Sea spiders. The sea spider is not a spider at all, but a crab

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4. Giant sea spider (Pantopoda)

Giant sea Spiders are a relatively little studied group of organisms. And they are only indirectly related to spiders. Pantopods They are called sea spiders only for their external resemblance; in fact, they are not spiders.

Pantopods widespread in the world's oceans. They live in northern seas, and in the south. Some of their species can be found in the surface layer of water, and some sea spiders have been found even at a depth of 7300 meters.

The structural features of these animals include a big difference in the length of the torso and limbs. For example sea ​​spider with a body size of 15-18mm. has limb lengths up to 240mm. Cephalothorax pantopods consists of 7-9 segments, followed by a rudimentary abdomen.

Due to the disproportionately small body, some internal organs pantopod are on their limbs.

Sea spiders are predators. They feed on the soft tissues of sea anemones, sponges, and hydroids.

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3. Vespa mandarinia (Asian giant hornet)

This Asian giant is one of the most dangerous animals on this list to humans. - most big hornet in the world. The body length of the average male is 51mm, and the wingspan is 75mm. These giants live in Southeast Asia - in Primorye, Japan, China, Korea, Nepal, India and in the mountainous regions of Sri Lanka.

The sting of this hornet can be fatal to humans. has a sting about 6mm long, which stings and releases a large amount of poison. The venom of these hornets is very toxic. But hornets rarely use their sting. Hornets hunt using powerful jaws, which are used to tear apart prey.

They feed in the same way as their smaller counterparts from the Vespa genus - their diet consists of various insects, fruits, and berries. Hornets do not disdain the meat of fish washed ashore.

Cause great harm to beekeepers. Just a few hornets can easily and quickly destroy an entire bee family. Beekeepers quite often suffer from invasion giant hornets, suffering huge losses. Therefore, whenever possible, beekeepers try to destroy hornet nests. When destroying a nest, hornets fiercely defend themselves, biting and stinging people. It is among beekeepers that the mortality rate from giant hornet bites is very high - dozens of people die every year around the world.

Sea spiders, or multi-elbow(lat. Pantopoda Gerstaeker, 1862) - class of marine chelicerata (Chelicerata). They live at almost all depths, from the littoral to the abyssal, under conditions of normal salinity. Found in all seas. Currently, more than 1000 are known modern species. Sometimes sea spiders are separated from chelicerates into an independent type.

External structure

The body of sea spiders consists of two sections (tagmas) - a segmented prosoma and a small unsegmented opisthosoma. The prosoma may have a cylindrical ( Nympnon sp.) or discoid ( Pycnogonium sp.) shape. In the second case, it is flattened in the dorsoventral direction. Pantopod length 1-72 mm; the span of walking legs is from 1.4 mm to 50 cm.

Prosoma

The midgut occupies a central position in the body. Lateral outgrowths - diverticula - extend from its central part. No specialized glands were found. The wall of this section is formed by single-layer intestinal epithelium. Cells contain a large number of granules that are stained with bromine-phenol blue and Sudan black B, which indicates the protein-lipid nature of the contents of the designated vacuoles. Cell nuclei are in most cases poorly distinguishable. In addition, there are cells in the cytoplasm of which the number of vesicles is not so large; the nucleus is well stained with Ehrlich's hematoxylin. The cells can form pseudopodia and capture food particles.

The posterior section is the shortest. It is a tube at the distal end of which is the anus. The boundary between the midgut and hindgut is marked by the muscular sphincter.

The suprapharyngeal ganglion of sea spiders is unified education, the peripheral part of which is formed by the bodies of nerve cells (neurons), and the central part by their processes, which form the so-called neuropil. The suprapharyngeal ganglion is located under the orbital tubercle, above the esophagus. Two (Pseudopallene spinipes) or four (Nymphon rubrum) optic nerves arise from the dorsal surface of the brain. They go to the ocelli located on the ocular tubercle. The distal part of the nerves forms a thickening. It may turn out to be the optic ganglion. From frontal surface Several more nerves depart - one dorsal nerve of the proboscis, a pair of nerves that innervate the pharynx, and another pair of nerves that serve the heliphores.

Separated respiratory organs No.

The circulatory system consists of a heart extending from the optic tubercle to the base of the abdomen and equipped with 2-3 pairs of lateral slits, and sometimes one unpaired one at the posterior end. The excretory organs are located in the 2nd and 3rd pairs of limbs and open on their 4th or 5th segment.

