Threats to sturgeon farming and caviar production

Poles have fallen in love with sturgeons and salmon. Our country has become the second in the European Union and the fourth in the world producer of caviar and sturgeon. Blooms of golden algae, which no one expected a few years ago, have turned our attention to lesser-known but dangerous organisms for fish. So it’s worth taking a closer look at the caviar-destroying exotics and their much more common relatives here, which infect many fish species. With the worldwide development of aquaculture and the implementation of increasingly industrial farming methods, the threat of disease will increase [1].

The left cavern

Polypodium hydriforme remains the bane of caviar producers. Ovsyannikov discovered them in 1871 in sterlet eggs. It was described as an unknown species by Ussow in 1885. It was he who gave it the Latin name, which is still valid today, and recognized it as an unusual parasitoid. Atypical because not only was it a parasite, it was also turned inside out, that is, with the digestive epithelium (gastrodermis) facing outward, and the ectoderm innate to the center of the body. Then there was a long lull in research on this peculiar animal. A renaissance of interest in the polypodium occurred in the 1940s, when it began to make economic losses, and then in the 1980s, when it was imported to North America [7, 9, 10].

Neither dog nor otter – a beast in disguise

Polypodium ‘s unusual morphology is accompanied by a unique life cycle and an unexpectedly high rate of genome evolution. It spends most of its life as a parasite in ripening caviar. Its most common victims are the sterlet Acipenser ruthenus, the paddlefish Polyodon spathula and the shovelnose Scaphirhynchus platorynchus. They are observed less frequently in the other sturgeon species Acipenser and the beluga Huso. Polypodium lives in the egg cell of the sturgeon as a two-nucleated cell 15-30 μm in diameter. Then it develops into a larva (embryo?), then transforms into a stolon – both forms with inverted tissues. Such transposition makes physiological sense: the polypodium is immersed in the yolk of its host’s egg, so it absorbs it and digests it with its entire body surface.

Infected oocytes are larger than uninfected ones (1.5-2.0 instead of 1.3 mm in healthy ones). This stage can last several years. Shortly before the hosts spawn, the polypodium overturns, acquiring the typical arrangement of body layers for cavernosaurs. At this point, the feelers (arms), previously scattered throughout the interior of the stolon, emerge to the surface. What’s left of the egg yolk enters the parasitoid’s absorptive-digestive cavity. Once the infected egg grains are released into the water, the stolon leaves the host body and is fragmented into a number of jellyfish-like progeny forms.

They are sexually reproducing predators, but with characteristic gametogenesis. Half of them, traditionally considered to be males, show fairly typical spermatogenesis. After that, however, it begins to carry out rather oogenesis. A kind of meiosis follows, but the resulting gametes are binucleate like ovules. One of the testes remains haploid while the other undergoes polyploidization. Protected by a special cap, the gonad becomes a gametophore full of binucleated gametes.

The other half of the jellyfish, classified as females, form two ovaries with oviducts in the endoderm. Despite years of intensive observation, no meiosis has been observed here. It is currently believed that females remain virgin. The significance of the gonads remains unclear. The binucleate gametes (or perhaps zygotes?) penetrate deep into the oocytes of female sturgeons and the cycle begins anew [9].

It is not uncommon for sturgeons to spend much of their lives in the seas, but they do spawn in freshwater. Polypodium hydriforme infects them only during spawning in rivers or streams. A freshwater lifestyle is a great rarity among parasitoids. In ponds and rivers, however, one encounters the Hydra centipede or the jellyfish Craspedacusta sowerbii. Both of these genera are quite distantly related to each other. Neither are cousins of the polypodium. It is noteworthy that freshwater forms are not found in the other clusters of typical caviomorphs (corals, crustaceans, columnomedusae, ossicles). Instead, they are not absent among the Myxosporidian Myxozoa – parasites formerly classified as protozoa, but today as caviomorphs [2, 5].

In a class by itself

The uncanny structure of the polypoid assured it a rather isolated position among the Metazoa tissues from the beginning. The presence of parsimony cells with nematocysts from the beginning supported the recognition of P. hydriforme as a bizarre, but undoubtedly a caviomorph. In 2003. This classification has been questioned. On the basis of gene analysis, it was concluded that the described parasite of sturgeons is more closely related to bipedally symmetrical animals than to radially symmetrical parasites. Later, the conclusions of these studies were questioned, pointing out the error of combining two long branches of the phylogenetic tree as sister groups. Some researchers considered the results indicating the affinity of myxosporidians with Polypodium and the affiliation of the two groups to the cavernosaurs to be an error, and at the same time an example of a research artifact. For others, the affinity was true, so it was an example of correct inference.

In any case, the caviar destroyer occupies a special position within the entire type. Indeed, it is necessary to create for it a separate genus Polypodium, family Polypodiidae, order Polypodiidea, and even a whole class (cluster) Polypodiozoa . As pathogens and pathogens of fishes important for active conservation, responsible for castration of icicles of umbrella and keystone species, it is of economic as well as conservation importance [5].

Incredible mix of features – lots to explain not only on the contentious issue

Myxosporidians (Myxozoa, myxozoa, myxosporidia) have accompanied fish as their parasites for nearly 500 million years. To date, some 2,200 species have been characterized, grouped into some 62 genera. Many more are waiting to be discovered and described. Perhaps as many as 30,000? It was only the industrialization of freshwater and migratory fish farming plus the domestication of more marine fish (such as tuna) that turned the eyes of the world to this group, hitherto categorized as protists and later moved to caecilians. The hit to the pocket prompted scientists to study this group. For more than a century, the most important features in classifying these pathogens into genus and species were the structure of spores.

