Recent advances in our knowledge of the Myxozoa

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.

Cladistic analysis of myxozoan species with known alternating life-cycles

The phylogenetic relationships of 15 myxozoan taxa with known alternating life-cycles were investigated in order to provide insight into the puzzling matches between myxosporeans and actinosporeans of the myxozoan life-cycle data. Phylogenetic analyses were performed using two partitioned data-sets of life-cycle stages, myxosporean stage from fish hosts versus actinosporean stage from annelid hosts, and a combined data-set of myxosporean and actinosporean stages. A cnidarian parasite of fish, Polypodium hydriforme Ussov, 1885, was used as the outgroup. The supraspecific level grouping in the conventional classification of actinosporeans was not supported in the analysis of the partitioned data from the actinosporean phase, which yielded two equally parsimonious trees. Analysis of the partitioned data from the myxosporean phase provided 24 equally parsimonious trees and did not support the current classification of myxosporeans. The analyses of the partitioned data of myxozoans by life-cycle stage revealed a lack of taxonomic congruence between the two life-stage partitions. Two equally parsimonious trees were obtained from analysis of the combined data. The suborder Variisporina of the Myxozoa was not supported by the total evidence trees, while the monophyly of the species of Myxobolus Butschli, 1882 and of the Myxidiidae were supported. The cladograms from the combined data revealed that these myxozoan species formed four major monophyletic groups. Among them, two were supported by the partitioned data of the actinosporean phase. The phylogenetic signals and the better resolution reflected by the trees of combined data suggest that the phylogenetic total evidence approach should be employed in future studies of the systematics of myxozoans.

Evolutionary patterns in life histories of Oxyurida

The Oxyurida comprises some 850 known species that occur in the intestine of arthropods and vertebrates (one species in annelids). Important arthropod hosts include Diplopoda, Blattodea, Gryllotalpoidea, Passalidae, Scarabaeida and Hydrophilidae. The major vertebrate hosts are lizards, tortoises, primates, rodents and lagomorphs. An underlying characteristic of the group is haplodiploid reproduction and like many haplodiploid groups, pinworms tend to have life histories that involve high levels of inbreeding. Unlike Strongylida, Ascaridida and Spirurida, which have diversified in tissue site and life cycle as well as hosts, pinworms show little variation in these features and have radiated only across host groups. Two explanations are advanced for this. Haplodiploidy and its concomitant inbreeding may act to canalise evolutionary change, although diverse groups such as the Hymenoptera belie this. Alternatively, Strongylida, Ascaridida and Spirurida are presumed to have arisen from skin-penetrating ancestors that were forced to undergo a tissue migration before reaching their primitive tissue site, the gut. This migration demanded they adapt to a variety of tissue sites and thus acted as a preadaptation to further diversification. The Oxyurida, in contrast, probably arose using oral contaminative transmission. The lack of exposure to other tissue sites may therefore have relegated pinworms to their position in the posterior gut.

[Recruiting modalities of Gymnophallus nereicola J. Rebecq and G. Prévot, 1962 (Digenea, Gymnophallidae) by Nereis diversicolor (author's transl)]

Metacercariae of Gymnophallidae are generally parasites of Molluscs. Only some species have metacercariae occurring in Annelid. Gymnophallus nereicola J. Rebecq et G. Prévot, 1962 is a mediterranean species. His life cycle is characterized by only one second intermediate host, Nereis diversicolor. In consideration of this atypical host, several peculiarities are necessary for the development of G. nereicola life cycle. These features are studied in this paper. Some have relation with the parasite: large number of cercariae; behaviour; swimming trajectory; cephalic glands. Some others have relation with the second intermediate host; cercarial attraction; metacercarial microhabitat. After all, others characters are ecological features: special parasite endemiotop, nearness of first and second intermediate hosts. All these particularities are in favour of a positive recruitment of Gymnophallus nereicola.

[New marine parasitic nematode, Abos bathycola, from priapulids and the position of the fam. Marimermithidae in the class Nematoda]

A new genus and species of the marine parasitic nematode Abos bathycola gen. et sp. n. are described. The diagnosis of the new order Marimermithida with one family Marimermithidae Rubtzov et Platonova including 6 genera of marine parasitic nematodes is given. The new order resembles the order Mermithida in its extraintestinal feeding but differs from it in the following characters. The mouth, oesophagus, rectum and anus are reduced or absent. The mid-gut is transformed into trophosome, without lumen as a rule. If the lumen is present, no food is traced in the trophosome. The location of rudiments of oesophagus, chords, undeveloped trophosome, rudiments of rectum, if present, is consecutive. Stichosome is absent in contrast to the order Mermithida, in which it is well developed and is located in parallel with trophosome. Nematodes of the new genus are parasites of marine invertebrates unlike mermithids which are parasitic in land and freshwater arthrpods and sometimes in mollusks.

Localization of larvae of Metastrongylus apri (Gmelin, 1790) Vostokov, 1905, the lungworm of pigs, in the annelid Eisenia foetida Savigny, 1826

The first stage larvae of Metastrongylus apri could be recovered from the crop of Eisenia foetida after 24 hours of their infection. These were found invading the calciferous glands of the annelid as early as 48 hours post-infection. The larvae were subsequently found localized in the calciferous glands. hearts, dorsal vessel, anterior part of crop and part of the oesophagus anterior to the calciferous glands of the annelid. In general, the larvae had a preference for the circulatory vessels of the annelid so that great majority of them inhabit in the vascular system and the blood sinuses of the above mentioned organs.

[2 new species of gregarines parasites of annelides: Pleurocystis eiseniellae of Eiseniella tetraedra Sav.)Oligochaeta), Pterospora petaloprocti of Petaloproctus terricola Quatr. (Polychaeta)]

The first species belongs to the genus Pleurocystis (in the Oligocheta host, Eiseniella tetraedra). It is characterized by early syzygies with lateral connection and navicular sporocysts with similar poles. It differs from the only species already described of the genus and is named Pleurocystis eiseniellae n. sp. The second one is related to the genus Pterospora (in the Polychaeta host, Petaloproctus terricola) but differs from the three species already known by its size and number of terminal digitations in specimens always found in syzygy. The sporocysts with dissimilar poles (one rounded, the other neck-shaped) show a loose epispore. This Gregarine in named Pterospora petaloprocti n. sp.