Interspecific comparison of experimental and natural routes of Lepeophtheirus salmonis and Caligus elongatus challenge and consequences for distribution of chalimus on salmonids and therapeutant screening

Although the gills have been reported as a site for settlement by sea lice attached stages in experimental challenges, the prevalence and abundance of infection on gills have not been reported in farmed fish and have been reported only rarely in wild salmonids. Disproportionate attachment to the gills may be significant in evaluation of new therapeutants and this study asks whether the efficacy data from studies of therapeutants based on experimental challenge can be extrapolated to the fish farm arena. The distribution of chalimus on gills, body and fins of farmed first sea year Atlantic salmon were examined in two Scottish sea lochs, each on two occasions (30 fish on each sampling date), fish from one farm with infection with Lepeophtheirus salmonis only and the other farm with exclusively Caligus elongatus. The dorsal fin was the primary attachment site of L. salmonis (50% and 33% in two samples respectively) with 29% and 32% on other fins and 20% and 29% on the body and head. Only one of 56 chalimus (1.8%) and four of 75 chalimus (5.3%) were attached to the gills. In contrast, the chalimus distribution of C. elongatus was predominantly on fins other than the dorsal, including the tail, 74% and 71%, on two sampling occasions, respectively, with only 4.2% and 3.6% of chalimus located on the dorsal fin. The pectoral fins were the most common location with 32%, followed by the tail 23%, pelvic fins 14% and only 3% on the anal fin. None of 287 and 111 chalimus in two samples, respectively, of C. elongatus was attached to the gills. The low abundance of lice on the gills indicated that non‐destructive sampling methods adequately represented lice infection. In three experimental challenges with L. salmonis, the gills were a major attachment location with 19.9–36.3% of chalimus, 9.9–26.9% on the body, 22–27% on the dorsal fin and 19–23% on the other fins. These differences with natural challenge in L. salmonis reported here and also in the literature suggest that the importance of the gills for chalimus location in the experimental model is atypical and an artefact of reduced host swimming speed in tanks that permits copepodids to attach to the gills. In natural infection, the flow over the gills is high and reduced flow is required for copepodid settlement. Greatest reductions (96%) in chalimus bath treated with cypermethrin occurred on the gills with no significant reductions in those located on the dorsal and other fins. It is possible that the efficacy of therapeutants may be higher in chalimus attached to the gills because of the higher concentration encountered of bath therapeutant and extrapolation of efficacy to farms may be misleading and progression to field testing is essential.     

Experiments on the infection dynamics of Caligus epidemicus (Copepoda: Caligidae) on the small marine fish, Ambassis marianus (Günther)

Abstract Copepodids of Caligus epidemicus survived for up to 7, 8 and 5 days at 19, 22 and 26 °C, respectively, without food. Activity was high up to 3 days and then declined, but 3- or 4-day-old copepodids were more infective than younger copepodids, suggesting an initial dispersal phase. Although the success of attachment to perchlets, Ambassis marianus , was higher in small volumes of water, there was no statistically significant association between water volume (1, 4, 60 and 2001) and success of attachment. Most copepodids and early chalimus stages in single and multiple infections were attached to the pectoral fins, but did not settle preferentially beside other copepodids on the skin of the host. The parasite was overdispersed (negative binomial distribution) when groups of fish were exposed to 100 copepodids. Perchlets were killed when exposed to large numbers of copepodids and adult C. epidemicus , but survived for at least 5 days after exposure to 25 or fewer copepodids and five or fewer adults (although not necessarily all attached).     

The histopathology associated with the juvenile stages of Lepeophtheirus salmonis on the Atlantic salmon, Salmo salar L.

Abstract. Atlantic salmon infected with juvenile stages of Lepeophtheirus salmonis were obtained from sea loch cage sites on the West coast of Scotland. Attached parasites on the skin were studied using light and electron microscopy. Behavioural activity was observed in attached stages maintained on skin in vitro. Pathological changes were associated with four main areas of interaction between the parasite and the host, namely second antennal attachment, filament attachment, maxilliped activity and feeding activity. The skin changes involved were initial mechanical disruption followed by epidermal hyperplasia. The filament material was secreted into the epidermis and spread laterally without breaching the basement membrane. Surprisingly little reaction to the filament occurred, but chalimus activity resulted in chronic changes which were more marked as the later chalimus stages were reached. Fibroplasia was a feature of lesions induced by redundant filaments resulting in small nodular lesions. After erosion of these nodules, all that remained were the melanized rings characteristic of old chalimus sites.     

