Shchitni (triops) - unusual spring aquatic animals that are older than dinosaurs

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Shrew (Triopsidae) is a genus of small crustaceans from the suborder Notostraca. Some species are considered living fossils, with origins dating back to the end of the Carboniferous period, namely 300 million years ago. Along with horseshoe crabs, shield crabs are the most ancient species. They have lived on Earth since the time of dinosaurs, and have not changed at all since those times, with the exception of decreasing in size. These are the most ancient animals existing today.

Origin of the species and description

Photo: Shchiten

The suborder Notostraca contains one family, Triopsidae, and only two genera, Triops and Lepidurus. By the 1950s, up to 70 species of shield bugs had been discovered. Many putative species are described based on morphological variation. There were two important revisions to the classification of the family - Linder in 1952 and Longhurst in 1955. These revised many of the taxa and defined only 11 species in two genera. This taxonomy was accepted for decades and was considered dogma.

Video: Shchiten

Interesting fact: More recent studies using molecular phylogenetics have demonstrated that the eleven currently recognized species harbor more reproductively isolated populations.

The shieldfish is sometimes called a “living fossil” because fossils belonging to the suborder were discovered in rocks of the Carboniferous period, some 300 million years ago. One extant species, the crustacean T. cancriformis, has remained virtually unchanged since the Jurassic period (about 180 million years ago).

There are many fossils of shieldfish found in a range of geological deposits. The absence of serious morphological changes that occurred in the family over the 250 million years of existence of these animals suggests that dinosaurs also saw shields in this form. Casachartra, an extinct group known only from Triassic and Jurassic fossils from western China and Kazakhstan, is closely related to shieldfishes and may belong to the order Notostraca.

Attention! Absolutely proven - shieldfish are living fossils

Shield snakes, in particular the species Triops cancriformis, are often called “living fossils.” Earlier in the Phanerozoic, I talked about an absolutely proven living fossil - the Comorian coelacanth, whose genotype and morphology allegedly remained virtually unchanged over many millions of years of evolution.

Shchitni, including the species T. Cancriformis, have a similar history, because according to the Russian-language Wikipedia they have not changed for about 200 million years! Moreover, this particular species of shieldfish supposedly lived already in the Late Triassic, as evidenced by the remains of this species found by scientists in sediments 237-200 million years old [1].

Fauna of the Triassic period

However, Wikipedia does not say that one of the discoverers of these fossils was paleontologist Ferdinand Trusheim, who documented these finds and gave them a different species name (T. cancriformis minor), due to the fact that these shieldfish were small in size compared to modern forms of T. cancriformis. [2].

The image shows different types of animals, with fossils of the shield at the bottom right

However, there is a study from 1997 in which the authors of the study prove morphological and phylogenetic stagnation in three groups of shield insects (T. granarius, T. longicaudatus, T. cancriformis), despite the fact that the genomic phylogeny showed differences in mitochondrial genomes in all populations.

Scientists also carried out a diagnostic morphological analysis, which noted changes between related groups of “living fossils”, but the authors still noted the insignificant role of these changes in the scale of the entire evolution of these animals, since most of these changes did not occur during speciation (separation of lines ), which means that morphological stasis, from the point of view of these scientists, are important evolutionary patterns, although they are not properties of the species level [3].

This very interesting conclusion is successfully argued by a study from 2017, which showed the difference in the ontogenetic sequence of fossil forms with that of modern forms.

A comparison of extinct and modern representatives of shieldfish supposedly of the same species (from the point of view of Wikipedia), although it showed a similar ratio in the length and width of the shield in the ontogenesis of these animals, significant differences were also discovered in the so-called “starting point” of their development processes.

Thus, fossil young shieldfish have a more rounded shield shape, which became more elliptical with age, while existing shieldfish forms already had an elliptical shield shape from the very beginning. Further differences between extant and fossil forms were discovered by comparing the ratio of shield to trunk, to the body itself, the size of the shields themselves, and so on.

All these differences are statistically significant, and they hint, to put it mildly, that the interpretation of living fossils in favor of shieldfish is erroneous. It is unlikely that T. cancriformis remained morphologically static for 237 million years [4], and scientists who ignore this fact are likely to be mistaken.

