ar Author's copy Silvia E. Guagliardo et al.30 70% alcohol. Part of the helminth sample was stained with Langeron’s hydrochloric carmine cleared in clove oil and mounted in Canada balsam. Diagrams were made at scale using a camera lucida. Measurements were all in micrometers (µm). Average value and range were indicated in brackets. Sucker ra- tio was expressed as the ratio = ventral sucker length + ventral sucker width/oral sucker length + oral sucker width. Parasite prevalence, abundance and intensity of infection were calculated following Bush et al. (1997). Dispersion co- efficient was calculated (DC: variance/media relationship ac- cording to Morales and Pino de Morales 1987). Male preva- lence was compared to female prevalence by means of log-likelihood “G” test (Zar 1999). Also, the total number of parasites in males was compared to that in females via non- parametric Mann-Whitney U test (Siegel and Castellan 1995). The qualitative association between pairs of parasite species was estimated via the correlation Φ coefficient (Combes 1983). The relationship between parasite intensity and host size was estimated using Spearman’s rank correlation coefficient (Siegel and Castellan 1995). The material analyzed in the present study was deposited in the Collection of Invertebrates (Helminths) in the Museo de Ciencias Naturales in La Plata, Argentina. Results Family Hemiuridae Looss, 1899 Subfamily Elytrophallinae Skryabin et Gushanskaya, 1954 Lecithocladium cristatum (Rudolphi, 1819) Looss, 1907 (Fig. 1, Table I) Site of infection: Stomach. Voucher specimens: MLP 5935. Prevalence of infection: 80.19%. Mean intensity: 22.32. Abundance: 17.90. Dominance: 78.30%. Comments The specimens in the present study were found to belong to the Lecithocladium genus (Rudolphi, 1819) Looss, 1907 consid- ering the following characteristics: body with strongly marked plications; oral infundibular sucker elongated pharynx; semi- nal vesicle with thick muscular wall located in the hindbody; narrow sinus-sac at the level of ventral sucker; not reaching the position of the seminal vesicle; absence of prostatic vesicle and tubular vitellarium composed of 7 elongated lobes. In a study on hemiurids in fish from the North Atlantic Ocean, Gibson and Bray (1986) revised the species of Lecitho- cladium and divided them into seven groups according to body size, sucker ratio and egg length. Based on these pa- rameters, the digenean species studied in our research could have been included in the group that Gibson and Bray (1986) designated as group F to which L. harpodontis belongs, con- sidering the total length of the body plus the ecsoma (both reaching a length of 6 mm), the similarity in size between the oral sucker and the ventral sucker (which can, in fact, be slightly larger) and the length of eggs (from 12 to 20 µm). However, our species differs from L. harpodontis because in the latter the oral sucker was longer than the ventral one and sucker ratio is 1:1. In contrast, in our study the ventral sucker was slightly larger than the oral one and sucker ratio was 1:1.21. The Lecithocladium species from S. porosa also differs from L. harpodontis because its seminal vesicle was saccular, the pars prostatica straight (vs S-shaped pars prostatica) and the Fig. 1. Lecithocladium cristatum (schematic, in toto). Scale bar = 0.5 mm Author's copy Digeneans of Seriolella porosa in Argentina 31 eggs were larger. The eggs were also larger than those of L. chaetodipteri and L. excisum (Fabio 1988) which have been reported to parasitize fish from the southwest Atlantic Ocean. In addition, two species of Lecithocladium have been found in congeneric fish, one of which corresponds to L. se- riolellae from New Zealand (Manter 1954) and the other was L. cristatum from Peru and Chile (Luque and Oliva 1993). The species found in our research differs from L. chaetodipteri due to the relation of the width of suckers, the length of the sinus- sac and the fact that the vitellarium does not penetrate into the