The duration of efficacy following oral treatment with emamectin benzoate against infestations of sea lice,Lepeophtheirus salmonis (Krøyer), in Atlantic salmon Salmo salar L.

The duration of efficacy of emamectin benzoate in the oral treatment of sea lice, Lepeophtheirus salmonis, infesting Atlantic salmon, Salmo salar L., was evaluated in a tank study. One group of salmon was treated at a nominal dose of 50 μg kg?1 biomass day?1 for 7 consecutive days and a second group was untreated. Fish were then redistributed to 16 tanks, each holding 17 control and 17 treated fish. On days 34, 41, 48, 55, 62, 69, 76 and 83, two tanks were challenged with L. salmonis copepodites. Eight to 14 days after each challenge, fish were anaesthetized and numbers of lice recorded. Treatment with emamectin benzoate prevented development of copepodites for up to 62 days from the start of treatment, and chalimus numbers remained low for 69 days. Treated fish, challenged from days 34 to 69, had significantly (P<0.01) fewer lice than control fish. Treated fish challenged at days 76 and 83 still had fewer lice than control groups, although differences were not statistically significant for both replicates. When chalimus appeared on treated fish challenged at days 69–83, survival of chalimus to adult stages was lower than on control fish. Louse egg production on treated fish challenged at days 62–83 was not reduced compared to control groups.     

Effects of baffles and isolation on dorsal fin erosion in steelhead trout, Oncorhynchus mykiss (Walbaum)

Abstract. Dorsal fin condition was evaluated in two groups of hatchery-reared steelhead trout, Oncorhynchus mykiss (Walbaum), and compared to wild fish. Hatchery fish were reared either in nursery tanks with baffles or in isolation (1 fish/rearing space). Evaluations were conducted to determine differences in dorsal fin condition between these groups and to monitor the progression and severity of fin damage. Isolated hatchery and wild steelhead trout maintained perfect dorsal fins with characteristic white margins. Hatchery fish reared in nursery tanks with baffles experienced dorsal fin erosion that started when the fish were about 40mm total length (30 days after being on feed). This erosion continued until dorsal fin length averaged only 2·3 mm in fish 161–200 mm total length in contrast to an average dorsal fin length of 20·9mm in wild fish of the same size.     

Efficacy of teflubenzuron (Calicide®) for the treatment of sea lice, Lepeophtheirus salmonis (Krøyer 1838), infestations of farmed Atlantic salmon (Salmo salar L.)

The efficacy of teflubenzuron (Calicide^®) for the treatment of farmed Atlantic salmon (Salmo salar L.) infested with sea lice [Lepeophtheirus salmonis (Krøyer)] was investigated on two commercial salmon farms. Teflubenzuron (Calicide^®) coated onto commercial feed pellets was administered orally at 10 mg per kg body weight per day for seven consecutive days. Fish were randomly sampled and lice numbers recorded from both treated and control groups on four sampling occasions post‐medication. Statistically significant reductions in the number of L. salmonis per fish were recorded. Maximum efficacy was observed toward chalimus and preadult stages of L. salmonis, and was achieved about 7 days post‐medication. No apparent effect was observed on adult lice. No adverse drug reactions or palatability problems were associated with the treatments.     

Spraguea (Microsporida: Spraguidae) infections in the nervous system of the Japanese anglerfish, Lophius litulon (Jordan), with comments on transmission routes and host pathology

Anglerfish from the genus Lophius are a globally important commercial fishery. The microsporidian Spraguea infects the nervous system of these fish resulting in the formation of large, visible parasitic xenomas. Lophius litulon from Japan were investigated to evaluate the intensity and distribution of Spraguea xenomas throughout the nervous system and to assess pathogenicity to the host and possible transmission routes of the parasite. Spraguea infections in L. litulon had a high prevalence; all fish over 403 mm in standard length being infected, with larger fish usually more heavily infected than smaller fish. Seventy percent of all fish examined had some gross visible sign of infection. The initial site of development is the supramedullary cells on the dorsal surface of the medulla oblongata, where all infected fish have parasitic xenomas. As the disease progresses, a number of secondary sites typically become infected such as the spinal, trigeminal and vagus nerves. Fish with infection in the vagus nerve bundles often have simultaneous sites of infection, in particular the spinal nerves and along the ventral nerve towards the urinary bladder. Advanced vagus nerve infections sometimes form xenomas adjacent to kidney tissue. Spraguea DNA was amplified from the contents of the urinary bladders of two fish, suggesting that microsporidian spores may be excreted in the urine. We conclude that supramedullary cells on the hindbrain are the primary site of infection, which is probably initiated at the cutaneous mucous glands where supramedullary cells are known to extend their peripheral axons. The prevalence of Spraguea infections in L. litulon was very high, and infections often extremely heavy; however, no associated pathogenicity was observed, and heavily infected fish were otherwise normal.     