Stages: a) the largest samples. b)-e) scale insects gradually decrease in size. df), the smallest shieldfish

As for specifically molecular data, supporters of living fossils, analyzing very close species of shieldfish to the species T. cancriformis, pointed to the unique different evolution of the genomes of the latter in relation to relatives from the point of view of the evolution of their Micro RNA, which was analyzed during ontogenesis (ovum, first— fourth larval instar, adult).

The T. cancriformis microRNA regions contained gene sequences that were more similar in characteristics to sequences in many arthropods than to sequences in closely related species that were significantly different. Although this does not negate their close relationship, nevertheless, the roles of their MicroRNAs in ontogenesis are different, which is not in favor of living fossils.

However, during the process of ontogenesis, sequences were also identified that were more consistent with the sequences of vertebrates than with arthropods, which is why the researchers did not destroy the concept of a living fossil, but simply noted the unique evolution of this species [5].

However, other researchers noticed this kind of discrepancy and conducted a number of studies of DNA genomes, which continued to note various genetic divergences between populations of different related species and a lack of support before the Mesozoic radiation [6;7], that is, these studies discovered genetic divergence and lack of genetic stasis in parts of shield groups [8;9].

However, there were also studies that showed a very slow evolution of the genomes of populations of shield insects of the species T. Cancriformis, reproducing by parthenogenesis, for example, in the winter ponds of Israel, which can be attributed in favor of the concept of “living fossils”.

But the researchers also noted the biological diversity for the neighboring population of the species L. aus, which reproduces by standard sexual means, and the influence of horizontal gene transfer was proven for both populations as a whole [10], which speaks against the hypothesis of genetic stagnation.

Other studies of this species show genetic diversity in European populations of T. cancriformis, suggesting that the subspecies should be reconsidered as separate species.

Although, using DNA markers, researchers note low allelic diversity within the various structures of these populations, compared to other branches of other species of shieldfish, which tells us in favor of genetic stagnation from the point of view of a superficial assessment of the globalization of the evolutionary process [11].

Also in favor of “living fossils” is a recent review study from 2021, which tells retrospective tales of rethinking the term “living fossils” with the goal that this term scientifically exists to generalize all “ancient” forms, not for categorization, but for answer questions from various studies, such as their origin or preservation/non-preservation in their unchanged form.

Although such answers have long been given in favor of non-conservation, for example, in the conclusions of the study of coelacanth by biologist Didier Kazan and many others after him [12]. They were simply partially ignored by the authors, or rather mentioned in passing [13].

image from this study.

However, in my opinion, the term “living fossils” is outdated and should not be reconsidered for the purpose of resuscitation, even in the spirit of combining it into an “agenda” topic. Attempts to make this term scientific look like attempts to reanimate a long-rotten corpse by dancing with tambourines, when attempts with a defibrillator and adrenaline have already failed.

Such an imitation of scientific activity is carried out by people with a conservative mentality stuck at the turn of the century who have difficulty accepting new data from young scientists “from other schools,” because it is already becoming difficult for them to analyze this data properly due to objective reasons.

Such trends are supported only by their young students, who are of the opinion that if there is one significant sign in relation to the proof of any hypothesis, then even if there are at least how many plausible and true signs against this hypothesis, if they are a little less weighty, these signs will amount to nothing to mean. However, I think this is not entirely true.

Regarding the same shields, too much “weightless” evidence has come out, which has acquired a very significant connotation, capable of not only shaking, but also burying the concept of “living fossils”.

The last stone in the stagnant genetic garden of prehistoric scale insects can rightly be considered a multiplex phylogenetic analysis of Branchiopoda, in a colossal study of all known species of Notostraca from 2013 [14], which was carried out using a well-tested Bayesian mathematical method in this kind of research [15], as well as the PartitionFinder method and laser for estimating divergence time [16].

This study finally showed that the concept of “living fossils” cannot be interpreted as genomes, since the researchers did not find any absence of evolutionary changes and, on the contrary, discovered these changes.

Contrary to the retrospective views of scientists (which I wrote about above), who still write about “living fossils”, only briefly highlighting the large-scale evidence of changes in the genomes of “living fossils [16], this study puts an end to this issue, since divergent analysis dating shows that shieldfish could be considered as “living fossils” only on the basis of morphological conservatism, if it had not been refuted in 2017 [4].