It usually measures 19-20 µm and has a round or egg-shaped, less often tubular, shape. It was noted as early as the 19th century that it consists of several cells and has some similarities to the cells of parsimony. Until the end of the 20th century, this did not prevent scientists from describing Myxozoa as protozoa and later protists. Depending on the species, a spore contains from one to thirteen (usually two) pole cells. Inside each of them are spirally twisted threads. Under certain conditions, they shoot outward, anchoring the spore in the host tissues. What does this remind us of? Parasitic cavernosaurs, especially their modifications in parasitic narcomedusae and polypodium.

An unexpected twist

Scientists, veterinarians and breeders have long been convinced that progeny spores form in diseased fish, which drop out and infect the next victims. It was assumed that infection with myxosporidia resembled malaria in humans or eimeria in cattle, because there too the pathogenic protist increases in leaps and bounds through schizogony. The similarities went further, for schizogony was supposed to be followed by sporogony, or the formation of a new generation of spores. In 1984. It has been experimentally proven that Myxobolus cerebralis spores do not cause infection in salmonids.

The parasite needs an intermediate host in the form of a tube worm (tubifex). In the body of this freshwater cousin of earthworms from a spore was formed Triactinomyxon, which develops in the intestinal epithelium of the invertebrate. A pansporocyst is formed from a pole cell, and from it actinospores that will infect the fish. Plasmodia will form at the site of infection. Trophic stages (trophozoites) travel through the body of the prey, using either the bloodstream or the nervous system.

Such a sophisticated life cycle, with two dissimilar sexual generations and two dissimilar survival stages (spores), is unique in the entire animal world. Over time, it has been proven that another culprit of hatchling molt in Pacific salmon and rainbow trout, Ceratonova shasta (Ceratomyxa shasta), undergoes a very similar life cycle. Here, too, an intermediate host belonging to the freshwater annelids is essential, this time the tiny polychaete Manayunkia speciosa. Massive, and deadly, infections are facilitated by the high density of M. speciosa, uncommon in the wild, but quite common in dam reservoirs. This is another argument for demolishing dams in Canada and the US.

Host change has also been documented for: Hofferellus, Myxidium, Sphaerospora, Thellohanellus and Zoschokkella, indicating either annelids or bryozoans. Fish are the most common definitive hosts of myxosporidia. Nevertheless, species have been discovered completing their life cycle in amphibians, reptiles, birds and mammals. Hosts are usually annelids, sometimes bryozoans. Again, a similar relationship to that of malarial spores, where humans are the intermediate host and mosquitoes the definitive host [2, 4, 6].

What a scandal!

Myxozoa are considered today to be extremely simplified caviomorphs in the course of evolution. Shifting to a parasitic lifestyle, they evolved from relatively large, predatory, free-living jellyfish into intracellular parasites, composed of a few and sometimes a single cell. The simplification of body structure was accompanied by the loss of many genes and proteins, not only those responsible for oxygen respiration, but also embryonic and larval development, coordination of the work of various organs and tissues, and communication between cells. The bodies of myxosporidians are among the tiniest tissues, and their genomes are among the smallest, most truncated among animals.

Particularly intriguing is the absence of genes with caspase and BCL2 domains, as well as homologs of p53 and apoptotic protease-activating factor-1 (Apaf-1). The exclusion of apoptosis pathways suggests that the ancestors of today’s myxosporidiospores were tumor-like formations, only that they were capable of surviving for some time outside the patient’s body and infecting new victims. Such tumors are still observed today, for example: in mollusks, dogs or Tasmanian devils. Hence the emergence of the SCANDAL (speciated by cancer development animals) hypothesis, concerning the emergence of new animal species, or even whole clusters, as independent cancers [2, 3, 4, 6, 11].

You can get wheelspin in such a squeeze

Long-term parasitism often mellows, approaching commensalism. This rule works quite well not only for tapeworms, but also for myxosporidia. On natural sites or in traditionally extensively managed fish farms, infection with myxosporidia is not uncommonly asymptomatic. The situation is different in modern, intensive aquaculture, or in new ecological systems created after invasions by alien species of fish and invertebrates. Here, the fish suffer and then die en masse, and the economic losses are sometimes enormous.

Most damaging worldwide are the hypertrophic kidney disease Tetracapsuloides bryosalmonae and trout keloid caused by Myxobolus cerebralis. There is no shortage of diseases affecting carp (e.g.: swim bladder inflammation caused by Sphaerospora dykovae, fin invasion by Thelohanellus nikolski) or burbot (the real one). The lack of reports of damage in Polish fish farms is an observational artifact rather than an actual lack of pathogens. Hatchling die-off caused by Myxozoa is overshadowed immediately by viral and bacterial causes. Salmonids and cyprinids from aquacultures are not tested for asymptomatic carriage in Poland [6, 8].

Buddenbrockia like a symbol of recycling

A cousin of Tetracapsuloides bryosalmonae turned out to be one of the world’s and Europe’s most puzzling freshwater animals: the Buddenbrockia buddenbrockia plumatellae . This microscopic worm that parasitizes bryozoans has rarely been observed since it was described in 1851, and more and more researchers began to consider it a myth or an observer’s mistake. The rediscovery of living specimens brought more questions than answers. Budenbrokia resembles the Möbius ribbon known from geometry – a topological manifold having only one side and only one edge, popularized as a symbol of recycling. Slightly more sophisticated mathematics helped with explaining the mysteries of budenbrokii. Only Bayesian inference was able to reconstruct its evolutionary affinities based on the sequence of nuclear proteins [8, 11].

In the article, I used, among other things. z :

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