ecsoma, and from L. excisum due to the size of the ventral sucker, which is, in fact, smaller than the oral sucker. Also, its sinus-sac was shorter and lies anteriorly to the ventral sucker. Our species was also different from L. seriolellae (Manter 1954) because the latter was smaller and also the sinus-sac begins an- teriorly to the ventral sucker, whereas in our species the sinus- sac rised exactly at the level of the ventral sucker. As for L. cristatum, the specimens studied in the present re- search show remarkable similarities mainly in their measure- ments (Table I), length of the vitellarium, length of the sinus- sac and sucker ratio, and in a tendency of the acetabulum to be slightly larger than the oral sucker. In view of all the above, it can be concluded that the species found in S. porosa coincides with L. cristatum. It is also important to note that L. cristatum was previously cited in S. violacea in Peru and Chile and was also found in the South Atlantic where it had been cited as L. falklandicus by Gaevskaya and Kovaleva (1978) after which synonymy be- tween the latter and L. cristatum was established by Luque and Oliva (1993). Elytrophalloides oatesi (Leiper et Atkinson, 1914) Szidat et Graefe, 1967 (Fig. 2, Table II) Site of infection: Stomach. Voucher specimens: MLP 5936. Prevalence of infection: 31.68%. Mean intensity: 14.37. Abundance: 4.55. Dominance: 19.92%. Comments The following characteristics allowed us to include Elytrophal- loides oatesi in the Elytrophalloides genus: presence of a long and thin sinus-sac, tegument with plications, long seminal vesi- cle with thick muscular walls and vitelline tear-shaped lobes. Bray (1990) recognized only two valid species, namely E. oatesi Szidat et Graefe, 1967 and E. humerus Bray, 1990. The measurements of the specimens in the present re- search coincide, in general, with the ranges of variability re- ported for E. oatesi, however, they are smaller than the spec- imens studied by Suriano and Sutton (1981). On the other hand, our specimens do not have the typical “shoulder-pads” that are present in other species of the genus. Elytrophalloides oatesi has been cited in 52 fish species (Bray 1990) all over the world. According to Rodyuk and Jessen (1981) it is the most commonly recorded parasite in the Subantarctic area. In Argentina there are records in 12 fish species (Szidat and Graefe 1968; Suriano and Sutton 1981; Gaevskaya et al. 1990; Szidat 1950, 1955, 1961; MacKenzie and Longshaw 1995; Rodyuk 1995; Sardella and Timi 1996). Table I. Comparative data on the morphometry of Lecithocladium cristatum (Rudolphi, 1819) Looss, 1907 Luque and Oliva (1993) Zdzitowiecki (1997) Present study (n = 15) Body length 4640 (1700–9400) 1300–4600 2608 (2060–3570) Body width 900 (400–1740) 500–1150 592.7 (410–910) Ecsoma length 1820 (130–4420) 600–1800 1503.57 (1375–1790) Pre-oral lobe – 10–20 Oral sucker length 200 (160–230) 226–320 282.7 (180–360) Oral sucker width 200 (160–230) 143–182 212.7 (140–370) Ventral sucker length 230 (200–240) 228–515 301.3 (210–430) Ventral sucker width 230 (200–240) 277–506 256.7 (160–410) Sucker ratio VO/VV 1:1.15 (width ratio) 1:0.89–0.93 (length ratio) 1:1.13 (1:0.9–1:1.34) Pharynx length – 167–330 175.2 (122.5–240) Pharynx width – 87–209 107.5 (65–170) Testicle length 130 (100–160) 156.2 (110–210) Testicle width 130 (100–160) 140.9 (90–180) Sinus sac length – 300–420 400–450 Sinus sac width – 30–42 38–40 Genital atrium length 100–120 Seminal vesicle length 210–495 310 (170–550) Seminal vesicle width 130–330 123 (80–180) Ovary length 150 (130–170) – 132.86 (70–190) Ovary width 122.86 (60–150) Egg length 19 18–24 16.27 (12.5–17.5) Egg width 9 11–14 10 (7.5–12.5) Author's copy Silvia E. Guagliardo et al.32 T a b le I I. C o m p ar at iv e d at a o n t h e m o rp h o m et ry o f El yt ro ph al lo id es o at es i ( L ei p er e t A tk in so n , 1 9 1 4 ) S zi d at e t G ra ef e, 1 9 6 7 S zi d a t (1 9 5 0 ) S zi d a t (1 9 5 5 ) S zi d a t a n d G ra ef e S u ri a n o a n d B ra y ( 