From a molecular data point of view, there is no support for limited diversification or relict status of these animals. Over the course of their long evolutionary history, shieldfish have undergone multiple global changes and high species turnover.

The researchers mildly criticized the position of scientists who draw conclusions about diversification patterns only from fossil evidence in groups where widespread morphological conservatism is represented by an incomplete fossil record that does not explain rampant speciation.

Therefore, we can say with confidence that shieldfish, including shieldfish of the species Triops cancriformis, are absolutely proven living fossils, though only in the eyes of retrospective scientists, conspiracy theorists and convinced creationists!

The shield is in a hurry to eat those who insult it with fossils. Links to sources:

1) https://ru.wikipedia.org/wiki/Shchitni

2) Triopsiden (Crust. Phyll.) aus dem Keuper Frankens. Von F. TRUSHEIM, Wiirzburg.1938. Mit 10 Textabbildungen und Tafel 13 und 14

3) https://academic.oup.com/biolinnean/article/61/4/439/2705816

4) https://link.springer.com/article/10.1007/s12542-017-0370-8

5) https://m.rnajournal.cshlp.org/content/21/2/230.short

6) https://link.springer.com/article/10.1186/1471-2148-7-161

7) https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0034998

https://academic.oup.com/jcb/article/34/6/795/2547980

9) https://link.springer.com/article/10.1023/A:1003168416080

10) https://www.tandfonline.com/doi/abs/10.1080/09397140.2016.1257404

11) https://www.semanticscholar.org/paper/Morphological-variation-and-genetic-diversity-of-of-Zierold/29225c617df233026af0dd901d6b2b2bc11140a1?p2df

12) Didier Casane, Patrick Laurenti. Why coelacanths are not 'living fossils': A review of molecular and morphological data // Bioessays. — 2013. — Vol. 35, No. 4. - P. 332-338. — doi:10.1002/bies.201200145

13) https://academic.oup.com/bioscience/article/68/10/760/5065827

14) https://peerj.com/articles/62/

15) https://academic.oup.com/sysbio/article/61/3/539/1674894

16) https://www.jstor.org/stable/2410522?origin=crossref&seq=1

Appearance and features

Photo: What the shield looks like

Shields are 2–10 cm long, have a wide carapace in the front and a long thin abdomen. This creates an overall tadpole-like shape. The carapace is dorsoventrally flattened and smooth. The front portion includes the head, and two rocky eyes located together on the top of the head. Two pairs of antennas are significantly reduced, and the second pair is sometimes absent altogether. The oral cavities contain a pair of single-branched antennules and no jaws.

Ventral side of the scale, showing up to 70 pairs of legs. The torso contains a large number of "body rings", which look like body segments but do not always reflect the underlying segmentation. The first eleven body rings make up the thorax and bear one pair of legs, each of which also has a genital opening. In the female it is modified, forming a “brood pouch”. The first one or two pairs of legs are distinct from the rest and probably function as sensory organs.

The remaining segments form the abdominal cavity. The number of body rings varies both within and between species, and the number of pairs of legs per body ring can be up to six. The legs gradually become smaller along the abdomen, and in the last segments they are completely absent. The abdomen ends in a telson and a pair of long, thin, multi-jointed caudal branches. The shape of the telson varies between the two genera: in Lepidurus a rounded projection extends between the caudal rami, while in Triops there is no such projection.

Interesting fact: Some species have the ability to turn pink when there is a large amount of hemoglobin in their blood.

The color of shieldweed is often brown or grayish-yellow. On the proximal side of the abdomen, the animal has many small hair-like appendages (about 60) that move rhythmically and allow the individual to direct food towards the mouth. Males and females differ in both size and morphology. Males tend to have a slightly longer carapace length and possess larger secondary antennae that can be used as clamps during breeding. In addition, females have an egg sac.

Now you know what a shield looks like. Let's see where this crustacean lives.

