The susceptibility of three-spined stickleback (Gasterosteus aculeatus), nine-spined stickleback (Pungitius pungitius) and flounder (Platichthys flesus) to experimental infections with the monogenean Gyrodactylus salaris

Three-spined stickleback (Gasterosteus aculeatus L.), nine-spined stickleback (Pungitius pungitius (L.)) and flounder (Platichthys flesus (L.)) are widespread teleosts, which all have behaviours involving migration between freshwater and brackish/sea water environments. Their importance in dispersal of the freshwater monogenean Gyrodactylus salaris Malmberg, 1957, which causes heavy losses of Atlantic salmon (Salmo salar L.) parr in infected Norwegian rivers, was tested indirectly by their susceptibility and resistance to the parasite in laboratory experiments. Gyrodactylus salaris attached to the three fish species, but no parasite reproduction was observed. The infections were eliminated after a maximum of 3 days on flounder, 6 days on nine-spined stickleback, and 8 days on three-spined stickleback. Thus these fishes are innately resistant to G. salaris, and are therefore of no importance concerning the population dynamics of G. salaris in freshwater systems. However, attachment of parasites indicates that these fish species may function as transport hosts, and theoretically play a part in the dispersal of G. salaris in nature. The freshwater monogenean Gyrodactylus salaris Malmberg, 1957 is well known for its devastating effects on Atlantic salmon (Salmo salar L.) populations in Norway (cf. Johnsen and Jensen 1991, Mo 1994). By the end of 1994 the parasite had been reported from a total of 38 rivers along the Norwegian coastline in addition to the same number of hatcheries (Soleng and Bakke 1995). The susceptibility of both non-salmonid and salmonid fish species to G. salaris has been examined previously (see Bakke et al. 1992, 1996) in order to study the potential host range and dispersal potential of the parasite. Earlier, Johnsen and Jensen (1986) found the distribution of G. salaris in Norway to be related to stocking of fish from infected hatcheries, but also the possibility of natural dispersal by migration of infected fish through fjord systems has been hypothesized (Anonymous 1982, Johnsen and Jensen 1986, Lund and Heggberget 1992). This latter is supported by Soleng and Bakke (1997) and Soleng et al. (1998) who examined the salinity tolerance and dispersal potential of G. salaris on salmon. Mo (1987) found two flounders (Platichthys flesus (L.)) in the River Litledalselva in western Norway infected with 2 and 3 specimens of G. salaris. Flounder is a common fish species in fjords and estuaries in Norway, especially in brackish water, and they frequently ascend rivers and migrate long distances upstream (Pethon 1985). Both estuaries and freshwater environments are reported as important nursery and feeding areas, especially for immature individuals (e.g., Ladiges 1935, Summers 1979, 1980). The widespread three-spined stickleback (Gasterosteus aculeatus L.) consists of both euryhaline anadromous and freshwater resident populations. Most populations of the trachurus type, which possess a complete row of lateral bony plates (Münzing 1963, Aneer 1973), is anadromous (Wootton 1976). Fish that overwinter in sea water migrate to fresh (or brackish) water environments in spring, and following breeding, back to the sea in the autumn (Wootton 1976). The leiurus type, which possess only few (or no) lateral plates in the anterior region of the body (Münzing 1963, Aneer 1973), is mainly resident in fresh water, but this is not absolute (Wootton 1976). The semiarmatus type, with lateral plates on both the pectoral and caudal part of the body, is said to be a hybrid between the two former types (Aneer 1973), and is found as both freshwater resident and anadromous populations (Wootton 1976). The ninespined stickleback (Pungitius pungitius (L.)) has a more limited distribution in Norway than the three-spined stickleback, and is more restricted to fresh and brackish water, seldom being found in pure sea water (Pethon 1985). The salinity tolerance of nine-spined stickleback is lower than that of the three-spined stickleback, but it is, however, still considerable (Heuts 