1 9 9 0 ) P re se n t st u d y (c it ed a s Pl er ur us sp .) (c it ed a s El yt ro ph al lo id es (1 9 6 8 ) S u tt o n ( 1 9 8 1 ) H o st : (n = 1 5 ) H o st : El eg in op s m er lu cc ii) H o st : H o st s: M .h ub bs i, M eu sc he ni a m ac lo vi nu s H o st : M er lu cc iu s h ub bs i Pa ra ch ae ni ch th ys An tim or a ro str at a, fr ey ci ne ti, ch ar co ti M ac ro ro nu s Al dr ic he tta m ag el la ni cu s, fo rs te ri Sa lil ot a au str al is, M .p ol ile pi s B o d y l en g th 9 5 0 (1 7 5 0 – 1 8 5 0 ) (1 7 0 0 – 2 6 0 0 ) (2 5 0 0 – 3 7 5 0 ) 1 1 6 0 /1 4 0 0 1 4 6 6 .8 8 ( 1 2 4 0 – 1 8 0 0 ) B o d y w id th 2 8 0 3 5 0 (4 0 0 – 6 0 0 ) (5 0 0 – 1 0 0 0 ) 1 8 5 /– 3 5 9 .3 8 ( 2 5 0 – 4 5 0 ) E cs o m a le n g th – – – (5 5 0 – 5 6 0 ) 1 8 /w it h d ra w n 4 1 0 P re -o ra l lo b e – – – – 1 0 /1 5 1 0 O ra l su ck er l en g th 8 0 3 0 (1 6 6 – 1 8 0 ) 9 0 /1 1 0 1 3 8 ( 1 0 0 – 2 0 0 ) O ra l su ck er w id th 1 0 0 1 0 0 1 8 0 (1 2 0 – 1 6 6 ) 8 5 /– 1 2 8 ( 1 0 0 – 2 0 0 ) V en tr al s u ck er l en g th – – – (2 4 8 – 3 2 0 ) 1 5 5 /1 8 5 1 8 3 .3 3 ( 1 3 0 – 2 7 0 ) V en tr al s u ck er w id th 1 5 0 (1 5 0 – 1 7 0 ) 3 0 0 (2 4 0 – 3 1 5 ) 1 4 5 /– 1 8 6 .6 6 ( 1 3 0 – 2 7 0 ) S u ck er r at io V O /V V 1 :1 .8 7 1 :1 .5 1 :1 .6 1 :2 1 :1 .8 2 /– 1 :1 .4 2 ( 1 .0 3 – 1 .8 5 ) P h ar y n x l en g th (4 0 – 5 0 ) – 9 0 (8 4 – 1 0 0 ) 5 0 /– 8 0 .3 5 ( 6 0 – 1 1 0 ) P h ar y n x w id th – (1 0 – 5 0 ) – (6 0 – 8 4 ) 4 5 /– 6 6 .6 1 ( 5 0 – 8 0 ) T es ti cl e le n g th – – – – an t. : 8 5 p o st .: 7 5 /– 7 0 ( 5 0 – 9 0 ) T es ti cl e w id th – – – – an t. : 7 0 p o st :6 0 /– 7 5 ( 6 0 – 1 0 0 ) S in u s sa c le n g th – – – – – /– (7 0 0 – 8 0 0 ) S in u s sa c w id th – – – – – /– (4 0 – 5 0 ) S em in al v es ic le l en g th – – – – 2 0 2 /– 3 5 3 ( 3 0 0 – 4 8 0 ) S em in al v es ic le w id th – – – – 5 0 /– 7 4 .4 4 ( 5 0 – 1 0 0 ) O v ar y l en g th – – – – 6 5 /– 1 0 0 O v ar y w id th – – – – 6 0 /– 8 0 E g g l en g th – 2 0 (2 5 – 2 7 ) (2 0 – 2 4 ) 2 2 /– 2 4 .4 4 ( 2 2 .5 – 2 5 ) E g g w id th 1 1 (1 0 – 1 2 ) (8 – 1 4 ) (9 – 1 0 )/ – 1 0 .2 7 ( 1 0 – 1 2 .5 ) Author's copy Digeneans of Seriolella porosa in Argentina 33 E. oatesi has also been cited in approximately 11 fish species from waters adjacent to the Malvinas (= Falkland) Islands (Zdzitowiecki 1997, Kohn et al. 2007). Our research reports the first citation of E. oatesi in Silver warehou, S. porosa. Family Lecithasteridae Odhner, 1905 Subfamily Lecithasterinae Odhner, 1905 Aponurus laguncula Looss, 1907 (Fig. 3, Table III) Site of infection: Stomach. Voucher specimens: MLP 5938. Prevalence of infection: 19.80%. Mean intensity: 2. Abundance: 0.40. Dominance: 1.3%. Comments These digeneans coincide in general with those in Aponurus Looss, 1907, whose type species is A. laguncula, due to the fol- lowing characteristics: relatively small body, absence of spines and ecsoma, vitellarium of tear-shaped lobes and absence of genital atrium. This genus has been considered to be very close to the genera Lecithophyllum Odhner, 1905 and Bra- chadena Linton, 1910, however, some authors have reported differences between Brachadena and Lecithophyllum, e.g., the presence of a different genital atrium (Margolis 1958) and Fig. 2. Elytrophalloides oatesi (schematic, in toto). Scale bar = 0.2 mm Fig. 3. Aponurus laguncula (schematic, in toto). Scale bar = 0.1 mm Author's copy Silvia E. Guagliardo et al.34 different egg size (Manter 1947). Later, Overstreet (1973) and Gibson and Bray (1979) considered the possibility of estab- lishing synonymy between Lecithophyllum and Aponurus. Al- though Margolis (1958) differentiated Brachadena from Lecithophyllum and Aponurus, as its vitellarium consisted of seven ventrally bound lobes, Overstreet (1973) analyzed the type species and found that the lobes of the vitellarium were not ventrally bound and therefore included it in the Aponurus genus. However, according to Bray and Cribb (2000) further studies on the genital atrium and the vitelline lobes may