Co-Existence of Myxobolus Spp. (Myxozoa) in Gray Mullet (Mugil Cephalus) Juveniles from the Mediterranean Sea
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Parasitology Research (2019) 118:159–167 https://doi.org/10.1007/s00436-018-6151-9 FISH PARASITOLOGY - ORIGINAL PAPER Co-existence of Myxobolus spp. (Myxozoa) in gray mullet (Mugil cephalus) juveniles from the Mediterranean Sea Galit Sharon1 & Michal Ucko1 & Ben Tamir1 & Arik Diamant1 Received: 21 June 2018 /Accepted: 12 November 2018 /Published online: 29 November 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Gray mullet (Mugilidae) occur in all seas and are farmed widely around the world, and thus, the risk of their parasites spreading through transport of aquaculture seed is a serious concern. Among others, gray mullets typically host a diversity of myxosporeans, a group in which spore morphometrics of genera has been consistently shown to be inadequate for determination of species. In this study, we investigated Myxobolus Bütschli 1882 (Myxosporea) species found in two fingerling stocks of Mugil cephalus caught in the wild off the coasts of the eastern (Israel) and western (Spain) Mediterranean. Although we observed similar morphological features, significant dissimilarities in spore size and differences in Myxobolus species SSU rDNA se- quences were noted. Genetic analyses demonstrated that multiple Myxobolus species, some with SSU rDNA sequences new to GenBank, infected the stock from Spain. In addition, Myxobolus DNAwas found associated with several types of host tissue (gill, tail, and internal organs), and sequence analyses indicated that multiple species of Myxobolus were also present, sometimes in different tissues from the same fish. The results suggest that the gray mullets supported a collection of several different Myxobolus species with similar morphology. Keywords Fish . Parasite . Myxosporea . Aquaculture Introduction have been documented from gray mullet (Eiras et al. 2005, 2007), of which 22 have been found in two or more different Mugil cephalus, gray mullet, is a catadromous, euryhaline fish host tissues (Shvedko and Aseev 2008). The genus Myxobolus widely distributed in coastal areas, commonly inhabiting es- Bütschli 1882 (Myxosporea) includes some important patho- tuaries and lower river systems (Lane et al. 2015). For centu- gens of marine and freshwater fish (Maíllo-Bellón et al. 2011), ries, it has been an important human food resource, and many of which are known to directly cause host mortality throughout the Mediterranean region, M. cephalus is farmed (Lom and Dyková 1995; Rigos et al. 1999;Brownand both intensively and semi-intensively. Culture ponds are fre- Bruno 2006). Several hundred species have been described quently stocked with wild caught gray mullet fry collected in to date (Eiras et al. 2014). Early studies and identification of shallow coastal and estuarine sites (Pruginin et al. 1975; Myxobolus species, as in other myxosporea, were typically Merella and Garippa 2001; Kim et al. 2013a). based on spore morphology as well as host and organ/tissue Gray mullets are infected by diverse parasites, including specificity. As molecular sequencing of SSU rDNA gene be- myxosporeans (Lom and Dyková 1992; Kent et al. 2001), came the accepted standard approach for differentiating be- some of which are serious pathogens (Yurakhno and tween myxosporean species with close spore morphology, this Ovcharenko 2014). To date, 26 different Myxobolus species method which is now widely used, enabling effective compar- ison of different fish tissues that may be infected with closely related species (Székely et al. 2009). Indeed, in the last Section Editor: Christopher Whipps 20 years, the taxonomy of Myxobolus has been subjected to * Galit Sharon extensive phylogenetic analyses (Liu et al. 2010). In this [email protected] study, we investigated Myxobolus spp. infections in gray mul- let fingerlings that originated from two distinct sources: (a) wild gray mullet captured as fry in a river estuary in northern 1 Israel Oceanographic and Limnological Research, National Center for Mariculture, P.O.Box 1212, 88112 Eilat, Israel Israel, and (b) wild gray mullet captured as fry in the Ebro 160 Parasitol Res (2019) 118:159–167 River Delta in Spain, and imported to Israel. These stocks, Microscopic examination of fish infected originating from the extreme west and east sides of the with Myxobolus spp. Mediterranean Sea, were introduced into the facilities of Israel Oceanographic and Limnological Research, National Fish were sedated with 50 mg/ml clove oil and euthanized by Center for Mariculture (IOLR-NCM) in Eilat, for experimen- decapitation. Fresh smears of plasmodia removed from the tal culture purposes. The Myxobolus spp. found parasitizing caudal fin were examined under a light microscope and mea- these fish are presented. sured. Air-dried smears were fixed in absolute methyl alcohol and stained with Giemsa, Hemacolor, or Gram stain. Photographs were taken with an Olympus 4040 digital camera Materials and methods