Histopathology caused by Caligus epidemicus Hewitt (Copepoda: Caligidae) on captive Acanthopagrus australis (Günther) (Pisces: Sparidae)

Abstract. Highest densities of copepodids (≤5.3 cm ^-2), chalimus (≤8.5 cm ^-2) and redundant filaments (≤6.7 cm^-2) of Caligus epidemicus were found on the anterodorsal and anteroventral flanks, head and pectoral fins of Acanthopagrus australis (Günther). Copepodids eroded the epidermis and usually attached their frontal filament to the basement membrane on the inter-ray region of fins, as well as the fin rays or scales. Chalimus stages were associated with a healed epidermis and little tissue response. The most extensive tissue response was associated with the redundant frontal filament, and was characterized by infiltrating macrophages, multinucleated giant cells, and epidermal and fibroblast proliferation. No significant pathology could be attributed to either pre-adult or adult copepods.     

A comparison of epidemiological patterns of salmon lice, Lepeophtheirus salmonis, infections on farmed Atlantic salmon, Salmo salar L., in Norway and Scotland

This paper examines two large national data sets collected over several years and contrasts the patterns of sea lice, Lepeophtheirus salmonis (Krøyer, 1837), infections, the use of treatments and the occurrence of chalimus peaks between Norwegian and Scottish farms. Infection levels in Scotland were significantly higher in general over the period under study. For the chalimus stage group in the first quarter of the year, Norwegian mean abundance stayed below 10 lice per m² while Scottish means reached 45 lice per m² of fish skin per m³ of water. Both countries had more chalimus in summer than at other times of year, but in the last 3 months of the year Scottish fish had, on average, two to four times as many chalimus as Norwegian fish. Peaks of chalimus abundance were more frequent in Scotland, particularly in winter, but the most prominent peaks occurred in summer in both countries. In Scotland a marked mid-year build-up of mobile pre-adult and adult stages was seen, and both countries showed a tendency for mobile counts on the second year fish to increase towards the end of the year. Scottish fish carried, on average, three times as many mobile lice per m² of skin as Norwegian fish in the last 3 months of the year. The difference in lice loads was reflected in the greater use of veterinary medicines on Scottish farms. The higher infection levels in Scotland may be due to shallower and more enclosed water bodies used for farming, smaller and shallower pens, differences in sea water temperatures or in access to appropriate medication. The results highlight the importance of ensuring that effective veterinary medicines are available in the UK for the control of infection.     

Induced breeding and larval rearing of the endangered Australian freshwater fish trout cod, Maccullochella macquariensis (Cuvier) (Percichthyidae)

Abstract. The endangered Australian freshwater fish trout cod, Maccullochella macquariensis (Cuvier), was the subject of a captive breeding programme to produce fry for reintroduction into the wild. Trout cod broodfish were maintained in earthen ponds for up to 5 years and underwent gonadal maturation each spring but did not spawn in the ponds. Infestations of the protozoan parasite Chilodonella hexasticha caused the death of at least 21 broodfish. Mature fish, removed from ponds when water temperatures had reached or exceeded 16°C, had a higher proportion of atretic oocytes and fewer fish spawned successfully compared to fish removed at lower temperatures. Ovulation was induced by a single injection of 1000–3000 iu/kg HCG. Between 1188 and 11338 eggs ranging from 2·5 to 3·6 mm in diameter were stripped from individual fish. Hatching commenced on days 5–9 and continued for up to 10 days (at 15·5–23°C). Larvae commenced feeding on days 21–25. Trout cod larvae were grown out to fry (363.3–48.6mm total length [TL]) in fertilized fry rearing ponds then released. Between 1986 and 1989, 8420 trout cod fry were released into several sites in the upper Murray River and upper Murrumbidgee River, and reports indicate that released fish are surviving.     