reval- idate the Brachadena genus. Our specimens do not seem to have a well-defined genital atrium, which is why they cannot be included in the Lecitho- phyllum genus. However, the first section of the hermaphro- ditic duct shows a dilation which looks like the so-called her- maphroditic vesicle described by Yamaguti (1971) for this genus. The morphological characters of our specimens share major similarities with those reported by Bray and MacKen- zie (1990) for Aponurus laguncula (Table III), showing, how- ever, slight differences in the ranges of body and egg length recorded by these authors. Interestingly, León-Regagnon et al. (1997) have recorded very different measurements for A. la- guncula in comparison to those recorded by Bray and Mac- Kenzie (1990), who considered it as a generalist due to its dis- tribution. In fact, A. laguncula has been cited in 60 species of teleost fishes belonging to 27 families, displaying a high preva- lence in clupeids (Mamaev 1970). The first citation of the genus in the South Atlantic belongs to Szidat (1950) who re- ported Aponurus sp. in Eleginops maclovinus. Later, in 1961, Szidat cited A. laguncula in Paralichthys patagonicus from waters of the Patagonian Shelf, which is the first record for Se- riolella porosa. Family Derogenidae Nicoll, 1910 Subfamily Gonocercinae Skryabin et Gushanskaya, 1955 Gonocerca cf. phycidis (Fig. 4, Table IV) Site of infection: Stomach. Voucher specimens: MLP 5937. Prevalence of infection: 0.99%. Mean intensity: 1. Abundance: 0.01. Dominance: 0.04%. Table III. Comparative data on the morphometry of Aponurus laguncula Looss, 1907 Manter (1947) Bray and MacKenzie León-Regagnon et al. Present study (1990) (1997) (n = 8) Body length 770 620 (500–705) 2900 (2600–3300) 866.87 (750–1005) Body width 240 192 (167–242) 660 (664–670) 202.18 (162.5–225) Oral sucker length 152 74 (68–81) 150 80.62 (62.5–87.5) Oral sucker width 90 78 (68-85) 250 80.94 (62.5–87.5) Ventral sucker length 119 137 (123–151) 450 (430–480) 131.78 (112.5–142.5) Ventral sucker width 68 134 (126–144) 440 (400–480) 153.44 (125–162.5) Sucker ratio VO/VV 1:2 1:1.71 (1.57–1.85) 1:2.19 1:1.68 (1:0.9–1:1.89) Pharynx length – 35 (32–45) – 34.68 (25–37.5) Pharynx width – 39 (32–47) – 40 (30–45) Testicle length – 72 (38–110) – 91 (87.5–100) Testicle width – 67 (48–85) – 89.16 (70–100) Sinus sac length – 35 – 30 Sinus sac width – 22 – 20 Seminal vesicle length – 45 – 50 Seminal vesicle width – 40 – 20 Ovary length – 67 (54–95) – 65.42 (50–77.5) Ovary width – 65 (48–98) – 63 (42.5–75) Egg length (32–33) 38 (32–42) 28 (27–30) 28.13 (25–30) Egg width (14–17) 18 (16–21) 16 (15–18) 13.75 (12.5–15) Table IV. Comparative data on the morphometry of Gonocerca cf. phycidis Suriano and Sutton Present study (1981) (n = 1) Body length 2600–4000 1850 Body width 850–1100 450 Oral sucker length 300–350 200 Oral sucker width 550–600 232.5 Ventral sucker length 700–750 310 Ventral sucker width 900–950 400 Sucker ratio VO/VV 1:2–3 1:1.64 Pharynx length 90–95 90 Pharynx width 150–160 80 Testicle length – 150 Testicle width – 150 Ovary length – 100 Ovary width – 95 Egg length 40 30 Egg width 20 16 Author's copy Digeneans of Seriolella porosa in Argentina 35 Comments In the present study this species was recorded only once and only one specimen was found. This species was included in the genus Gonocerca Manter, 1925 in view of the following char- acteristics: presence of an elongated body without spines, tes- ticles in tandem or symmetric, median genital pore located in the posterior margin of the oral sucker, hermaphroditic duct and absence of a sinus-sac, thin genital atrium, well-developed pars prostatica, free external club-shaped seminal vesicle and medium-sized pre-testicular ovary and vitellarium in the form of two pre-testicular masses lateral to the ovary and more or less lobulated. Zdzitowiecki (1997) recognized two species of Gonocerca in Antarctic and Subantarctic waters, namely G. lobata and G. phycidis, both showing differences in egg size. Based on