Co

Parasite species

Definitive hosts

Intermediate hosts

Discocotyle sagittata

Crepidostomum farionis

Salmo spp., Onchorhynchus, Coregonus, Salvelinus, Prosopium (Kennedy, 1978; Hoffman, 1998)

Species of Coregonus, Salmo, Salvelinus, Thamallus, Perca, Etheostoma, Lepomis, Lota, Leucichthys, Notropis, Oncorhynchus Cristivomer, Prosopium, Gasterosteus (Hoffman, 1998).

None

First intermediate host: Pisidium sp„ Sphaerium corneum (Brown, 1927, Hoffman, 1998), Pisidium casertanum, Poli (Awachie, 1968). Hopkins (1934) suggested that due to widespread distribution of this species, it may adapt to other invertebrate hosts, although it will probably remain dependent on Sphaeriidae as molluscan host Second intermediate hosts: Sialis lutaria, Ephemera danica (Brown, 1927; Crawford, 1943; Hopkins, 1934; Robertson, 1953), Gammarus pulex (Baylis, 1931; Awachie, 1968), mayfly nymph (Hoffman, 1998)

Crepidostomum metoecus Species of Coregonus, Cotius, Esox, Lota, Salvelinus, First intermediate hosts: Pisidium sp. (Noller, 1928; Moravec, 1982), Thymallus and Salmo Lymnaea peregra (Awachie, 1968)

Second intermediate hosts: Gammarus pulex (Awachie, 1968),

Ramellogammarus vancouverensis (Margolis and Moravec, 1982), Ephemera danica (Moravec, 1982), dead and dying metacercariae in Ecdyonurus torrentis, Baetis rhodani, Paraleptophlebia submarginata, Leuctra spp. and Sialis lutaria (Awachie, 1968)

Phyllodistomum simile Salmo trutta, Salmo salar and Anguilla anguilla in UK First intermediate host: Sphaerium corneum (Thomas, 1956, 1958a,b, (Kennedy, 1974), Thymallus thymallus (Bykhorskaya- 1964b,c) metacercariae also encyst in sporocysts Pavlovskaya, 1962)

Table 5 continued

Parasite species

Definitive hosts

Intermediate hosts

Neoechinorhynchus rutili

Circumpolar distribution, in species of Ambloplites, Carrasius, Catastomas, Cottus, Couesius, Culaea, Cyprinus, Dallia, Esox, Fundulus, Gasterosteus, Gila, Hyperhychus, Ictalurus, Lota, Micropterus, Morone, Mylocheilus, Notemigonus, Notropis, Onchorhynchus, Perca, Pimephales, Prosopium, Ptychocheilus, Pungitius, Richardsonius, Salmo, Salvelinus, Semotilus, Stizostedion, Thymallus and Umbra (Hoffman, 1998)

Intermediate hosts include ostracods such as Cypria reptans (Dezfuli, 1996) and Sialis lutaria (Lassiere, 1988)

Diphyllobothrium ditremum Fish-eating birds including cormorants

Procercoids in copepods (Hoffman, 1998; Heckmann and Ching, 1987) plerocercoids in species of Salmo, Salvelinus, Onchorhynchus, Gasterosteus

Bothriocephalus claviceps

Adults in Anguilla anguilla, Anguilla rostrata, species of Ambloplites, Chaenobryttus, Lepomis, Micropterus Percopis, Stizostedion and Gasterosteus

Procercoids in copepods; small fish as carriers or paratenic hosts

Raphidascaris acus Species of Ptychocheilus, Abramis Onchorhyncthus,

Esox, Salmo, Lota, Salvelinus, Stizostedion, Ictalurus (Hoffman, 1998); Perca, Hucho, Salmothymus, (Moravec, 1994)

Only fish or amphibia act as obligate intermediate hosts. Larvae encyst in liver. Fish hosts include 70 species, mainly cyprinids, bullheads, loaches, chub, minnows, pike (Moravec, 1994), and young trout (Alvarez Pellitero, 1979d). Invertebrates including oligochaetes, Asellus, Macrocyclops, snails, chironomidae, Trichoptera and Ceratopogonidae act as paratenic hosts

Table J continued

Parasite species

Definitive hosts

Intermediate hosts

Cycullanus (T.) truttae

Species of Onchorhynchus, Prosopium, Salmo, Salvelinus, Hucho, Coregonus, Thymallus, Brachymystax, Salvelinus, Stenodus, Onchorhynchus, Coregonus, adult Lampetra (gut or adominal cavity) In other fish such as Anguilla anguilla, Perca fluviatilis and Lota lota do not develop to maturity (Moravec, 1994)

Larval Lampetra planeri, Lampetra fluviatilis and Petromyzon sp. (Moravec, 1994) act as obligatory intermediate host. Attempts to infect various invertebrate and fish species unsuccessful. Other salmonids act as paratenic hosts or as postcyclic hosts Postcyclic hosts include predatory fish, mainly salmonids (homopost-cyclic hosts), also pike, perch, catfish and burbot (heteropost-cyclic hosts) feeding on adult lampreys or salmonids (the definitive hosts)

Cystidicoloides ephemeridarum

Species of Salmo (S .tratta, S. salar), Salmothymus, Onchorhynchus, Salvelinus, Hucho, Thymallus, Brachymystax, Coregonus, Prosopium. Postcyclic hosts include Esox, Lota, Anguilla, Leuciscus, Barbus and Acipenser

Ephemeropterans such as Ephemera spp., Leptophlebia,

Habrophlebia, Hexagenia and Polymitarcys. Forage fish such as species of Cottus, Noemacheilus, Phoxinus as paratenic or paradefinitive hosts

Capillaria sp.