Pleistophora infestation in fathead minnows, Pimephales promelas (Rafinesque)

Pleistophora infestation was observed in adult fathead minnows, Pimephales promelas, held under laboratory conditions. Fish were clinically healthy, and presented no gross findings at necropsy. Histopathology revealed parasitic stages only in the ovaries. Spores within sporophorous vesicles were mainly encountered in late vitellogenic oocytes and were ultrastructurally identified as a microsporidian parasite. Heavily parasitized oocytes underwent degeneration followed by the release of spores into the ovarian interstitium. Degenerating oocytes and interstitial spores caused ovarian inflammation. Male fish showed no parasites in the testes. Parasitic infestation was compared with body length, body weight, gonadal weight, gonadosomatic index and plasma vitellogenin levels, and revealed no statistically significant differences between non-parasitized and parasitized females. The isolated holding conditions of the fish and the presence of parasitic stages in the ovaries suggested that an infestation with Pleistophora ovariaeSummerfelt, 1964 was more probable than that with Pleistophora mirandellae (Vaney & Conte, 1901).     

The efficacy of emamectin benzoate as an oral treatment of sea lice, Lepeophtheirus salmonis (KrÒyer), infestations in Atlantic salmon, Salmo salar L.

The efficacy of emamectin benzoate as an oral treatment of sea lice, Lepeophtheirus salmonis (KrÒyer), infestations in Atlantic salmon, Salmo salar L., was evaluated in a dose titration study and two dose confirmation studies. Replicated groups of salmon with induced infestations of sea lice were given emamectin benzoate on pelleted feed at doses of 0, 25, 50 and 100 μg kg⁻¹ biomass day⁻¹ for seven consecutive days. Sea lice were counted at 7, 14 and 21 days from the start of treatment, and comparisons made with control fish given the same diet without emamectin benzoate. Total numbers of sea lice were significantly reduced at all doses in all three studies when compared to control fish. There was no significant difference between doses of 50 and 100 μg kg⁻¹, but the 50 μg kg⁻¹ dose resulted in significantly fewer lice than the 25 μg kg⁻¹ dose. Emamectin benzoate was highly effective in reducing numbers of preadult and adult lice and prevented the maturation of chalimus to motile stages. The optimum therapeutic dose was selected as 50 μg kg⁻¹ day⁻¹ for seven days. Treatment reduced the incidence of epidermal damage by sea lice and, in one study, survival of treated fish was 48% higher than in control groups. No fish mortalities or adverse effects were attributed to treatment with emamectin benzoate at any of the doses tested.     

Distribution and structure of the population of sea lice, Lepeophtheirus salmonis Krøyer, on Atlantic salmon, Salmo salar L., under typical rearing conditions

Abstract. Samples of Atlantic salmon, Salmo salar L., held in sea cages and tanks with running water were examined for presence of sea lice, Lepeophtheirus salmonis. The number of lice, their sex stage (only post-chalimus stages) and location on the fish body were recorded. A new method has been used to express parasitic intensity in fish of different size. Effects of different factors on the degree and character of infestation (structure of the lice population), such as fish size, habitat and chemical treatment, have been studied. Different lice categories have shown different susceptibility to delousing with dichlorvos. Second pre-adults are most susceptible, and males generally more than females. Running water has been found to'flush out'the lice from the fish body. Pre-adults are flushed out easier than adults, and males generally easier than females. Adult males show high tolerance to water of lowered salinity and great mobility manifested in occasional transfer between hosts. The higher activity level of adult males may partly explain a drop in proportion of males after delousing and in running water. Frequency of mating has been found to increase with parasitic intensity.     