this characteristic, the digenean species found in our study resembles G. phycidis. The latter has been cited in deep-water fish (Bray et al. 1999, Karlsbakk et al. 2002, Klimpel et al. 2001). It has been cited in 14 fish species from the Antarctic region and in 15 fish species from the Subantarctic region (Zdzitowiecki 1997) of the Atlantic Ocean. In Argentina it was cited by Szidat (1955), Gaevskaya and Kovaleva (1977), Suriano and Sutton (1981), Laskowski and Zdzitowiecki (2005). The measurements of the only specimen found in this study are included in Table IV. It is comparatively smaller than those studied by Suriano and Sutton (1981). Nonetheless, this difference could be attributed to intraspecific variations not in- cluded here in the absence of a range of measurements. Further studies on a larger number of specimens will therefore be nec- essary to confirm that this digenean specimen belongs to the species G. phycidis in which it is temporarily included. Quantitative aspects A total of 2,309 specimens of digeneans were recorded in the hosts examined in the present study, L. cristatum being the most dominant, prevalent and abundant. All species were community components with prevalences higher than 10%, with the exception of G. cf. phycidis in which only one individual was recorded, showing an overdis- persed distribution. The comparison of the prevalences between host sexes only revealed significant differences in A. laguncula in which males were found to be more parasitized. A similar pattern was observed when the total number of parasites in each sex were compared (U = 825.50, p = 0.042). Nonetheless, these values may simply represent a sampling effect (65 males vs 32 fe- males) and may have no particular biological significance. None of the species showed any interspecific association with each other (coefficient Φ: L. cristatum-A. laguncula = 0.082; L. cristatum-E. oatesi = –0.0015; A. laguncula-E. oatesi = –0.018 and therefore they might follow different patterns of colonization (i.e., different intermediate hosts, and seasonal and/or spatial distribution of the infective stages). The three component species showed a significant correlation with host size although this correlation was only marginally positive for E. oatesi (L. cristatum: r s = –0.23, p = 0.010; A. laguncula: r s = –0.021, p = 0.036; E. oatesi: r s = 0.23, p = 0.022). Discussion All parasite species analyzed showed a type of overdispersed distribution. The lack of statistical association between para- site species may suggest different pathways of parasitic colo- nization. This agrees with one of the most common parasite distribution patterns in parasite populations (Esch and Fer- nandez 1993). The host size and age are important variables in explaining the variability in parasitofauna and therefore in- fracommunity parameters, suggesting modifications in the ecological relationships throughout the ontogeny of the hosts (Riffo 1991). Also, the fact that intensity was positively cor- related with host size in E. oatesi seems to indicate that this species is transmitted by some selectively incorporated trophic item with higher abundance in larger hosts. Fig. 4. Gonocerca cf. phycidis (schematic, in toto). Scale bar = 0.2 mm Author's copy Silvia E. Guagliardo et al.36 Aponurus laguncula was more prevalent in male hosts which harbour a higher parasite numbers than female hosts. In addition, Perrier and Di Giácomo (2002) confirmed that re- productive aggregations with a male-female ratio of 2:1 were present in the study area, although the sample of hosts was un- balanced. This agrees with the male-female ratio in the sam- ple found in our study and it may be the reason for the higher parasitism record shown by males which, as they are more abundant in the ecosystem, seem to be more exposed to meta- cercariae-infected preys. The different species of Lecithocladium in the Northern Atlantic are transmitted by Philine aperta, an opisthobranch gastropod, as first intermediate host (Køie 