The Effect of Light on the Settlement of the Salmon Louse, Lepeophtheirus Salmonis, on Atlantic Salmon, Salmo Salar L
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Journal of Fish Diseases 2004, 27, 701–708 The effect of light on the settlement of the salmon louse, Lepeophtheirus salmonis, on Atlantic salmon, Salmo salar L. H I Browman, K Boxaspen and P Kuhn Institute of Marine Research – Austevoll, Storebø, Norway Abstract Introduction The salmon louse, Lepeophtheirus salmonis,isan The salmon louse, Lepeophtheirus salmonis,isan ectoparasitic copepod that infests both wild and ectoparasitic copepod that infests both wild and farmed salmonid fish. Salmon lice are a major dis- farmed salmonid fish, mainly of the genera Salmo, ease problem in the farming of Atlantic salmon, Salvelinus and Oncorhynchus (Pike & Wadsworth Salmo salar L., and the possibility of salmon lice 1999). Salmon lice are a major disease problem in the playing a role in the decline of wild anadromous farming of Atlantic salmon, Salmo salar L., and they stocks has also been raised. Lepeophtheirus salmonis have been implicated in the decline of some wild can detect a range of stimuli (pressure/moving anadromous stocks (e.g. Birkeland 1996; Finstad, water, chemicals and light) in the external envi- Bjørn, Grimnes & Hvidsten 2000). Lepeophtheirus ronment. However, the response thresholds to salmonis hatch as nauplius I larvae from egg strings various stimuli, and the spatial and temporal scales carried by adult females (which are attached to the over which they operate in the context of host host) and immediately commence a free-swimming location, are largely unknown. In this context, we planktonic lifestyle. The speciesÕ life cycle consists of attempted to determine whether salmon lice cope- several larval stages – two naupliar, one copepodid podids settle onto hosts more effectively, or at dif- and four chalimus – before passing through two pre- ferent locations on the fish’s body, under different adult stages, which culminate in male and female qualities of light. Lice settlement trials were con- host-resident adults (Johnson & Albright 1991). The ducted under three lighting conditions; L1: unpo- copepodid is the primary infective stage; the sole larized under ultraviolet A (UVA)-through visible; purpose of this free-living larval form is to locate and L2: unpolarized without UVA (control); L3: 100% attach to a suitable host. In recent years, the search linearly polarized without UVA. A dark control was for effective and long-term solutions to the problems also conducted. No statistically significant differ- caused by salmon lice – and other parasites of fish – ence in lice settlement was found. While changes in has turned from delousing treatments to improving light intensity are involved in host detection at our knowledge of their biology. One aspect of this spatial scales on the order of metres, the results work focuses on the host-associated stimuli that presented here suggest that it is not the primary parasites might use to locate and discriminate a sensory modality underlying host location at smaller compatible host (e.g. Buchmann & Nielsen 1999; spatial scales (cm to mm). Novales Flamarique, Browman, Be´langer & Box- aspen 2000; Buchmann & Lindenstrøm 2002; Haas, Keywords: Lepeophtheirus salmonis, parasite host- Stiegeler, Keating, Kullmann, Rabenau, Scho- finding, polarization vision, spectral reflectance, namsgruber & Haberl 2002; Haas 2003; Luntz ultraviolet vision, vision. 2003; Mikheev, Pasternak & Valtonen 2003, 2004). Lepeophtheirus salmonis can detect a range of Correspondence Dr H I Browman, Institute of Marine Research – Austevoll, N-5392 Storebø, Norway environmental and host-related stimuli – pressure/ (e-mail: [email protected]) Ó 2004 Blackwell Publishing Ltd 701 Journal of Fish Diseases 2004, 27, 701–708 H I Browman et al. Effect of light on salmon louse settlement moving water, light, salinity, temperature and spectral sensitivity of the L. salmonis retina is semiochemicals (Wootten, Smith & Needham unknown (but see spectral response results pre- 1982; Bron, Sommerville & Rae 1993; Heuch & sented by Novales Flamarique et al. 2000). Numer- Karlsen 1997; Devine, Ingvarsdottir, Mordue, Pike, ous investigators have observed positive phototaxis Pickett, Duce & Mordue 2000; Novales Flamari- behaviour in salmon lice and strong responses to que et al. 2000; Ingvarsdo´ttir, Birkett, Duce, Mor- both shadows and flashes of light (Wootten et al. due, Pickett, Wadhams & Mordue (Luntz) 2002a; 1982; Bron et al. 1993; Bron & Sommerville 1998; Ingvarsdo´ttir, Birkett, Duce, Genna, Mordue, Aarseth & Schram 1999; Novales Flamarique et al. Pickett, Wadhams & Mordue (Luntz) 2002b; 2000). Luntz 2003). However, the response thresholds to The morphological basis (orthogonal microvilli) these stimuli, and the spatial and temporal scales also exists for the eye of L. salmonis to act as a over which they operate in the context of host polarization (POL) detector, as has been reported location, are largely unknown. The following is a for other invertebrates with virtually identical eye brief overview of behavioural studies on salmon lice structure (Bron & Sommerville 1998; Wehner that are related to host location. 