Polypodia


Polypodia

Here is polypodia ( Polypodium hydriforme ) - the amazing intestinal an animal whose larva parasitizes within oocytes 3-3-33298. sturgeon-shaped fish (3–3–335. sturgeons 3–3–3298. and 3–3–337. paddlefish 3–3–3298. Old and New Worlds). That is, it is an intracellular parasite with a multicellular structure!

The photo captures the moment of transition of polypodia to free life in water. A long “sausage” with tentacles is the so-called stolon (see Stolon ); such a “sausage” comes out from each infected egg that has fallen into the water during spawning of the host fish. After some time, the stolon is divided into many "jellyfish" organisms, as on the right side of the main photo. The appearance of these "jellyfish" is very unusual: they have no sense organs (for example, 3-3-3343. Statocysts 3-3-33298.) And an umbrella, characteristic of the mobile stages 3-3-345. bowing (Cnidaria). However, this does not prevent polypodia from slowly moving along the bottom with the help of 12 tentacles and even hovering in the water column. Polypod tentacles do not surround the mouth with a corolla, as in other bowing ones, but are located on the sides of the body: on each side there are four thin tactile tentacles and two thicker supporting ones. Therefore, the symmetry of the body in polypodia is not 3–3–347. radial as usual with Cnidaria, and biradial . Polypodia also lacks the ability to regenerate inherent in stereotypes.

"Medusoid" polypodia reproduce asexually - are divided in the longitudinal direction, starting from 3-3-3353. aboral pole (opposite the mouth), on which are laid six new tentacles. As a result of this division, in most cases, each new individual receives 12 tentacles: six old tentacles from the mouth and six new ones from the opposite. But there are individuals with 24 tentacles in which separation has not occurred, or 6-tentacle polypodia, which for one reason or another did not have the laying of new tentacles before division.


Polypodia

The structure of the "jellyfish" polypodium before dividing into two - at the aboral pole a "bundle" of 12 new tentacles is visible (3-3-33275. Nt 3-3-33276.). mc - the mouth hidden behind the tentacles, st - sensory, or tactile, tentacles; wt - supporting tentacles. Photo from an article by E. V. Raikova et al., 2007.3-3r381. Muscular system of a peculiar parasitic cnidarian Polypodium hydriforme : A phalloidin fluorescence study


The polypodia is also distinguished from most relatives by a freshwater lifestyle. Although there are freshwater jellyfish among those who like it (for example, 3–3–3301. 3–3–392. Craspedacusta sowerbii 3–3–3298. 3–3–3302.) And freshwater 3–3–395. hydra However, the vast majority of them are marine animals. Polypodia parasitizes exclusively on freshwater fish, respectively, and free-living polypodia swim in fresh water and "walk" along the river bottom, like hydra.

Despite the fact that discovered polypody for a long time - back in 1871 - some details of his lifestyle are still a mystery. For example, it is not known exactly how and what free-floating polypodia feed on. It is suggested that they can hunt with the tentacles on small invertebrates - small bristle worms , turbellarium and 3-3-3105. rotifers . For example, when growing in captivity polypods 3-3-33287. willingly eat small worms. Polypodia is hunted using tentacles dotted with 3-3-3109. stinging cells , or nematocytes.

Free-floating stolon with tentacles from an oocyte of fish of the family of paddlefish 3-3-3303.

The first time after parting with an oocyte, polypodia exists due to the reserves of nutrients accumulated in the previous parasitic larval stage. The larva of all barkers is planula - leads a free-floating lifestyle and serves for resettlement, even in parasitic 3-3-3123. drug addict . But in polypodia, a planar-shaped larva is located inside the fish’s oocyte, and is “turned inside out”: the epidermis with nematocytes is turned inside the body, and the gastrodermis (3-3-3125. Gastrodermis 3-3-33298.), Which will later line the gastric (intestinal) cavity, is outward. Moreover, the larva does not freely lie in the oocyte, but is located inside a special cell - trophamnion. With its help, the developing larva actively absorbs the nutrients of the oocyte.

Gradually, the larva grows into a stolon, on which the kidneys are formed - the rudiments of future "medusoid" individuals. Such kidneys in polypodia in sterlet oocytes have 3-3-3129. up to 60 , and in larger sturgeon oocytes - 90. Tentacles are formed on each kidney, while the stolon's kidneys and tentacles are turned inside his body. Immediately before spawning of the host fish, the polypodium “turns out”: the kidneys with tentacles go outside. In this case, the remaining yolk of the oocyte is “drawn” into the gastric cavity, the parasite destroys the nucleus of the oocyte and completely depletes its reserves. Considering that almost all oocytes can be affected by polypodia, this parasite significantly reduces the reproductive success of fish. For humans, polypodia is not dangerous, even if you eat infected caviar, nothing will happen. But distinguishing infected eggs from normal ones is quite easy - they are larger than uninfected ones and differ in color (mature infected eggs are lighter than uninfected ones).