Potential for sea ranching rainbow trout, Oncorhynchus mykiss (Walbaum): evidence from trials in two Norwegian fjords

Abstract. In total, 10014 hatchery-reared 1+ and 2+ rainbow trout, Oncorhynchus mykiss (Walbaum), were released near the mouth of the River Imsa, south-western Norway, and in the Oslofjord, south-eastern Norway. The reported recapture rate was significantly higher in the Oslofjord(15·4%) than at Ims(3·6%). Between 70 and 80% of the recaptures were made in the year of release. The fish were recaptured in rivers and the sea along the Norwegian coast, most of them near the release sites. In total, 60% were recaptured in salt water and 40% in fresh water. Fish released in the Oslofjord were largely captured with rod and line. Of those released at Ims most of the freshwater recaptures were made in a fish trap in the River Imsa. In salt water most fish were captured by nets. Specific growth rate varied inversely with body weight at release, decreased with time after release and decreased faster for fish released in the Oslofjord than at Ims. In general, the proportion of fish recaptured increased with mean fish weight at release. Recaptured biomass per 1000 fish released was higher when released in the Oslofjord (87·12kg) than at Ims (24·42kg). Stocking of rainbow trout in fjords is feasible and may contribute to local recreational fisheries. Yield was highest when relatively small (∼50g) fish were released. Economically, the best results may be obtained when the fish are released in nutrient rich fjords.     

Macrobenthos: before, during and after a fish farm

Abstract. We have investigated the effect of a salmon farm on the local benthos and the benthic environment. The bottom fauna was examined before establishment in March 1988, during operation in September 1989 and April 1990, and after closure in March-April 1991. During this time 2417 tonnes of salmon were produced. The samples were collected from two stations within 250m of the farm site. The sediment contained mainly clay and silt, the organic content varied from 9·1 to 17·3% and there was no increase in the organic content during the sampling period.
The bottom fauna in 1988 was rich and relatively similar at both stations. The species number close to the farm declined from 65 in 1988 to only 11 in 1989. A year after the fish farm was moved, the number of species had risen to 29.
The fish farm clearly influenced the bottom fauna in the immediate vicinity of the site but no effect was found at 250 m distance. The environmental conditions had improved in 1991 after abandonment of the site, but the fauna had not returned to the status measured before operations began in 1988.     

The Irish sea trout enhancement programme: a review of the broodstock and ova production programmes

The Irish sea trout, Salmo trutta L., broodstock programme was instigated in response to the collapse of sea trout stocks in the west coast of Ireland in 1989 and 1990. Wild sea trout kelts and post-smolts were successfully reconditioned and used as broodstock to produce eyed ova for distribution to affected fisheries. From 1991 to 1999, a total of 8.2 million green ova from four separate stocks were produced. The number of females stripped increased from 34 in 1991 to a peak of 1435 in 1997. Green ova survival to the eyed ova stage ranged from 65 to 96%. The eyed ova produced were distributed to 23 affected fisheries along the west coast. The average cost per eyed ova decreased from €2.02 at the start of the programme to €0.04 at the end. The growth rates recorded for broodstock held in captivity were comparable with those recorded for wild sea trout. Relationships between fish size, egg size and number of eggs produced were examined for each stock and for each stock in each year. Significant positive relationships were found between fish size and egg number and fish size and egg size, with the exception of the first year in which fish were stripped, when a negative correlation between fish size and egg size was found.     

Life cycle of Caligus rogercresseyi, (Copepoda: Caligidae) parasite of Chilean reared salmonids

Caligus rogercresseyi, [Contrib. Zool. 69 (2000) 137] is the only caligid known to affect the salmon industry in Southern Chile. Economic losses due to reduced fish quality, cost of chemical treatment and outbreaks of other diseases such as the Piscirickettsiosis occur. The life cycle of C. rogercresseyi is described in rainbow trout reared in seawater tanks from observations made under natural conditions of light and temperature between January 1997 and April 1998. Fish were infected with laboratory-cultured larvae obtained from ovigerous females. Rainbow trout were periodically slaughtered for parasite collection and identification. C. rogercresseyi life cycle includes the following stages: two nauplius, one copepodid, four chalimus and the adult. No preadult stage was observed. Timing of the different stages of development was directly dependent on water temperature. The maturation of the eggs or the time for a complete life cycle took place at 45 days in July at 10.3 °C, 31–32 days in April at 12.4 and 12.8 °C, respectively, and at 26 days in November at 15.2 °C. In January, at 16.7 °C, only the appearance of first eggs were observed at 18 days. A simple degree–day (dd) model is proposed for each developmental stage between 4 and 17 °C, where the development rate is a linear function of the average temperature of water. Using this degree–day model, the proportion of fourth stage chalimus was maximum at 172 dd of effective temperature, adult males at 193 degree–days, adult females at 208 dd. The minimum temperature threshold is at 4.2 °C where there is no development of the parasite. The appearance of first eggs occurred at 231 dd and the first pigmented eggs at 277 dd. The temperature-independent degree–days value allowed to predict the timing of C. rogercresseyi life cycle at any temperature within the evaluated range.     