The sexes are separate; the testes look like bags and are located in the body on the sides of the intestines, and behind the heart they are connected by a jumper; in the 4th-7th pairs of limbs they give off processes reaching the end of the 2nd segment, where on the 6th and 7th pairs (rarely on the 5th pair) they open with the genital openings; The female genital organs have a similar structure, but their processes reach the 4th segment of the legs and open outward on the second segment for the most part of all legs; in males, on the fourth segment of the 4th-7th pairs of limbs there are openings of the so-called cement glands that secrete a substance with which the male glues the testicles laid by the female into balls and attaches them to his limbs of the third pair.

Development

Ecology

Panthopods are exclusively marine arthropods. Meet on different depths(from the lower littoral to the abyssal). Littoral and sublittoral forms live in thickets of red and brown algae, on soils different textures. The body of sea spiders is often used as a substrate by numerous sessile and sedentary organisms (sessile polychaetes (Polychaeta), foraminifera (Foraminifera), bryozoans (Bryozoa), ciliates (Ciliophora), sponges (Porifera), etc.). Periodic molting allows the body to get rid of fouling organisms, but sexually mature (non-molting) individuals do not have this opportunity. Egg legs, if any, are used to clean the body.

IN natural conditions sea ​​spiders slowly move along the bottom or algae, clinging with claws located one at a time on the last segment (propodus) of each walking leg. Sometimes sea spiders can swim short distances, moving in the water column, pushing off with their limbs and slowly moving them. To sink to the bottom, they take a characteristic “umbrella” pose, bending all walking legs at the level of the second or third coxal segment (coxa1 and coxa2) to the dorsal side.

Sea spiders are primarily predators. They feed on a variety of sessile or sedentary invertebrates - polychaetes (Polychaeta), bryozoans (Bryozoa), coelenterates (Cnidaria), nudibranchia (Nudibranchia), benthic crustaceans (Crustacea), sea cucumbers (Holothuroidea). Filming Pantopoda in natural environment habitats showed that they favorite treat- sea anemones. When feeding, sea spiders actively use heliphores, at the distal end of which there is a real claw. At the same time, the sea spider not only holds prey with them, but can also tear off pieces from it and bring it to the mouth. Forms are known whose heliphores have undergone reduction. This can be expressed in a reduction in size ( Amothella sp., Fragilia sp., Heterofragilia sp), disappearance of the claw ( Eurycyde sp., Ephyrogymma sp.) and even completely ( Tanystilla sp.) of the entire limb. Apparently, this reduction may be associated with an increase in the size of the proboscis (the so-called compensatory effect). Nothing is known about the feeding habits of such forms.

The feeding process of sea spiders Nymphon, Pseudopallen It is easy to observe in laboratory conditions, but do not forget that these organisms are capable of prolonged fasting (up to several months) without visible damage to the body. To maintain a living culture of sea spiders, colonial hydroids and small sea anemones are used as food.

All the behaviors and examples described above interspecies relations refer exclusively to littoral and sublittoral forms. The ecological features of the inhabitants of the bathyal and abyssal are unknown.

Phylogeny

The Pantopoda group has an unclear taxonomic position. There are several hypotheses in this regard.

• Sea spiders as a group related to the Chelicerata.

Many modern researchers adhere to this point of view. And this assumption was made by Lamarck in 1802, and at the beginning of the century before last he placed the group Pycnogonides in Arachnida, considering them originally terrestrial spiders that secondarily switched to an aquatic lifestyle. However, there is no actual evidence for this, other than purely external resemblance, Lamarck did not bring it.
Later, in 1890, Morgan, studying the embryonic development of representatives of the Pantopoda group, came to the conclusion that there are many similarities in the development of terrestrial spiders and sea spiders (for example, the formation and development of the body cavity - myxocoel, the structure of the eyes, the organization of the digestive system - the presence diverticulum). Based on these data, he suggests the possibility of a relationship between sea and land spiders.