mounted on phase contrast and Nomarski interference micro- scopes. For histology, infected tissue bits were fixed in 10% Fish buffered neutral formalin, dehydrated, and embedded in par- affin blocks. Five micron sections were stained with hematox- The Israeli Mediterranean (IM) and Spanish Mediterranean ylin and eosin, Gram, or Ziel–Neelsen. For transmission elec- (SM) stocks arrived in two separate batches at IOLR-NCM, tron microscopy (TEM), the plasmodia were fixed in 2.5% Eilat. Both were examined for parasites and other disease glutaraldehyde in 0.1 M sodium cacodylate buffer, dehydrated agents upon arrival. in a graded ethanol series, and embedded in Epon 812 blocks. The imported SM gray mullets, upon initially arriving Grids were stained with uranyl acetate and lead citrate and in Israel in mid-March 2015, were stocked in outdoor examined in a Jeol 100CX transmission electron microscope ponds in a fish farm in northern Israel, after which on at 60 kV. March 29, 2015, 6000 of these fish were transported to the Reproduction Department at IOLR-NCM, Eilat, for experimental purposes (Project no. 9207/11; study ap- Extraction, purification, and precipitation of DNA proved by the Animal Experimentation Ethics Committee, Agricultural Research Organization, Volcani Whole plasmodia from both IM and SM batches were stored Center, P.O. Box 6, Bet Dagan 50250, Israel). All fish at − 80 °C until processed. From SM fish no. 6, a parasite were subjected upon arrival to veterinary inspection and plasmodium was carefully removed from the caudal fin and then a routine quarantine procedure. The fish were in addition, two infected scales were removed from SM#6 and placed in 6-m3 containers with artificial aeration fed SM#7. These samples were processed for rDNA sequencing. with UV-treated flowing seawater (40 ± 0.5 ppt salinity) In addition, random organ samples from five heavily infected at a flow rate of ten water changes per day, water tem- fish from the SM batch were sampled; these included caudal perature average was 25 °C ± 1, and water quality pa- fin, intestine, spleen, liver, kidney, skin, mesentery fat, mus- rameters (dissolved oxygen, pH, salinity, and tempera- cle, and gill. From the IM batch samples were taken from the ture) were monitored constantly. A subsample of 200 caudal fin, intestine, skin, and gills. All samples were stored in fish 6.33 g avg. weight, 84 mm total length (TL), were − 80 °C until processing. examined. Some individuals displayed minute, cyst-like Plasmodia and/or tissue were ground with 300 ml of grind- lumps on their caudal fins. These fish were separated, ing buffer (100 mM Tris-HCl pH 9, 100 mM EDTA, and 1% and microscopical examination indicated presence of SDS). The homogenate was incubated for 30 min at 70 °C. parasite plasmodia. Accordingly, individual fish were Forty-two microliters of 8 M potassium acetate was then visually scored as having light (1–2), medium (3–5), added; the mixture was placed on ice for 30 min, then centri- or heavy (> 6) infection. After approximately 4 months fuged at 12,000 g for 15 min at 4 °C. To eliminate traces of on July 23, 2015, a second shipment of gray mullet pellet, the supernatant was transferred to a fresh tube and fingerlings (for the same project no. 9207/11) of ap- centrifuged again for 5 min. DNA was precipitated with 1 proximately the same size and weight was received at volume of isopropanol and left for 15 min at room tempera- NCM. This stock (IM) was comprised of 3000 wild ture. Pelleted DNA was washed twice in 70% ethanol, and the gray mullet fingerlings caught in a river estuary in air-dried pellet was dissolved in 50 μl of double-distilled H2O. northern Israel during the first week of July 2015 and DNA quantity and quality (260:280 ratios) were estimated in a kept for 2 weeks in a separate outdoor holding pond at microplate spectrophotometer (PowerWave™ XS, BioTek, the same farm where the SM batch was kept. In the IM Winooski, VT). PCRs were performed with a Programmable stock too, plasmodia were visible on fish caudal fins. Thermal Controller (PTC-100; MJ Research, San Francisco, Visibly infected fish were isolated and the number of CA) in a final volume of 50 μl containing 1 U of Taq DNA plasmodia per caudal fin was recorded and infection polymerase (Promega, Madison, WI) following Diamant et al. intensity scored as above. (2005). Parasitol Res (2019) 118:159–167 161 Amplification, visualization, and sequencing of PCR Results products Dark, spherical plasmodia typical of the genus Myxobolus The small-subunit ribosomal DNA (SSU rDNA) gene cluster (Myxosporea) were observed embedded in the dorsal and cau- was targeted for amplification using PCR previously de- dal fins of both IM and SM mullet stocks (Fig. 1a, b). A total scribed by Hallett and Diamant (2001). A combination of of 40 out of 200 examined SM fish (20%) had visibly affected primers designed on the basis of SSU rDNA sequences of fins. The intensity of infection was variable ranging from 1 to myxozoan species (ACT1f, ACT1r, MYX4r) (Hallett and 10 plasmodia/fish.