Giant water bug: description

Let's start with the sizes. These insects grow up to 17 cm in length. They have different colors - from green to brown - depending on where they live. These bugs have their front legs (or first pair) turned into powerful pincers with hooks. It is with these that they firmly hold their prey until the injected toxin takes effect. They are active predators, mostly aquatic. Fish, frogs and even turtles are what the giant water bug feeds on. It even attacks small waterfowl! The toxin in the bedbug's saliva is similar in composition and effect to the venom of some snakes, but is not as toxic. It paralyzes the victim and dissolves its tissue, and the liquid contents are absorbed by the bug. In hunting, these predators are helped by retinal eyes and a hearing organ on the middle part of the body (tempanal organ). The bug can fly and loves the light of lanterns.

Because they fly towards light, they are called “electric lighting beetles.” It is in the light that people catch them in large quantities. Breathing is carried out through two tracheal tubes that open on the abdomen. These creatures have to surface every 7-8 minutes to take a breath. In case of danger, the giant bug pretends to be dead and becomes like a fallen leaf, which, however, can spray the secret of odorous glands located in the abdominal region at the attacker.

Where does the shieldfish live?

Photo: Common shrew

Shrews can be found in Africa, Australia, Asia, South America, Europe (including the UK), and parts of North America where the climate is suitable. Some eggs remain unaffected from the previous group and hatch when rain soaks the area where they are located. This animal has calmly adapted to existence on all continents except Antarctica. It is found on most of the islands in the Pacific, Atlantic, and Indian oceans.

The habitat of the shieldweed is located in:

• Eurasia, 2 species live everywhere there: Lepidurus apus + Triops cancriformis (summer shieldweed);
• America, such species as Triops longicaudatus, Triops newberryi, etc. have been recorded;
• Australia, several subspecies live everywhere under the combined name Triops australiensis;
• Africa has become home to the species - Triops numidicus;
• the species Triops granarius has become a favorite in South Africa, Japan, China, Russia and Italy. Shield beetles are found throughout the world in freshwater, brackish or saline waters, as well as in shallow lakes, peat bogs and heathlands. In rice fields, Triops longicaudatus is considered a pest because it liquefies sediment, preventing light from reaching rice seedlings.

Mostly, scale insects are found at the bottom of warm (on average 15 - 31 ° C) reservoirs. They also prefer to live in highly alkaline waters and cannot tolerate a pH lower than 6. The water pools in which they live must retain water for a month and not experience significant temperature changes. During the day, shieldfish can be found in the soil of a reservoir or in its thickness, digging and collecting food. At night they tend to burrow into the mud.

Shchitni in the aquarium

Shields are very unpretentious animals, they are content with any food, and are undemanding to the quality of water; Temperature fluctuations are also not a big problem for them. Only in the first days of life is it necessary to provide them with “greenhouse” conditions.

Typically, three types of shieldfish are bred in aquariums: the so-called. American - Triops longicaudatus

, European -
Triops cancriformis
and Australian -
Triops australiensis
.
There are known successful attempts to keep the European Lepidurus apus
, but such an aquarium requires special equipment: in nature, this species lives in cold melt water with a temperature no higher than 15 °C.

These creatures can be an interesting addition to a freshwater aquarium (provided, of course, the right conditions for shieldfish are met, for example, they need fine sand on the bottom), since shieldfish eat everything they find and, in addition, are unusual in appearance. They may, however, pose a certain danger to small fish, fry or other crustaceans and may in turn be eaten by larger fish. They can also damage the roots of aquarium plants.

In Europe, children's toy stores often sell special kits for breeding shieldbills as a science project or a beginner's aquarium. Usually the kit includes a suitable container, a thermometer, a hose, an aerator, food and, in fact, shieldfish “eggs” (pure or mixed with sand). Managing them is quite simple - the sand mixture or cysts in their pure form from such a set are placed in distilled or spring water and they hatch very quickly - often in a day or less. Growth varies significantly from one day to the next until they reach their maximum size of approximately 5-8 cm. In addition to their unpretentiousness, the short life span of these crustaceans is also an attractive feature - children can observe the full life cycle of shieldfish even before they have time they'll get bored.

What does the shield eagle eat?

Photo: Crustacean shield

Shields are omnivores, and they also dominate as predators in their niche, eating all animals that are smaller than them. Individuals tend to prefer animal detritus over plant detritus, but will eat both. Insect larvae, as well as various zooplankton, are also the subject of their dietary preferences. They prefer mosquito larvae over other insect larvae.

