Helminth parasites of freshwater fishes from Cuatro Ciénegas,Coahuila, in the Chihuahuan Desert of Mexico: inventory and biogeographical implications

As part of an ongoing inventory of the helminth parasites of freshwater fishes in Mexico, 570 individual fish were collected between Apr 2008 and Oct 2011 in 26 localities along the Cuatro Ciénegas region in Coahuila State, northern Mexico. Seventeen species of hosts, mostly corresponding to Nearctic freshwater elements, were studied. A total of 8324 individual worms were collected during this survey, representing 25 species of helminths, of which 9 were digeneans, 3 monogeneans, 3 acanthocephalans, 9 nematodes and 1 cestode. Most of the records in this checklist represent new host or locality records. The information provided in this checklist may be helpful for our understanding of the biodiversity and historical biogeography of this host–parasite system, because in the Cuatro Ciénegas region occur a Nearctic freshwater fish fauna, along with Neotropical and endemic elements, and from a biogeographical point of view, this may represent a transitional area.     

Toxicity of aqueous aluminium to the ectoparasitic monogenean Gyrodactylus salaris

Both laboratory and field experiments have demonstrated that aqueous aluminium can act as a paraciticide to the monogenean Gyrodactylus salaris Malmberg, 1957 infecting freshwater salmonids. However, the reproductive conditions of gyrodactylids surviving to cessation of an aluminium exposure is unknown. Therefore, Atlantic salmon (Salmo salar L.) parr infected with G. salaris was experimentally exposed to elevated concentrations of aqueous aluminium for more than 1 month. During this period, the infection increased the first week before it peaked and started to steadily decline approaching elimination. When almost all parasite specimens were eliminated, the water quality was adjusted to normal aluminium-poor freshwater. During the next 3 weeks, the surviving G. salaris were shown to clearly resume their reproduction. The experiment demonstrates the potential of the gyrodactylids to reproduce after cessation of an aluminium exposure. Thus, the as-yet unknown mechanism behind the toxic effects of aluminium on G. salaris survival and reproduction might to some extent be reversible. The present study gives further support for this metal as a parasiticide to G. salaris but demonstrates at the same time that in order to eliminate the parasites totally, the aluminium treatment must eradicate all G. salaris from the skin of the infected hosts. Furthermore, the potential development of resistance to aluminium should also be studied.     

Infection dynamics of Neoheterobothrium hirame (Monogenea) on juvenile olive flounder, Paralichthys olivaceus (Temminck & Schlegel), in coastal waters of Japan

A recent epidemic of the invasive monogenean Neoheterobothrium hirame is suspected to have caused the significant decline of the commercial catch of olive flounder, Paralichthys olivaceus, in Japan. To clarify the causal link between parasite invasion and host depletion, we monitored the infections of N. hirame on juvenile flounder for 2 years at two widely separate bays, Obama Bay, experiencing a devastating reduction in flounder catches, and Miyako Bay with a stable catch. Prevalence, mean intensity and abundance of developmental stages of N. hirame were compared between the bays and between fish year classes. Fishes captured at Obama Bay harboured three times more worms than those at Miyako Bay and had a significantly higher overall prevalence of infection. In addition, there was a negative correlation between parasite intensity and host condition factor at Obama Bay, suggesting a causal link between the N. hirame epidemic and the reduction of local flounder populations.     

A Checklist of Metazoan Parasites from Rainbow Trout (Oncorhynchus mykiss)

An extensive literature survey on metazoan parasites from rainbow trout Oncorhynchus mykiss has been conducted. The taxa Monogenea, Cestoda, Digenea, Nematoda, Acanthocephala, Crustacea and Hirudinea are covered. A total of 169 taxonomic entities are recorded in rainbow trout worldwide although few of these may prove synonyms in future analyses of the parasite specimens. These records include Monogenea (15), Cestoda (27), Digenea (37), Nematoda (39), Acanthocephala (23), Crustacea (17), Mollusca (6) and Hirudinea (5). The large number of parasites in this salmonid reflects its cosmopolitan distribution.     