1943, Nelson 1968). As with the three-spined stickleback, the salinity tolerance of the nine-spined stickleback is lowest during the breeding season when they are found only in fresh or brackish water (Wootton 1976). Some populations of Address for correspondence: A. Soleng, Zoological Museum, University of Oslo, Sars gate 1, N-0562 Oslo, Norway. Phone: ++ 47 22 85 17 62; Fax: ++ 47 22 85 18 37; E-mail: [email protected] FOLIA PARASITOLOGICA 45: 270-274, 1998 Soleng, Bakke: Susceptibility of sticklebacks and flounder to G. salaris 271 nine-spined sticklebacks also overwinter in the sea (Wootton 1976). In order to assess the possible role of flounder, threeand nine-spined sticklebacks in dispersal of G. salaris, we examined the susceptibility and resistance of these fish species to the parasite in laboratory experiments. MATERIALS AND METODS The flounders and three-spined sticklebacks were caught with a beach seine in a brackish water area (salinity 18 ‰) at the inner part of Bunnefjorden near Oslo, Norway, while the nine-spined sticklebacks were caught in a small freshwater stream draining into the same area. All fishes were caught in the end of August 1995. All three-spined sticklebacks were of the trachurus type with well developed lateral bony plates (see Münzing 1963, Aneer 1973). All fishes were gradually acclimated to active-charcoal filtered dechlorinated laboratory fresh water in grey plastic tanks (1 x 1 x 0.30 m) during a period of 24 hours. Prior to the experiment the fishes were disinfected with a 0.009% formaldehyde solution for one hour, and further acclimated for one week to the experimental conditions at 12.5°C (range 12.4-12.6°C). Twenty-one flounders (mean length 3.8 cm, range 2.9-5.5 cm; mean weight 0.9 g, range 0.3-3.5 g), 42 three-spined sticklebacks (mean length 5.9 cm, range 5.1-6.6 cm; mean weight 1.7 g, range 1.1-2.6 g) and 42 nine-spined sticklebacks (mean length 4.1 cm, range 3.4-4.8 cm; mean weight 0.5 g, range 0.3-0.7 g) were experimentally infected with G. salaris in small plastic boxes (0.38 x 0.27 x 0.12 m). This was accomplished by exposing 21 uninfected fish to four living heavily infected salmon parr and the fins from three heavily infected salmon parr per box for 24 hours. The G. salaris strain used originated from wild salmon parr caught in the River Lierelva, Buskerud County, approximately one month prior to the start of the experiments. After being infected, all fishes were individually isolated in plastic boxes (0.17 x 0.11 x 0.05 m) with mesh bottoms floating in grey plastic tanks. A total of 21 fish was used in each large fish tank which had a continuous flow of water (2 l/min). Aquarium filters (Fluval 403 without filter medium; 1200 l/hour) circulated the water. To prevent fish escapees a transparent lid was placed over the boxes, while a semitransparent lid was placed over the fish tanks as shelter. The experiments were performed under constant dim illumination, and the fishes were not fed. We used 10 hatchery reared Atlantic salmon parr (1) of the River Lierelv stock as control fish. These fish had not previously been exposed to any Gyrodactylus infection, and had been routinely disinfected against ectoparasites and fungus in the hatchery. They were experimentally infected by exposure to fins from only one heavily infected salmon parr for 24 hours. All fishes were anaesthetized with 0.04 % chlorobutanol in petri-dishes, and later examined under a stereo-microscope in laboratory water each day. The number of G. salaris was assessed by counting the parasite specimens on the skin and fins (not gills, olfactory organs, opercular and oral cavity). Anaesthesia may be stressful to the parasites. Harris et al. (1994) stressed, however, that the mortality and fecundity schedules of G. salaris derived experimentally by Jansen and Bakke (1991) are close to those occurring in natural infections, despite the complications of frequent anaesthesia. Eight three-spined sticklebacks died during the first day of the experiment, and are not included in the results. The statistical tests used are in accordance with Zar (1984), and the level of significance was 5 % (α = 0.05). All parasitological terms used follow the definitions given in Margolis et al. (1982).