1991). Nonetheless, metacercariae do not show high specificity to their hosts. Køie (op. cit.) demonstrated experimentally that Lecithocla- dium uses copepods as secondary hosts, whereas a ctenophore species and a polychaete have been found infected in the wild and they therefore behave as disseminators of infective stages. On the northeast coast of Africa, Reimer et al. (1975) found metacercariae of the Lecithocladium genus in two types of medusae, in Pleurobrachia sp., in two calanoid copepods and in six species of the chaetognath Sagitta. This genus has been also recorded in Australia, the Indian Ocean and China, among other places (Korotaeva 1969, Gu and Shen 1978, Bray and Cribb 2004). This shows the large amplitude of suscep- tible intermediate hosts and therefore increases the spatial dis- tribution pattern of species, thus justifying the high prevalence observed in the sample studied in the present research. A similar situation occurs with A. laguncula. Although noth- ing is known about its life cycle, adult samples have been recorded in definitive hosts not only in a diverse variety of de- finitive hosts, but also within a vast distributional range (Bray and MacKenzie 1990). In spite of the fact that E. oatesi has been recorded in several teleosts from different geographical areas, this species is not considered as a cosmopolitan (Rocka 2006). The pronounced differences in abundance and dominance between L. cristatum and the other three species described here and the absence of any association between the species might be due to the fact that each digenean species chooses different intermediate host species or because each digenean species uses its host species in a different way. Iannacone (2003) did not find any digenean species para- sitizing Seriolella violacea from Callao, Peru. However, Tan- talean and Huiza (1994) cited the Lecithocladium genus in Pe- ruvian waters, although they did not give any details about the host species. Rohde et al. (1980) only cited one digenean species, Syncoelium filiferum, in Seriolella brama from New Zealand, and Bray and Cribb (2003) reported Cephalolepi- dapedon warehou in the gut of Seriolella punctata from Tas- mania. All these differences might be attributed to the differ- ent trophic preferences of the Seriolella species in each of the study area, and therefore, to a strong ecological component in the structuring of community assemblages. As stated above, G. phycidis has been reported from deep- water hosts (Bray et al. 1999, Klimpel et al. 2001, Karlsbakk et al. 2002). However, in our study area, Silver warehou has been described as a species of pelagic and gregarious habits in the neritic region (Forciniti and Pérez Macri 1992). As to the co-occurrence of G. cf. phycidis and E. oatesi, it is notewor- thy that Oliva (2001) also recorded both species in Macruronus magellanicus in Chilean waters. The presence of Silver warehou in the northern area of San Matías Gulf and the fact that this region is used as a spawning area by this fish species might be related to the availability of food (Ramirez 1996). It seems likely that carriers of larval di- geneans form part of the group of euphausiids and hyperiidean amphipods, which are food items for Silver warehou (Forciniti and Pérez Macri 1992). Another group of potential larval di- geneans might be that of jellyfishes and chaetognaths, all of which, in general, leave no tracks as trophic items except for the parasites they transport. The results obtained from the present study contribute to knowledge about the biodiversity in San Matías Gulf, as well as identifying the role that the Silver warehou plays as a de- finitive host to at least three digenean species in the ecosystem. This group of helminths in San Matías Gulf might establish the boundaries of different metapopulations, since no available parasite records for Seriolella species have shown as much species diversity as in the present study. Acknowledgements. 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