Further, in 1899, Meinert pointed out the possible homology of the proboscis of sea spiders and the rostrum of spiders, as well as the arachnoid glands of sea spider larvae and the venom glands of arachnids. Subsequently, more and more new facts appeared that were used as evidence of the kinship of the groups in question. And every researcher whose area of ​​interest was directly or indirectly related to this strange and little-studied group considered it his duty to add at least one piece of evidence to his collection. For example, it has been shown that the body of sea spiders and modern Chelicerata consists of a small number of segments. In addition, the nervous system is characterized by the fusion of the ganglia of the ventral nerve cord and the absence of the deutocerebrum (the middle part of the supra-pharyngeal ganglion). However, it should be noted that the last statement is untenable. According to modern neuroanatomical studies, all representatives of Chelicerata have a well-defined deutocerebrum, in contrast to older ideas about its reduction. This part of the brain innervates the first pair of limbs - cheliphores in pycnogonids and chelicerae in chelicerates. In addition, it is customary to homologize the limbs of sea spiders and arachnids. From this point of view, the cheliphores of sea spiders correspond to the chelicerae, and the palps to the pedipalps. The number of walking legs in both groups is eight. However, researchers avoid a number of obvious problems. The egg legs of sea spiders have no homologs in arachnids. It is also known that in the fauna of sea spiders there are forms with five ( Pentanymphon sp.) and even six ( Dodecalopoda sp.) with pairs of walking legs, which does not fit into this concept at all. In addition, it is not entirely clear how much

The systematic and evolutionary position of which has not yet been fully determined. Despite the name, sea spiders (Pycnogonida) have no relation to real spiders, although they are considered an early separated group of the chelicerate subtype, which includes arachnids and merostomids, that is, horseshoe crabs and crustaceans.

Sea spiders are a relatively small group, currently numbering about 1,300 species. The earliest record of a sea spider as a larva dates back to the Cambrian period, and there are also descriptions of finds from Silurian and Devonian deposits.

These are very strange animals, morphologically unlike anything else, consisting almost entirely of only legs. Their body is so tiny that not even half of it fits in it. internal organs, which normal animals should have there. Therefore, for example, sexual and digestive system sea ​​spiders are located entirely in the legs. And their legs, although luxurious, are rather frail due to weak muscles, so sea spiders are very leisurely creatures and can spend 40 minutes without moving at all. Because of this, bryozoans and all sorts of polyps grow on them, and amphipods and sea goats gladly use these stilts as a substrate. Particularly leisurely individuals even manage to fall into a trap - they do not move for so long that a sponge manages to grow around their legs. But their long legs allow them to move on any, even the softest, substrate, and sea spiders can be found almost everywhere, from the intertidal zone to deep-sea habitats.

The life of a sea spider is that of a leisurely bottom wanderer. Any mobile prey is faster than this predator, and therefore its food is mainly attached soft organisms like hydroid polyps. At the front end of the spider's body there is a tiny head with a rigid trunk and heliphores armed with claws. The spider uses its trunk to suck out polyps, and with its claws it tears off soft pieces from the victim, which are then digested in the processes of the midgut located in the legs (!). It must be said that real spiders also have intestines with lateral processes, but they are much shorter and do not extend into the limbs. By the way, it is interesting that sea spiders do not have any gas exchange organs - it is believed that with such a leisurely lifestyle, the tiny volume of oxygen that is absorbed through the surface of the body is sufficient.

On the sea spider's tiny head is a small eye tubercle with two pairs of eyes that detect light and shadow and possibly the outlines of objects. With the help of these eyes, the male spider finds the female, slender legs which is filled with ripening eggs, sits on top of it and rides on it, waiting for the eggs to ripen. Most sea spiders are dioecious, but one hermaphroditic species is also known - Ascorhynchus borderoi.

Unlike other arthropods, sea spiders have several pairs of genital openings, and they are located on walking legs. After the eggs mature, the female lays them, and the male immediately fertilizes the clutch. Then the male collects the eggs into cocoons, fastening them with a gelatinous substance, which is secreted by cement glands, also located on his legs, and puts them on special egg-bearing legs. Mating of sea spiders lasts from half an hour to several hours, and in some species it can last for weeks. After this slow process is completed, care for the offspring falls entirely on the shoulders of the male, and in the literal sense: he carries the cocoons on himself until the very late stages of embryonic maturation. Moreover, during a season, a male can mate with several females, and then on his egg-bearing legs there will be several cocoons from different mothers.

See also:
Sea spiders, “Nature”, No. 8, 2006.

Veronica Samotskaya