Blind sea spiders. Sea spiders. Lifestyle and nutrition of sea spider, scorpion
Order - perciformes Family - sea dragons Maximum length - 40 cm Fishing places - shallow water with a sandy bottom Fishing method - small path The sea scorpion (Trachinus araneu; in Italian - sea spider) has a more “squat” body shape than its relatives, a massive head , the mouth is large, almost vertically cut, relatively small eyes, in front of which there are two pointed outgrowths. 
On the back rises the first dorsal fin of seven spiny rays with poison-producing glands, the second, longer one, is supported by soft rays. The anal fin is very long, the ventral fins are medium in size, and the tail is spade-shaped. 
The gill covers have spines with poison-producing glands. The body color is brown or yellow-brown, the upper part is covered with various round and oval spots, forming longitudinal stripes on the sides.
Reproduction and size of sea scorpion, spider
The sea scorpion spawns in the spring and summer months, with the maximum length of adults reaching 40 cm.
Lifestyle and nutrition of sea spider, scorpion
The sea scorpion lives in shallow water on a sandy bottom, where it burrows and, merging with environment, awaits prey. This predatory fish feeds on crustaceans, mollusks and fish larger than itself. Usually, a sea scorpion, having attacked its prey, plunges its spines into it*, injects poison into the prey, which paralyzes it, and it quickly dies. This fish is also very dangerous for humans, since a prick from its spines can cause very painful allergic reactions.* The sea scorpion uses its spines exclusively for self-defense
How to catch sea scorpion, spider
Track. It is most convenient to catch sea scorpion in coastal waters on a small bottom path, using natural bait. The tackle uses a sinker mounted on a fishing line and attached with a block to a leash 5 m long. By lowering the nozzle to the bottom, they try to lure the sea scorpion out of its hiding place. In order to fish on the track, you need to move one and a half miles away from the shore, but with artificial baits you can swim more than three miles. A hooked sea scorpion reacts quite quickly, but it is usually not difficult to pull it out. When the fish is already in the boat, remove the hook very carefully, trying not to get hurt by its dangerous spines. You can catch sea scorpions all year round, but it is best to do this in the spring. The most favorable hours for such fishing begin at dawn and end at noon. Baits. The sea scorpion cannot resist all sorts of sea worms, whole sardines or pieces, crustaceans, tentacles and strips of squid or cuttlefish. The most catchy spoons are curved spoons, especially shiny ones, 2-3 cm long.
Sea spiders, they are spider crabs, they are also marble crabs, they live in the Mediterranean, Black Sea and Atlantic Ocean, near the coast of Morocco and France. They are found on the Crimean peninsula and the Caucasus coast, at shallow depths with a rocky or rocky bottom.
Sea spiders are members of the Grapsidae family. These crabs are called "spider" crabs because of their long, dark legs, and the name "marble" comes from the distinctive pattern on their shell.
Description of the sea spider
The spider crab is small and agile, its body length reaches only 38 millimeters and its width is 43 millimeters. The carapace is square and flat. The anterior edge, located between the eyes, is especially wide and straight, with 3 sharp teeth on each side. Upper part the shell can become overgrown with small crustaceans called balanuses, as well as algae.
The skeleton is external, respiration is carried out using gills. The left claw has small teeth that tightly close together. The right claw is larger than the left, the teeth are curved, and there is a gap between them. Externally, the right claw resembles forceps. The marble crab is a decapod crustacean, has 10 long, strong legs, covered with hairs. The color of the shell ranges from brownish-greenish to brownish-violet. The shell is decorated with a wavy pattern reminiscent of marble.
Spider crab lifestyle
Sea spiders live in the coastal zone; they stay at the very edge of the water and can even leave the water at a distance of up to 5 meters. This is the only Black Sea crab that can run out of the water. In the sea they can live at depths of up to 10 meters.
Marble crabs tolerate drying out well and love to bask on rocks in the sun. Spider crabs build their own homes. The crab selects a stone and begins to climb under it, throwing grains of sand out from under the stone with its claws; the crab hides in the resulting niche. Having accumulated reserves and having eaten well, the sea spider hides in a safe shelter.
Sea spiders feed on the remains of plants and animals, plankton, mollusks and polychaetes. They climb onto rocks protruding from the water and clean their surface. In case of any danger, the crab instantly hides in any crevice, and if there is none, it rushes into the water.
At night, he carefully crawls out of his old shell. At night they can climb onto rocks to a height of 3-5 m. They cannot burrow into the sand, but they are perfectly camouflaged among algae and mussels. If a crab loses a leg or claw, the lost organ is restored after 2-3 molts. The lifespan of a spider crab is 3 years.
Reproduction of marble crabs
The breeding season for sea spiders occurs in July-August at a water temperature of about 17 degrees.
