R. J. G. Lester

Recent advances in our knowledge of the Myxozoa

Abstract In the last few years two factors have helped to significantly advance our understanding of the Myxozoa. First, the phenomenal increase in fin fish aquaculture in the 1990s has lead to the increased importance of these parasites; in turn this has lead to intensified research efforts, which have increased knowledge of the development, diagnosis, and pathogenesis of myxozoans. The hallmark discovery in the 1980s that the life cycle of Myxobolus cerebralis requires development of an actinosporean stage in the oligochaete, Tubifex tubifex, led to the elucidation of the life cycles of several other myxozoans. Also, the life cycle and taxonomy of the enigmatic PKX myxozoan has been resolved: it is the alternate stage of the unusual myxozoan, Tetracapsula bryosalmonae, from bryozoans. The 18S rDNA gene of many species has been sequenced, and here we add 22 new sequences to the data set. Phylogenetic analyses using all these sequences indicate that:1) the Myxozoa are closely related to Cnidaria (also supported by morphological data); 2) marine taxa at the genus level branch separately from genera that usually infect freshwater fishes; 3) taxa cluster more by development and tissue location than by spore morphology; 4) the tetracapsulids branched off early in myxozoan evolution, perhaps reflected by their having bryozoan, rather than annelid hosts; 5) the morphology of actinosporeans offers little information for determining their myxosporean counterparts (assuming that they exist); and 6) the marine actinosporeans from Australia appear to form a clade within the platysporinid myxosporeans. Ribosomal DNA sequences have also enabled development of diagnostic tests for myxozoans. PCR and in situ hybridisation tests based on rDNA sequences have been developed for Myxobolus cerebralis, Ceratomyxa shasta, Kudoa spp., and Tetracapsula bryosalmonae (PKX). Lectin-based and antibody tests have also been developed for certain myxozoans, such as PKX and C. shasta. We also review important diseases caused by myxozoans, which are emerging or re-emerging. Epizootics of whirling disease in wild rainbow trout (Oncorhynchus mykiss) have recently been reported throughout the Rocky Mountain states of the USA. With a dramatic increase in aquaculture of fishes using marine netpens, several marine myxozoans have been recognized or elevated in status as pathological agents. Kudoa thyrsites infections have caused severe post-harvest myoliquefaction in pen-reared Atlantic salmon (Salmo salar), and Ceratomyxa spp., Sphaerospora spp., and Myxidium leei cause disease in pen-reared sea bass (Dicentrarchus labrax) and sea bream species (family Sparidae) in Mediterranean countries.

First Report of Three Kudoa Species from Eastern Australia: Kudoa thyrsites from Mahi mahi (Coryphaena hippurus), Kudoa amamiensis and Kudoa minithyrsites n. sp. from Sweeper (Pempheris ypsilychnus)

Abstract Fish species around the world are parasitized by myxozoans of the genus Kudoa, several of which infect and cause damage of commercial importance. In particular, Kudoa thyrsites and Kudoa amamiensis infect certain cultured fish species causing damage to muscle tissue, making the fish unmarketable. Kudoa thyrsites has a broad host and geographic range infecting over 35 different fish species worldwide, while K. amamiensis has only been reported from a few species in Japanese waters. Through morphological and molecular analyses we have confirmed the presence of both of these parasites in eastern Australian waters. In addition, a novel Kudoa species was identified, having stellate spores, with one polar capsule larger than the other three. The SSU rDNA sequence of this parasite was 1.5% different from K. thyrsites and is an outlier from K. thyrsites representatives in a phylogenetic analysis. Furthermore, the spores of this parasite are distinctly smaller than those of K. thyrsites, and thus it is described as Kudoa minithyrsites n. sp. Although the potential effects of K. minithyrsites n. sp. on its fish hosts are unknown, both K. thyrsites and K. amamiensis are associated with flesh quality problems in some cultured species and may be potential threats to an expanding aquaculture industry in Australia.

PARAMOEBIC GILL INFECTION AND ASSOCIATED PATHOLOGY OF ATLANTIC SALMON, SALMO SALAR, AND RAINBOW TROUT, SALMO GAIRDNERI, IN TASMANIA

ABSTRACT: Severe branchitis associated with Paramoeba infection has emerged as the major infectious disease of sea-caged salmonids in Australia. Although amoebae can be found on the gills of fish when water temperatures are in the vicinity of 10°C (winter and spring), clinical disease is associated with water temperatures of 15 to 20°C (spring, summer, and autumn) and salinities of 35 ‰. Affected fish suffer from anorexia and respiratory distress. The histopathology of the disease as well as the morphology of the causative agent are being reported. Also included are notes on the epidemiology, clinical manifestations, treatment, and control of this serious disease.

A new Perkinsus species (Apicomplexa, Perkinsea) from the abalone Haliotis ruber

A new protozoan of the genus Perkinsus is described from the muscle and hemolymph of the blacklip abalone, Haliotis ruber, from South Australia. It occurs in the muscle of the adductor and mantle, and free and in brownish masses in the hemolymph. Cells cultured in thioglycollate medium produced a prezoosporangium which stained blue-black with iodine. The parasite differs from Perkinsus marinus, the only other member of the class, in having a much larger trophozoite, an eosinophilic vacuoplast when present, a short discharge tube, and appears to be uninfective to oysters.

Reappraisal of the use of parasites for fish stock identification.