Circumpolar distribution. Parasites of freshwater fish including C. salvelini in salmonid fish.

Paratenic in oligochaetes (Hoffman, 1998). Oligochaete worms act as obligate intermediate hosts or homoxenous (or direct development) in fish without intermediate host may occur

Paraquimperia tenerrima Anguilla anguilla (monoxenous species;

Moravec, 1994)

Life cycle not known (Moravec, 1994)

1998). The claim made by Awachie (1968) that Lymnaeaperegra is also a host of C. metoecus therefore requires verification. However, as there is evidence that several species of Pisidium and Sphaerium may be implicated as alternative intermediate hosts of another allocreadiid trematode, Bunodera luciopercae (Andrews and Chubb, 1980), this low level of specificity may also apply in the case of Crepidostomum species.

The level of specificity showed by digenetic parasites to the second intermediate hosts is generally lower than that to the molluscan host. In the case of Crepidostomum species they include Gammarus pulex and Ramellogammarus vancouverensis as well as insects belonging to the orders Plecoptera, Ephemeroptera and Megaloptera (Table 5). The presence of Crepidostomum farionis in Arctic Charr from Greenland and Iceland (Due and Curtis, 1995), where amphipods and Ephemeroptera are absent, indicates that this parasite has some flexibility in its choice of second intermediate host.

During 1951-1952 it was shown that the freshwater bivalve Sphaerium corneum acted as the molluscan host of Phyllodistomum simile (Thomas, 1956, 1958a, 1964c). Although it was shown that the metacercariae in the sporocysts infected the trout it is possible that insect larvae, including anisopteran dragonfly larvae, may also have been implicated as an optional secondary intermediate host as is the case with other Phyllodistomum species (Thomas, 1958a; Ginetsinskaya, 1961). In contrast, the sporocysts of Phyllodistomum dogieli, which develop in Dreissena polymorpha, leave the host after the metacercariae develop and are presumably swallowed directly by the definitive hosts, Rutilus rutilus or Abramis brama (Ginetsinskaya, 1961). As Phyllodistomum simile was also found in the trout in the Teifi in 1998, in the apparent absence of Sphaerium corneum, it would appear that this trematode might also use Pisidium species as alternative molluscan hosts.

Small crustacea, namely ostracods, serve as the intermediate hosts of Neoechinorhynchus species, including N. cristatus, N. cylindricus, N. sagina-tus and N. rutili (Merritt and Pratt, 1964; Walkey, 1967; Valtonen, 1979; Dezfuli, 1996; Hoffman, 1998). Larvae of N. rutili in the alder-fly Sialis lutaria have also been shown to be infective to rainbow trout (Lassiere, 1988; Hoffman, 1998) but it is possible that these may be acting only as paratenic hosts (Walkey, 1967). Lassiere and Crompton (1988) also demonstrated the involvement of the stickleback Gasterosteus aculeatus in the postcyclic transmission of N. rutili into rainbow trout. Other small crustacea, the copepods, also serve as hosts for the procercoids of the two cestode species, D. ditremum and B. claviceps. Small fish act as carrier or paratenic hosts for B. claviceps while the plerocercoids of D. ditremum, the only allogenic parasite in the community, develop in species of salmonidae (Table 7).

Larger benthic invertebrates are implicated as hosts of the nematode species Capillaria and Cystidicoloides ephemeridarum. According to Moravec (1994) oligochaete worms may be obligate intermediate hosts for Capillaria sp. or alternatively development may be homoxenous or direct in the fish host. Several species of mayflies may act as intermediate hosts of C. ephemeridarum (Table 5) while small forage fish may act as paratenic hosts. Moravec (1994) showed that ammocoete larvae of lampreys are the obligatory intermediate hosts of Cucullanus T. truttae the dominant nematode parasite of the trout in the Teifi, but salmonid fish may act as paratenic and as postcyclic hosts. During 1950 the ammocoete larvae were found to coexist in depositing sediments with Sphaerium corneum in the River Teifi. Fish species, including Salmo trutta, also serve as intermediate hosts for R. acus although a number of invertebrate species (Table 5) may act as paratenic hosts (Alvarez Pellitero, 1978; Moravec, 1994).

According to Moravec (1994) the life cycle of P. tenerrima remains to be elucidated. Much work remains to be done to achieve a complete identification of the intermediate hosts that are involved in the transmission of the helminth parasites in the Teifi, Pysgotwr and other river systems. This problem is greatly exacerbated by the low level of specificity. Presumably this has arisen as a result of the stochastic and unpredictable nature of existence in freshwater ecosystems. However, such information is essential for a full understanding of the mechanisms that are involved in regulating their population dynamics.

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