ASIAN PANGASIIDS—AN EMERGING PROBLEM FOR EUROPEAN INLAND WATERS? SYSTEMATIC AND PARASITOLOGICAL ASPECTS

Some species of the pangasiid catfishes are economically very important and have a large culture potential. They have been successfully bred and cultured in Thailand and Vietnam (Lerssutthichawal et al. 1999), from where they are exported to the European and American markets, processed mainly into fillets. Those fish, commonly referred to as “panga”, have hitherto been unknown to Europe and the USA. The most valuable catfish species are: sutchi catfish, Pangasianodon hypophthalmus (Sauvage, 1878), and basa, Pangasius bocourti Sauvage, 1880, which can be raised in the river-based cages or earth-constructed ponds. Their hybrids are also of high economical value (Hung et al. 2003). Pangasiids were also experimentally cultured in Puerto Rico during 2003–2004. They were successfully reproduced and cultured with results similar to those achieved in Asian aquaculture (McGee 2005). Moreover, Pangasius sp. is regarded as a new species for aquaculture (however not new to market or consumers) in central and eastern Europe (Varadi 2008). It is anticipated that in the next 5 years worldwide aquaculture production of Pangasius sp. could be similar to that achieved with tilapia and salmon (McGee 2005). ACTA ICHTHYOLOGICA ET PISCATORIA (2009) 39 (2): 131–138 DOI: 10.3750/AIP2009.39.2.08

Juveniles of P. hypophthalmus called sutchi catfish, shark-catfish, iridescent or mystic shark are commercial in aquarium-trade as well (McGee 2005).
Two specimens of pangasiids were captured in a pond, situated in the area of the city of Szczecin. They were probably released there by ornamental fish keepers, because of their large size (over 33 cm). We suspect that many "oversized" pangasiids are released into the wild because very few home fish keepers can cope with fishes attaining sizes up to 130 cm.
The literature on pangasiid catfishes taxonomy is very scarce. Pouyaud et al. (2000) who gave a contribution to the phylogeny of pangasiid catfishes based on allozymes and mitochondrial DNA, concluded that the available morphological and osteological information on Pangasiidae remains very limited. A detailed study, using a large variety of characters, is urgently needed.
The current paper was aimed at presenting an invasive species of catfish with its corresponding parasite new to the natural waters of Poland. This study broadens the knowledge on pangasiids. It verifies also if along with the change of host's habitat, the parasite changed their microhabitat as well. A positive response would leave us to evaluate whether an adaptation to the new conditions of life may influence the morphological changes of the parasite body.

MATERIALS AND METHODS
Two specimens of pangasiids were captured in a small natural pond situated in a park within the city limits of Szczecin (Poland, lat 53 o 28΄N, long 14 o 16΄E), in April 2005. The coloration of these specimens was uniformly grey. The total length (TL) and weight were as following: 42.5 [1] and 33.2 cm [2] and 611.3 and 266.0 g, respectively (specimen numbers are given in square brackets).
In addition 8 sutchi catfish [3] obtained from a petshop in Szczecin, imported directly from Thailand (5.88 -7.91 cm TL; 0.8-4.7 g) were also examined, with intention to compare their parameters with those of the two fish caught in Szczecin. With similar purpose, we studied one specimen of Pangasianodon hypophthalmus [4] (as determined by a fish-breeder from the Lower Mekong River drainage, Vietnam), and one specimen of Pangasius bocourti [5] (46.0 cm, 1170 g and 44.5 cm, 800 g, respectively). Measurements followed the methods described by Gustiano (2003), markedly modified ( Fig. 1 Measurements were taken with an aid of electronic calliper and microscope Nikon SMZ 1000, coupled with the Lucia Measurement System, with the accuracy of 1 mm, except for the sample of 8 small specimens, where the accuracy was to 0.1 mm.
The following meristic characters were studied: number of dorsal, anal, pelvic and ventral fins and number of vertebrae. Total vertebral number included the four anteriormost vertebrae, associated with the Weberian apparatus, and the hypural fan centrum. Abdominal and caudal vertebrae were tabulated separately. Gill rakers were counted on the first branchial arch. The shape of dentition was examined both on the vomerine and palatine plates.
Additionally, during necropsy, the following organs were checked for the presence of parasites: skin, vitreous humour and eye lens, buccal cavity, gills, heart, gonad, liver, spleen, gall bladder, alimentary tract, kidney, swimbladder, and the peritoneum. The parasites found were identified from wet mounts and they were fixed and preserved in 75% alcohol, mounted in Hoyer's Medium on microscopic slides and identified. The slides of mucus and parenchymal tissue of the internal organs were exam- ined under Olympus BX 50 microscope with differential interference contrast microscopy (DIC) Nomarski. The stomach contents were studied by the decantation method. The parasites were preserved in the Canada balsam and then alum-carmine stained. Measurements of hard elements of the parasite haptor were obtained using a PZO 15 KM measuring eyepiece.