Analysis of the geographical distributions of parasites is an excellent source of information about the movement of host fish. Occasionally such analysis can also allow us to discriminate between fish populations, though unequivocal discrimination usually requires methods with a genetic basis. In assessing fish movement, parasites have two advantages over conventional tags: They can more readily detect mass migration and, because the fish only have to be caught once, the data are usually cheaper to obtain. The most important criterion in selecting a parasite to be used as a tag is its longevity in the fish; short-lived parasites give information on short-term movements of the fish, long-lived parasites on more extensive migrations.

Checklist of Parasites from Heron Island, Great Barrier Reef

The parasites found in 185 host species, including 122 fishes, at Heron Island and adjacent reefs in the Capricorn-Bunker group are listed. Parasites are identified to family or below. Many of the 580 records have not previously been published; the present location of these specimens is given.

A review of methods for estimating mortality due to parasites in wild fish populations

Six methods are described for detecting mortality due to parasitic infections in natural fish populations. They are: (a) through autopsies; (b) by determining the frequency of infections known to be eventually lethal; (c) by observing a decrease in the prevalence of a long-lived parasite (or permanent scar from a parasite) with host age; (d) by observing a decrease in the variance/mean ratio for the parasites with host age; (e) by comparing the observed frequency of a combination of two independent events with the calculated probability of their occurrence; and finally (f) by comparing the observed frequency distribution of the parasite, with a projected frequency based on data from lightly infected fish. In this technique, negative binomials are fitted to the data and truncated at various points. Some advantages and disadvantages of the different methods are given, together with examples. The methods do not necessarily provide definitive answers, but they are indicative of whether or not significant parasite-related mortality may be occurring, and in some cases provide an estimate of its probable magnitude in terms of the total host mortality rate.

Dynamics of the interaction between the parasitic isopod, anilocra pomacentri, and the coral reef fish, chromis nitida

Field and laboratory investigations into the effect of the parasitic isopod Anilocra pomacentri (Cymothoidae) on the population dynamics of the reef fish Chromis nitida (Pomacentridae) were carried out at Heron Island, Great Barrier Reef, Australia. Fish carried a single adult parasite just posterior and dorsal to the eye either to the right or to the left of the midline. The adult parasite was overdispersed among fish on patch reefs (dispersion factor, k = 0.69). Sequential field observations on a single cohort of fish showed that parasites significantly depressed growth, reproduction, and survivorship. The von Bertalanffy growth coefficients (a measure of somatic growth) were 0.10 for parasitized fish compared with 0.17 for non-parasitized fish. Female fish carrying the parasite produced only 12% of the number of eggs produced by non-parasitized fish of the same size. In the field, the mortality of infected juvenile C. nitida (LCF 15-30 mm) was estimated to be at least 88% in the first 70 days after recruitment of the fish. The mortality of uninfected recruits over the same period was 66%. In laboratory trials, the mortality associated with the infection of juvenile fish by larval parasites ranged from 78% for small fish (mean LCF 15.0 mm) to 28% for larger fish (mean LCF 24.9 mm) within 4 days of experimental infection. This is one of the few studies that evaluates the effect of a parasite on a population of fish in the field.

Stock discrimination of orange roughy, Hoplostethus atlanticus, by parasite analysis

The parasite fauna of the viscera of 1251 orange roughy,Hoplostethus atlanticus, collected in 1983 to 1986 from eight areas off southern Australia and three areas off New Zealand, was examined for evidence of discrete host populations. Fish from each area were divided into three length groups which averaged close to 28, 37, and 42 cm. Canonical multivariate analysis of data on larval nematodes (Anisakis spp.,Terranova sp., and a spirurid) and larval cestodes (Hepatoxylon trichiuri andCallitetrarhynchus sp.) discriminated five Australian and three New Zealand stocks. These were for Australia: (1) Great Australian Bight (2) South Australia/west Victoria/west and south Tasmania, (3) Cascade Plateau/Tasman Rise, (4) north-east Tasmania, (5) New South Wales; and for New Zealand: (1) north-east New Zealand, (2) south-east New Zealand, (3) west New Zealand. No significant differences in parasite fauna were detected between samples of fish taken within the spawning season and those taken outside the spawning season in the same area. In one southern Australian stock there was a north-south cline in the numbers ofAnisakis spp. This was apparent in both small (immature) and medium-sized (mature) fish. We conclude thatHoplostethus atlanticus is a sedentary species with little movement between fishmanagement zones.

Detection of the initial infective stages of the protozoan parasite Marteilia sydneyi in Saccostrea glomerata and their development through to sporogenesis

DNA probes were used in in situ hybridisation on histological sections of oysters exposed for defined intervals to Marteilia sydneyi infection to reveal the early development of the parasite in the oyster host, Saccostrea glomerata. The initial infective stages enter through the palps and gills whereupon extrasporogonic proliferation results in the liberation of cells into surrounding connective tissue and haemolymph spaces. Following systemic dissemination, the parasite infiltrates the digestive gland and becomes established as a nurse cell beneath the epithelial cells in a digestive tubule. Here, cell-within-cell proliferation results in the eventual liberation of daughter cells from the nurse cell into spaces between adjacent epithelial cells. None of these stages had previously been described. Proliferation is associated with host responses, including haemocytic infiltration of the connective tissue and diapedesis across tubule epithelia. The responses cease as sporogenesis begins.