Epizootics of metazoan gill parasites did not threaten feasibility of farming southern bluefin tuna (Thunnus maccoyii) in a trial extending over summer months

Tuna farming off Port Lincoln, Australia, involves catching wild 2–4-year-old southern bluefin tuna in summer and then fattening for periods of 2–8 months. As fresh product is not available year-round, the feasibility of maintaining tuna for longer periods was trialled, including over a summer season, when temperatures may exceed 24 °C. As the rates of growth and reproduction in ectoparasites of fishes are usually most rapid during warm temperatures, parasite epizootics at this time may adversely affect the health of tuna. We collected epidemiological data on burdens of metazoans on the gills of tuna from the time of stocking in April 2005 through to final harvest in August 2006 (N = 220). We document an epizootic of the copepod Pseudocycnus appendiculatus, characterised by a significant increase in both prevalence and mean intensity in the first winter, followed by a decline in these parameters over the next 12 months. This epizootic pattern appears to be independent of seasonal changes in temperature. For two other species, a second copepod (Euryphorus brachypterus) and a polyopisthocotylean flatworm (Hexostoma thynni), there were no clearly discernible trends in infections. As the high water temperatures over the summer period did not lead to increased infections of any species of gill parasites, we conclude that they do not threaten the feasibility of farming of Thunnus maccoyii.     

Comparative analyses within Gyrodactylus (Platyhelminthes: Monogenea) mitochondrial genomes and conserved polymerase chain reaction primers for gyrodactylid mitochondrial DNA

In this study, we describe the complete mitochondrial genomes of Gyrodactylus brachymystacis and Gyrodactylus parvae infecting rainbow trout (Oncorhynchus mykiss) and the invasive topmouth gudgeon (Pseudorasbora parva), respectively. The two circular genomes have a common genome organization found in other Gyrodactylus species. Comparative analyses of mitochondrial genomes from six Gyrodactylus species were carried out to determine base composition, codon usage, transfer RNA and ribosomal RNA genes, major non‐coding regions, and nucleotide diversity within the genus. We also provide the first universal models of the secondary structures of rrnS and rrnL for this group thereby promoting utilization of these genetic markers. Universal primers provided herein can be used to obtain more mitochondrial information for pathogen identification and may reveal different levels of molecular phylogenetic inferences for this lineage.     

Metazoan parasites on gills of Southern Bluefin Tuna (Thunnus maccoyii) do not rapidly proliferate after transfer to sea cages

A relatively new and highly valuable aquaculture industry focuses on three species of bluefin tunas, which are captured from the wild and fattened for several months in sea cages. In teleost aquaculture, mortalities and extra production costs are very commonly associated with metazoan ectoparasites. In tuna, however, the production value lost due to diseases associated with ectoparasites is unknown. We collected epidemiological data on burdens of metazoans on the gills of farmed southern bluefin, Thunnus maccoyii, in a series of monthly samples of tuna from the time of stocking through to harvest (March to August, 2004; N = 210) in five sea cages on a farm off Port Lincoln, Australia. Three species were recorded; for one (a copepod, Pseudocycnus appendiculatus), there was a gradual, significant increase in both abundance (from a mean of 0.1 in March, to 3.83 in August) and prevalence (from 10% to 67.5%). For the other two species (a second copepod, Euryphorus brachypterus, and a polyopisthocotylean flatworm, Hexostoma thynni) there were no discernible trends in prevalences and abundances. These results contrast markedly with those of other intensively cultured species of finfishes, in which parasite epizootics are frequent. This finding may indicate that despite the stresses of captivity, tuna mount a robust immune response to ectoparasites; the relatively low stocking densities at which tuna are farmed may facilitate this. The fall in water temperature during farming (22 °C to 13 °C) may also reduce the reproductive rate of these ectoparasites.     