RESULTS
Fish examination. We were not able to promptly identify the two pangasiids captured from the wild in Szczecin, Poland, when they arrived to our laboratory. The first assumption that they represent P. hypophthalmus, was rejected after measurements (results are summarized in Table 1). Meristic characters of the pangasiids studied are presented in Table 2. The majority of metric and meristic characters, as well as the presence of the monogenoid Thaparocleidus caecus indicate that the animals are members of Pangasianodon hypophthalmus, however, some features, especially those pertinent to the ratio between the length of fins to the body-or total length and the ratio of eye diameter to the length of the head, differed markedly from the species description (Roberts and Vidthayanon 1991) (Fig. 2). Pangasiids examined [1, 2] have longer head (25.6% of SL) and larger eye diameter when compared to the specimens P. hypophthalmus (21.6 and 30.0 cm long, respectively) demonstrated in the paper of Roberts and Vidthayanon (1991). The dorsal fin of the specimens studied is very high (23.3% of SL), with relatively short base. Pectoral fins are very long (20.9% of SL), overlapping with the origin of anal fin.
When compared to the juveniles studied, specimens captured in the pond in Szczecin have longer pectoral and ventral fins, as well as higher dorsal fin and smaller eye diameter. Meristic measurements in both samples overlap, except for the gill-rakers count. The number of gill-rakers are markedly lower in the sample of juveniles, however it Unbranched rays in P (Pu) 1-1 1 1-1 1 1 1 Branched rays in P (Pb) 9-9 9 8-10 9.13 14 11 Unbranched rays in V (Vu)   Table 3 Length (µm) of some skeletal elements of T. caecus (n = 20) recorded in pangasiid catfish examined is probably increasing in number with size of the fish, as in Clarias gariepinus (Siluriformes: Clariidae) (see : Teugels 1984).
When meristic data of "wild" pangasiids is compared with those available in the literature (Roberts andVidthayanon 1991, Gustiano 2004), they fit in the ranges of P. hypophthalmus, with exception of a slightly lower number (by 1) of abdominal and caudal vertebrae. The palatal tooth bands provide very useful distinctions among species of pangasiids. Two specimens examined [1, 2] have dentition typically Pangasianodon hypophthalmus (Fig. 3), i.e., the palatine and vomerine plates of each side are more or less perfectly joined into a single curved toothplate, but the vomerine portions of each side are more or less widely separated at the midline (Roberts and Vidthayanon 1991). Gill rakers were typical for P. hypophthalmus (Fig. 4), i.e., smaller rakers alternated with large ones (Roberts and Vidthayanon 1991). In the sample of 8 juvenile specimens from aquarium studied, the oral dentition as well as arrangement and shape of gill rakers on the first gill arch were typical for P. hypophthalmus, too.
The shape of adipose fin seems to have significant utility as a diagnostic character. It differs in shape between the hypophthalmus-like pangasiids studied and the bocourtilike. In the hypophthalmus-like pangasiids the adipose fin is broad with a wide base while in bocourti-like the base is narrow and clearly round distally (Fig. 5).
Therefore we suggest that the two specimens collected in the natural environs of Poland are hybrids, presumably of P. hypophthalmus with another species, of unknown identity and origin, most likely from a pet-shop, also imported from Southeast Asia. This proposal is supported by the presence of an Asian monogenoids.
Two specimens, initially declared as P. hypophthalmus [4] and P. bocourti [5], used in the comparative study, obtained from Vietnamese aquaculture, appear to be hybrids as well. The morphology of the first specimen [4] is hypophthalmus-like, except for the teeth (vomerine and palatine) which is P. djambal-like. This result is consistent with Gustiano (2004), who described hybrids of the two species mentioned above, as an effect of artificial hybridization in the Indonesian aquaculture. The morphology of the gill rakers is typical for P. hypophthalmus, however their number (45), according to Roberts and Vidthayanon (1991) exceeds both that of P. hypophthalmus (29-38) and P. djambal (24-35), while according to Gustiano (2004), it fits the upper range. Similarly, the morphology of the second specimen [5] is bocourti-like, again, except for the teeth, that is hypophthalmus-like, according to the key of Roberts and Vidthayanon (1991). Gill rakers were typical for P. bocourti, i.e., rakers of equal size. Their number (36) falls in the lower range reported for this species by Roberts and Vidthayanon (1991) (36-46).
Parasite examination. On the gill filaments of the one pangasiid collected in the natural waters of Poland [2] numerous monogenoids were found. A total of 372 individuals were collected, ranging from 97 on the external arch to 131 on the fourth arch. The monogenoids are 812µm long, pigmented eye spots, 14 hooks, ventral anchor smaller than dorsal, dorsal and ventral bars v-shaped. Based on detailed morphological examinations of 20 specimens the species was determined as Thaparocleidus caecus (Mizelle et Kritsky 1969) Lim 1996 (Monogenoidea: Ancyrocephalidae) (Table 3).
No other parasites were detected in other organs and tissues of the fish examined.