Polypodia

Left : A - sterlet oocyte with a planulike larva (P) inside, T - trophamnion; Y - oocyte yolk; B - a general view of the stolon from the sterlet infected oocyte, tentacles have not yet formed; C - stolon with tentacles turned inward; D - a fragment of the stolon, visible inward tentacles (TE). Right : General view of the ovary of a sterlet. Four mature oocytes infected with polypodia are clearly visible - they are much larger than uninfected ones. Images from an article by E. V. Raikova, 2002.3-3r3277. Polypodium hydriforme infection in the eggs of acipenseriform fishes


The transition from a free-floating lifestyle to intracellular parasitism is associated with sexual reproduction and has presented scientists with many surprises and mysteries. First of all, it should be mentioned that the floating "tentacle jellyfish" are dioecious, but their number is 3-3-3171. gametogenesis 3-3-33298. quite unusual. In gonads The formation of germ cells in males begins as typical spermatogenesis, but then 3–3–3277. occurs two reduction divisions, or meiosis as with oogenesis. In the second division, only nuclear division occurs without separation of the cytoplasm. As a result, male germ cells become dual-core. The gonad is completely filled with such cells and closed with a plate of 3-3-3179. ectoderm carrying stinging cells - “cap”. At this stage, it is called a gametophor. Gonads of females consist of two ovaries with oviducts, but gametes do not form in them - there are neither reduction divisions nor accumulation of nutrients in the cells. Female gonads can degrade immediately after laying, and can begin to collapse after they are fully formed as organs. It is surprising that females, despite the presence of rather complicated gonads, do not take any part in reproduction. Probably, multiplication of polypodia occurs 3-3-3181. parthenogenetic 3-3-33298. - due to males!


Polypodia

The gametophor of the "male" individual, filled with binuclear cells (3-3-33275. BC 3-3-33276.) And closed by a "lid" with nematocytes (3-3-33275. N 3-3-33276.). Image from an article by E. V. Raikova, 2002. Polypodium hydriforme infection in the eggs of acipenseriform fishes


It is the male gametophor that is responsible for the continuation of the polypody life cycle - for this they must fall on thin integuments prelarvae (free embryos emerging from eggs) of sturgeon fish that have not yet switched to self-nutrition and 3-3-33211. live off stocks of yolk eggs. As soon as this happens, the “cap” stinging cells shoot out, and the “cap” itself is destroyed. The contents of the gametophor - a lot of dual-core germ cells - somehow, until a scientifically known method, enters the body of the host fish. The earliest stage of polypody, 3-3-33213. discovered in fish, binuclear cells in young oocytes. Each of them later gives a planulike larva and trophamnion cell.

The polypody life cycle is closely synchronized with the development of the host fish. Stolons enter the water at the end of spring, during sturgeon spawning. By the time the fish larvae appear in the rivers, the "jellyfish" just have time to switch to a free lifestyle and use up all the nutrients. The larvae of prelarvae, unlike adult fish, are delicate and thin: apparently, gametes are easier to penetrate into fish at this stage of development. Sturgeons grow very slowly, and the larval development of polypodia inside the fish stretches for several years - until the moment when the oocytes of the host fish mature and spawning begins. This is creeping with the slowest embryogenesis!


Polypodia

Polypody life cycle. Parasitic stages of development are depicted inside the circle, free-living - outside the circle. I - XII - months of the year. A - stolons with the epidermis turned inside the body and tentacles inside the oocyte. B - a stolon with inverted tentacles, leaving the oocyte during spawning of the host fish. C - Stolon freely floating in water. D - a fragment of the stolon. E - 12-tentacle single floating individual. F - 24-tentacle single swimming individual. G - 6-tentacle single swimming individual. H - 24-tentacle female, question mark shows that her further fate remains incomprehensible. I - 12-tentacle male, question mark between I and J means that the method of penetration of the parasite into the fish remains not fully understood. J - a dual-core hematophore cell. K - a larva surrounded by a second trophamnion inside the oocyte. L - M - planula-shaped larva (trophamnion not shown). N - stolon without tentacles. O - Stolon with tentacles facing the inside of the body. Figure from an article by E. V. Raikova, 2002.3-3r3277. Polypodium hydriforme infection in the eggs of acipenseriform fishes


The unusual structure and lifestyle of polypody from the very moment of discovery of cause a lot of controversy among taxonomists. Now, based on DNA analysis, the polypody is carry to bowing with a sister taxon Myxozoa - endoparasites of fish that were previously considered as part of 3-3-33293. protozoa .

Photo © Ekaterina V. Raikova from inaturalist.org .

[i] Ira Demina 3-3-3302.

08 июнь 2020 /
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