Effects of temperature, salinity, desiccation and chemical treatments on egg embryonation and hatching success of Benedenia seriolae (Monogenea: Capsalidae), a parasite of farmed Seriola spp

The effects of temperature and salinity on the embryonation period and hatching success of eggs of Benedenia seriolae were investigated. Temperature strongly influenced embryonation period; eggs first hatched 5 days after laying at 28 degrees C and 16 days after laying at 14 degrees C. The relationship between temperature and embryonation period is described by quadratic regression equations for time to first and last hatching. Hatching success was >70% for B. seriolae eggs incubated at temperatures from 14 to 28 degrees C. However, no B. seriolae eggs embryonated and hatched at 30 degrees C and <2% of eggs hatched when incubated at 24 degrees C after transfer to 30 degrees C for 48 h. Embryonation period was similar for eggs incubated in sea water at 25, 30 and 35 per thousand salinity, but increased for eggs incubated at higher or lower salinities. When incubated at salinities ranging from 25 to 45 per thousand, more than 70% of B. seriolae eggs embryonated and hatched. Hatching success was lower at 20 and 50 per thousand salinity and few or no eggs hatched at 10 and 15 per thousand. Hatching of B. seriolae eggs can be prevented by desiccation for 3 min, by immersion in water at 50 degrees C for 30 s or by treatment with 25% ethanol for 3 min.     

Recruitment and effects of Discocotyle sagittata (Monogenea) infection on farmed trout

Farmed rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta were monitored over 3 years for infection with the blood-feeding gill fluke Discocotyle sagittata. Parasite transmission is seasonal: new infections take place during summer/autumn, and transmission is generally negligible during winter/spring. There are 2 sources of infection for naïve fish-of-the-year: limited invasion when fish are in the raceways by riverborne larvae originating external to the farm; and internally, within the farm, when 0+ fish are transferred to ponds previously occupied by older cohorts of infected fish. Thereafter, infection levels continue to increase in rainbow trout primarily through transmission within the farm. Prevalence rose to 100% in 1+ fish by the end of their second summer. In O. mykiss, mean abundance reached 194 worms/host for 1+ fish (up to 489 worms/host) and 160 worms/host for 2+ fish. By contrast, in S. trutta, parasite prevalence never exceeded 85% and, after the first year's invasions, infection levels decreased over time: in 1+ and 2+ brown trout, parasite mean abundance was < 4 (maximum 15) worms/host. We present evidence of the detrimental effects of D. sagittata on the host: high burdens are associated with pale gills, decreased body condition and host mortality. Parasite burdens become overdispersed during the warmer part of the year, as prevalence and mean abundance increase. However, the degree of parasite overdispersion decreases over winter; we cannot distinguish whether decreased aggregation is due to parasite losses from infected fish (including immune-mediated parasite mortality) or parasite-induced host mortality.     

Polystomatidae (Monogenea) of southern African Anura: Polystoma channingi n.sp. parasitic in two closely related Cacosternum species

Polystoma channingi is described as a new species of polystomatid flatworm (Monogenea) parasitic in the urinary bladder of Cacosternum nanum in the Eastern Cape Province, South Africa. In a locality where C. nanum and C. boettgeri occur sympatrically the parasite has been found in both species. This finding of a Polystoma sp. in two closely related host species questions the strict host-specificity generally ascribed to the genus, but supports the hypothesis that host-specificity is determined by the oncomiracidium's ability to recognize the tadpole as a suitable host. This is the first polystome to be described from the genus Cacosternum and the ninth species of Polystoma from South Africa. The species is distinguished by its huge haptor and suckers relative to the body size. The haptor length/body length ratio of 0.38 is by far the largest for the known southern African polystomes. The prevalence of infection with P. channingi n.sp. was 25% for C. nanum and 40% for C. boettgeri, while the mean intensity of infections in adult frogs was 1.5 for C. boettgeri and 1.0 for C. nanum.