Fun Fact: When food is scarce, some species of shieldfish resort to cannibalism, eating juveniles or using their pectoral appendages to filter food into their mouths. The thrips longicaudatus species is particularly adept at chewing the roots and leaves of germinating plants such as rice.

Mostly shieldfish are at the bottom, rummaging in the ground in search of food. They are active around the clock, but they need lighting to spend their time productively. It happens that shieldfish are on the surface of the water with their belly upside down. It is not clear what influences this behavior. The original theory about a lack of oxygen was not confirmed. A similar behavior is observed in shieldfish in water saturated with oxygen. This is probably how the animal looks for food and bacteria that have accumulated near the surface.

Some parasitic bacteria of the genus Echinostoma use T. longicaudatus as a host organism. Additionally, more nutrients are provided by this crustacean's constant digging into the pond substrate and raising the sediment. Shitneys are known to significantly reduce the size of mosquito populations by consuming mosquito larvae.

Means and methods of control

In the fight against scale insects and other plant pests, agrotechnical measures are used, including spraying plants with insecticides. To achieve this, work is being carried out in the following directions:

1. Development of optimal crop rotation.
2. Chemical soil treatment to kill pests.
3. Optimization of irrigation systems to prevent moisture stagnation in the fields.

Optimal crop rotation

When planning plantings, it is necessary to exclude sowing the field with rice every season. This culture is worth

alternate with dry grains: oats, corn. Some legumes (soybeans, clover) and buckwheat are also suitable. Before sowing, it is necessary to carry out work aimed at drying out the soil: drainage, creation of furrows and grooves. This will not only get rid of aquatic crustaceans and insects, but also improve the nutritional properties of the soil and create favorable conditions for its saturation with useful elements.

When sowing rice, it is necessary to constantly monitor the water level in the field. During the germination period, it is important to prevent it from rising. Along the edges of the field, as part of phytosanitary treatment, weeds must be pulled out or burned; after harvesting, complete destruction of plant residues is also necessary. If shieldworms are detected, it is recommended to reduce the water level to a minimum for 1-2 days.

Chemical treatment

The following drugs are used as chemical control agents:

• "Simition." It is an organophosphorus intestinal poison directed against a wide range of pests. Consumption per 1 ha is 0.5-1 liter. A 50% emulsion concentrate is used. It is used by spraying the crop.
• "Karate Zeon". Affects arthropods through contact and intestinal means. Recommended for spraying crops during the growing season. 0.15-0.2 liters of solution are spent per 1 hectare.

Correct setup of irrigation systems

For better drainage and irrigation of the soil, it is necessary to use fall plowing, which ensures timely drying of the top layer of soil. The optimal depth is 20-25 cm.

Water supply channels must be promptly cleared of silt and plant debris, which can serve as food for shield bugs. It is recommended to use slotting to drain excess liquid. The frequency of cracks should be calculated based on the microrelief and can reach 20 cm in places of excess water accumulation (lowlands, ditches).

Shield bugs can pose a real threat to rice crops in a number of regions of our country. If they are discovered, it is necessary to take all measures to destroy them. Otherwise, the animals will have time to lay cysts, which will give rise to the next generation of individuals.

Features of character and lifestyle

Photo: Summer Shield

Shields are relatively solitary species; their individuals are found separately in different areas of water bodies. This is due to the higher levels of predation that occur when they are in large groups. These small crustaceans use appendages called phyllopods to propel themselves forward in the water. They constantly move throughout the day and are found floating in the water column.

These crustaceans have exopods that allow them to dig through the mud in search of food. They are more active during the day. Research has shown that scale insects can reduce their metabolic rate during times when food becomes scarce or when other environmental conditions are unfavorable. They shed constantly, especially often shedding their tight shell early in their lives.

They most likely use their eyes to identify food items and potential mates (if reproduction occurs sexually). Behind the eyes is the dorsal, occipital organ, which is most likely used for chemoreception, that is, for the perception of chemical stimuli within the body or in the environment.

Shield bugs have a relatively short lifespan, both in the wild and in captivity. Their average lifespan in the wild is 40 to 90 days, unless the temporary pond dries out first. In captivity it can live on average from 70 to 90 days.