1997). Further, the arrangement of the tapetal cells Heuch & Karlsen (1997) observed that copepo- behind the louse ocelli (Land 1981; Bron & dids (the infective stage) are sensitive to hydro- Sommerville 1998) could improve the efficiency dynamic cues. Vibrations of 3 Hz produced the of the POL detection system (Novales Flamarique ) greatest response: swimming speeds of 9 cm s 1 & Hawryshyn 1998). In many invertebrates, and ) compared with a background speed of 2 cm s 1. some vertebrates, polarization vision is used (in a This can be interpreted as a mechanism that would manner analogous to colour vision) for object facilitate contact with a potential host fish swimming recognition, signal detection and discrimination, nearby (Hevrøy, Boxaspen, Oppedal, Taranger & contrast enhancement and camouflage breaking (see Holm 2003). Cronin, Shashar, Caldwell, Marshall, Cherokee & The response of L. salmonis to host-derived Chiou 2003; Horva´th & Varju´ 2004). The scales chemical stimuli appears to vary according to life on the sides of salmon are highly reflective across stage. Adult male lice increased their swimming the visible spectrum (Fig. 1). Thus, it is possible activity, and were attracted to, salmon conditioned that they produce a distinct POL reflection – water (SCW) (Ingvarsdo´ttir et al. 2002a,b). It different from the background underwater POL should be noted, however, that lice were given field – that could serve as a host detection cue for 10 min to orient in these Y-maze trials; too long to the louse (sensu Shashar, Hagan, Boal & Hanlon be considered relevant in the context of host 2000; Land 1991; Rowe & Denton 1997; Cronin location, as a louse would never have more than a et al. 2003). In an analogous manner, ultraviolet A few seconds to locate and attach to a salmon swimming nearby. Copepodids, however, did not respond to numerous host chemical sources, inclu- ding bile, blood, faeces/urine, mucus and skin (Bron et al. 1993). Nonetheless, in one way or another, fish parasites do generally respond prefer- entially to host-specific chemicals (e.g. Haas et al. 2002; Buchmann & Nielsen 1999; Buchmann & Lindenstrøm 2002; Luntz 2003). The structure of the L. salmonis eye suggests that it is important for the copepodid and/or that it plays a major role in later developmental stages. The optic photoreceptor of the copepodid is comprised of a median nauplius eye consisting of two lensed dorsolateral ocelli and a single unlensed ventral ocellus. Each dorsolateral ocellus has a slightly Figure 1 Spectral reflectance (280–800 nm) of the Atlantic salmon body surface from the ventral (along the lateral line larger maximum transverse diameter than other groove between the pectoral and pelvic fins), dorsal (in front of species reported and, thus, has a far higher eye to the dorsal fin), and caudal areas. A perfect reflector would body length ratio (Bron & Sommerville 1998). The generate a reflectance of 1.0 at any given wavelength. Ó 2004 Blackwell Publishing Ltd 702 Journal of Fish Diseases 2004, 27, 701–708 H I Browman et al. Effect of light on salmon louse settlement (UVA, 320–400 nm) radiation is thought to Experiments conducted in an analogous manner by enhance target contrast (e.g. Browman, Novales- Glover, Hamre, Skaala & Nilsen (2004), concluded Flamarique & Hawryshyn 1994, and see Discussion that normalizing lice abundance by fish surface area in Johnsen & Widder 2001). Thus, the UVA is a suitable method of removing fish size bias reflection pattern of salmon (Fig. 1) may also among groups challenged with lice. Thus, despite improve host location/settlement, if the louse can the difference in fish size, we could still compare perceive UVA radiation. settlement in our two experiments. In the work reported here, we tested whether lighting conditions – including POL and UVA – Infection protocol affect the overall settlement, and/or location of settlement, of infective stage L. salmonis (copepod- The infection protocol used in these experiments ids) on salmon in experimental tanks. conforms to that used in other studies of settlement by L. salmonis (Glover et al. 2004 and references cited therein). Round tanks (1.5 m diameter, filled Materials and methods with 1582 L of water) were chosen to avoid the possibility of still water refuges forming in corners Collection of Lepeophtheirus salmonis where copepodids could aggregate. Salmon were Adult L. salmonis with egg strings were collected allowed at least 2 days to acclimatize (at 8 °C) in from live salmon maintained in sea cages, or from the tanks before beginning the infection experi- ) salmon at the slaughterhouse. Adult lice were ment. The water supply (30 L min 1) was transported to the laboratory where egg strings turned off 5 min before introducing infective-stage were separated from the female using a scalpel. L. salmonis copepodids. During this time, the water Detached egg strings were placed in a hatching level in the tank was lowered to 1/3 that of full container (a 100 lm sieve of 50 cm diameter) volume and the water inflow was shut off. Even at suspended in a running water bath at 8 °C and this reduced water level, there remained enough under a 24 h light photoperiod.