DISCUSSION
The occurrence of exotic or non-native fish species in the natural water environment of Europe can be a really serious problem. Invasion of the non-native fishes, a process with significant social and economic impacts, is one of the main causes of the decline of native freshwater fishes worldwide (Pimentel et al. 2000). Major sources of new fish invasions are: 1) recreational fisheries, increasing markedly in the last few years; 2) introduction of fishes to support fisheries; 3) new species in the aquaculture and their escapes; 4) aquarium trade and live fish markets. Ornamental fishes, which are troublesome in the aquarium culture (e.g., pumpkinseed, Lepomis gibbosus) or reach unexpectedly large size (e.g., pirapitinga Piaractus brachypomus) (see : Boeger et al. 2002, Więcaszek et al. 2007, are often released to natural bodies of water by ornamental fish keepers. Successful overwintering in Polish waters (except for the warm-water canals of the power plants) would not be possible for the tropical exotic fish species, however it could take place in the warm waters of southern Europe. It is a two-sided problem: firstly these fishes may exert an adverse ecological effect after introduction as potential pests and, secondly, they may disseminate exotic parasites, capable of colonizing native fish species. However according to the so-called "rule of tens" only 10% of introductions end with permanent establishment and only 10% of cases of successful naturalization may be regarded as "pests" or "weeds" (Williamson 1996). Nonetheless, there is a growing concern about a potential impact of the above-mentioned parasites or pathogens on native fish species (Kennedy 1975).
Monogenoids in the Southeast Asia often present specificity a given group of hosts, at subclass, family, genus, as well as species levels. This suggests that, in the majority of cases, the hosts and their monogenoids have co-evolved (Lim 1998). The monogenoid Thaparocleidus caecus was described from Pangasianodon hypophthalmus (see: Lerssutthichawal et al. 1999). However it was reported also from P. larnaudii and P. conchophilus (see: Lerssutthichawal et al. 1999). Therefore, it is conceivable that T. caecus can be also present in the hybrids of P. hypophthalmus with other species.
The first species of Monogenoidea described from Pangasius spp. was Haplocleidus pangasi Tripathi, 1957 from India. Twenty-one years later, Gusev redescribed the species and transferred it to another genus as Silurodiscoides pangasi (Tripathi 1957) Gusev, 1978(Gusev 1978. Thaparocleidus caecus was reported for the first time in USA from the gills of an undetermined aquarium fish species. The authors of the species description, Mizelle and Kritsky (1969) included the parasite in the genus Ancylodiscoides Yamaguti, 1937 as Ancylodiscoides caecus. A redescription of Ancylodiscoides caecus was provided by Lim (1990) from the gills of P. hypophthalmus imported from the Thailand and cultured in Malaysia. The new redescription of Silurodiscoides caecus (Mizelle and Kritsky 1969) by Gusev (1978), was supplemented by the newly recorded structures of the parasite. The same author (Gusev 1996) stated that Silurodiscoides Gusev, 1976 was considered a junior subjective synonym of Thaparocleidus Jain, 1952, based on priority (Lim 1996). Hence, the parasite described has changed its specific name to T. caecus and under this name, along with T. siamensis (Lim 1990) Lim 1996, it is mentioned as a monogenoid from P. hypophthalmus of Thailand (Lerssutthichawal et al. 1999). The dimensions of the hard elements of the haptor of the species studied herein are slightly larger than those reported by Mizelle and Kritsky (1969), and Lim (1990) (Table 3). The mean total length of the parasite from this study, however, is slightly smaller. A very high value of mean intensity of infection in the sample investigated herein indicate that the length of the parasites may be strongly influenced both by their large density and a strong food-competition among them. The change of macrohabitat did not affect the presence of living parasites in one specimen. The first fish specimen examined, however, was not parasitized by T. caecus nor any other parasites.
Identification and morphological examinations of pangasiid catfishes seems to be very difficult, because in the areas of natural distribution (southeast Asia) escapes from the ponds and cages are common. Refugees are more frequently hybrids from aquaculture and artificially stocked (for anglers) water basins. Similarly, there is no pure species in aquarium trade; therefore fishes imported to Europe simply as "sutchi catfish" and reported in our natural water environment-could be hybrids of unknown parental individuals, not possible to detect precisely without detailed genetic studies (Maurice Kottelat, personal communication).
The parasite of gills, found in the fish examined, is host-specific, and arrived to Polish waters from the Far East along with its fish host. The above-mentioned fish hosted no other parasites-generalists, typical for its new water environment.
Monogenoid Thaparocleidus caecus is an autogenic species, for which a fish is a final host and the whole development cycle takes place in the water environment. Hence it is able to colonize new water basins only by the natural migration or the introduction of its host by the humans. The latter method was the most likely the reason of its presence in Europe, where it has never been recorded.