Social structure and reproduction

Photo: A pair of shields

Within the suborder Notostraca, and even within species, there are significant differences in the mode of reproduction. Some populations reproduce sexually, others exhibit self-fertilization of females, and still others are hermaphrodites combining both sexes. Therefore, the frequency of males in populations varies greatly.

In the sexual population, sperm leaves the male body through simple pores and there is no penis. The cysts are released by the female and are then retained in a cup-shaped brood pouch. The cysts are retained by the female only for a short time before being laid, and the larvae develop directly without going through metamorphosis.

The female keeps the eggs in an egg sac for several hours after fertilization. If conditions are favorable, the female lays white eggs/cysts on various substrates present in the pond. If conditions are not favorable, the female will modify the eggs so that they go into a dormant state and will not hatch until conditions improve. In either case, the first larval stage after deposition is the metanauplius (the larval stage of crustaceans).

At this early stage they are orange in color and have three pairs of limbs and one eye. A few hours later, they lose their exoskeleton and the telson begins to form into plankton. After another 15 hours, the larva again loses its exoskeleton and begins to resemble a miniature adult scale insect.

The juveniles continue to molt and become adults over the next few days. After seven days, the crustacean has the color and shape of an adult and can lay its eggs because it has reached full sexual maturity.

Other Common Features

Aquatic bugs, unlike their terrestrial counterparts, have no scent glands or they are very poorly developed (in the giant water bug). They breathe atmospheric oxygen with the help of tracheal tubes that penetrate the entire body of the insect, and spiracles that open in the thoracic or abdominal part of the body.

Bedbugs reproduce sexually, the female lays from 10 to 200 eggs. Development - with incomplete transformation. The eggs will hatch into larvae that look very similar to the imago (adult). They molt 4-5 times, increasing in size and acquiring adult organs that the larvae do not have (wings, antennae, segments of the legs). After a few weeks, the larva turns into an adult. A special feature of bedbug eggs is the presence of a cap at the top. When the larvae open these caps, but do not yet emerge from the egg, the sight looks very funny.

Natural enemies of shieldfish

Photo: What the shield looks like

These small crustaceans are the main source of food for waterfowl. Many bird species prey on cysts and adults. In addition, wood frogs and other frog species often prey on shield frogs. In times when food is scarce, these crustaceans may resort to cannibalism.

To reduce intraspecific predation, shieldbills tend to be solitary, becoming a smaller target and less visible than a larger group. Their brown coloration also acts as camouflage, blending in with the sediment at the bottom of their pond.

The main predators that hunt shieldfish include:

• birds;
• frogs;
• fish.

Shield bugs are considered human allies against West Nile virus because individuals consume culex mosquito larvae. They are also used as biological weapons in Japan by eating weeds in rice fields. T. cancriformis is used most often for this purpose. In Wyoming, the presence of T. longicaudatus usually indicates a good chance of frogs hatching.

Purchased scalefish are often kept in aquariums and feed on a diet consisting primarily of carrots, shrimp pellets and dried shrimp. Sometimes they are fed live shrimp or daphnia. Since they can eat almost anything, they are also fed regular lunches, crackers, potatoes, etc.

Classification

The shield system is far from complete, since very few biologists are working on them. For 2008, 16 species have been preliminarily identified:

Genus Lepidurus (Leach, 1819)

• Spring shieldweed ( Lepidurus apus
  Linnaeus, 1758)
• Lepidurus arcticus (Pallas, 1776)
• Lepidurus batesoni (Lonhorst, 1955)
• Lepidurus bilobatus (Packard, 1883)
• Lepidurus couesii (Packard, 1875)
• Lepidurus cryptus (Rogers, 2001)
• Lepidurus lemmoni (Holmes, 1894)
• Lepidurus lynchi
• Lepidurus mongolicus (Vekhoff, 1992)
• Lepidurus packardi

Genus Triops (Schrank, 1803)

• Triops australiensis (Spencer & Hall, 1895)
• Summer or common or crustacean ( Triops cancriformis
  Bosc, 1801)
• Triops granarius (Lucas, 1864)
• Triops longicaudatus (LeConte, 1846)
• Triops numidicus
• Triops newberryi (LeConte, 1846)