Post-haemorrhagic anaemia in sea bass,Dicentrarchus labrax (L.), caused by blood feeding of Ceratothoa oestroides (Isopoda: Cymothoidae)

The effects of the fish parasitic isopod, Ceratothoa oestroides (Risso), on haematological parameters of its cage-cultured sea bass host, Dicentrarchus labrax (L.), were studied. Analyses of blood parameters (cell counts, haemoglobin content and haematocrit) were carried out on parasitized and unparasitized sea bass from a fish farm in Turkey. Parasitized fish had significantly lowered erythrocyte counts, haematocrit and haemoglobin values and significantly increased leucocyte counts. Blood feeding by C. oestroides thus produces a post-haemorrhagic anaemia and the fish appear to mount an immune response to the presence of parasites.     

Effects of experimental infection by Caligus rogercresseyi (Copepoda: Caligidae) on the common jollytail, Galaxias maculatus (Osmeriforme: Galaxiidae) (Jenyns, 1842) in Chile

Galaxias maculatus, the common jollytail, is a native smelt fish with a lacustrine or diadromous life cycle. In Chile, the rearing cycle of this fish includes a freshwater and a marine phase. Several diseases and parasites reported for the freshwater phase could be avoided by rearing the fish in salty or brackish water. Nonetheless, this alternative could result in Caligus rogercresseyi infections. The objective of this study was to evaluate experimentally the capacity of C. rogercresseyi to infect the common jollytail and the effects this parasite may cause to the fish. Prevalence and intensity of experimental C. rogercresseyi infections on G. maculatus were estimated and the effects of this parasitosis on the fish were evaluated on fish survival, weight loss, swimming behavior, and skin damage. Two experiments were carried out with adult fish taken from the Maullín Estuary. Fish were acclimatized to a salinity of 32 ppt and divided into two groups of 20 fish each. The experimental group was infected with copepodid stages of C. rogercresseyi and the other was used as a control. The fish were kept in individual tanks until observing ovigerous female copepods. C. rogercresseyi recognizes G. maculatus as a host under experimental conditions and optimal salinity for the parasite. Results show that under experimental condition copepodid stage can successfully infect the fish, developing to the adult stage, mating, and producing eggs (females). C. rogercresseyi caused alterations in fish behavior, superficial body damage, weight loss, and mortality. Rearing the common jollytail in salty water should take into consideration measures to control these infections as they can damage fish and affect surrounding wild fish populations. There are no records of this parasite in natural common jollytail populations; however, our understanding of the ecological role played by the common jollytail in C. rogercresseyi transmissions among sympatric host species could be greatly improved through further research related to host preferences and epidemiological data for different sympatric host species.     

Nematode parasites infecting the starry batfish Halieutaea stellata (Vahl) (Lophiiformes: Ogcocephalidae) from the East and South China Sea

The starry batfish Halieutaea stellata (Vahl) is a small, benthic fish found in Indo‐West Pacific Oceans. However, our present knowledge of the helminth parasites of this fish is still fragmentary. In this study, a total of 29 fish collected from the East and South China Sea were examined to determine the prevalence, intensity and species composition of helminth parasites in H. stellata. Using morphological and molecular approaches, four species of nematodes were found parasitic in this fish host, including the adults and fourth‐stage larvae of Raphidascaroides nipponensis Yamaguti 1941; adults and third‐stage larvae of Raphidascaris lophii (Wu 1949), third‐ and fourth‐stage larvae of Hysterothylacium larval type IV‐A of Shamsi, Gasser & Beveridge 2013 and third‐stage larvae of Hysterothylacium amoyense (Hsü 1993). Halieutaea stellata represents a new host record for the three last‐named nematodes. Raphidascaroides nipponensis with the highest prevalence (82.5%) and intensity (mean = 13.5) of infection was considered as the dominant parasite species in H. stellata. The detailed morphology of the different developmental stages of the four nematode species was studied using light and scanning electron microscopy. All nematode species were also genetically characterized by sequencing and analysing the internal transcribed spacer (ITS) of the ribosomal DNA. This study provides further data on the occurrence of nematode parasites in H. stellata and also contributes to facilitate an accurate and rapid diagnosis of the infection by these little‐known nematodes.