One female lays up to 87 thousand eggs. Incubation lasts 25 days. Crab larvae eat plankton. Metamorphosis takes place in 4 stages. Puberty in females it occurs at 2 years.
Marble crab population
Like other Black Sea crabs, sea spiders are used to make souvenirs, but they are not commercial species.
Spider crabs are included in the Red Book of Ukraine, because lately their numbers decreased sharply. These crabs are protected in nature reserves Karadag and Cape Martyan.
Closest relatives of spider crabs
There are more than 10 thousand species of decapod crabs with five pairs of legs and bulging, stalked eyes. For example:
Stone crabs are the most large crabs on the Black Sea. The width of the stone crab's shell is about 10 centimeters. They prefer to live deeper, but can be found close to the shore;
The hairy crab looks like a stone crab, but is smaller in size, and its carapace is covered with numerous yellowish hairy bristles. They live closer to the shore, under the rocks;
Mediterranean or grass crabs have a green shell, which is why they are called “grass crabs.” Grass crabs are shallow water dwellers;
Water crab or lilac crab. It is slower and prefers to live exclusively in shallow water;
The swimming crab loves to burrow into the ground. Its small hind legs look like blades; with their help, the crab throws sand on itself. The crab also uses these legs for swimming; the swimming crab is the only one among the Black Sea crabs that can swim;
Blue crab came to the Black Sea from the Mediterranean in the 60s. He arrived in our latitudes with the ballast waters of ships. But the Black Sea water is too cold for young blue crabs, so they are extremely rare;
The invisible crab got its name because it is almost impossible to notice it in algae. These long-legged and skinny sea creatures they know how to camouflage themselves perfectly;
The Pea Crab typically lives among the mussels and can sometimes even crawl inside the shell. It is extremely difficult to see this crab, since an adult is no larger than a ten-kopeck coin;
Freshwater crab is an unusual Crimean crab. It differs not in size, but in origin and lifestyle. From the name it is clear that it lives in fresh water: in mountain rivers and ponds.
Freshwater crabs cannot be spread by currents, so they must travel on land at night. They once crossed the entire continent on foot in this way; it is believed that they originated in Southeast Asia.
Keeping marble crabs in an aquarium
Sea spiders do not dig holes, they prefer to hide under stones, so the bottom of the terrarium is covered with pebbles or sand, and there should be a variety of shelters at the bottom, for example, driftwood, stones, and ceramics. To make the aquaterrarium look more beautiful, it can be enlivened with the help of plants.
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. The cells contain a large number of granules, which 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 a single formation, 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. Several more nerves extend from the frontal surface - one dorsal nerve of the proboscis, a pair of nerves that innervate the pharynx, and another pair of nerves that serve the heliphores.
There are no separate respiratory organs.
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 mostly 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. They are found at different depths (from the lower littoral to the abyssal). Littoral and sublittoral forms live in thickets of red and brown algae, on soils of various 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.
Under 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 their natural habitat showed that their favorite food is 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 elements of behavior described above and examples of interspecific relationships relate 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
© Bogomolova E.V., Malakhov V.V.
Sea spidersE.V. Bogomolova, V.V. Malakhov
Vladimir Vasilievich Malakhov, corresponding member RAS, prof., head. department Invertebrate Zoology, Faculty of Biology, Moscow State University. M.V. Lomonosov.
Ekaterina Valerievna Bogomolova, Ph.D. biol. sciences, scientific co-workers the same department.
In order not to mislead anyone, let’s make a reservation right away - there are no spiders in the sea. They are generally extremely reluctant to leave land; Only one species leads an aquatic lifestyle - living in fresh waters silver spider ( Argyroneta aquatica). Sea spiders are a special group of invertebrates, which, along with all the familiar arachnids, crustaceans and insects, is included in the phylum of arthropods - the most numerous and diverse multicellular animals in the modern biosphere, which have mastered all environments on Earth.
In zoology, sea spiders are called Pantopoda(from the Greek panioV - whole and podi - leg), i.e. “consisting of only legs”, or Pycnogonida(from the Greek pucnoV - frequent, dense and gwnic - angle), i.e. “polygonal” or “multi-jointed”. Although sea spiders have been known to zoologists since the mid-18th century. (in our country they were studied by outstanding scientists V.M. Shimkevich and V.A. Dogel) and more than 1200 species have already been described, but the pycnogonid fauna of many regions is still poorly studied and the classification is poorly developed (there is not even a generally accepted division into orders).
Sea spiders live in all areas of the World Ocean, at all depths from the littoral to the abyssal and on any soil. They usually live in normal conditions oceanic salinity, only a few species are able to exist in the desalinated waters of seas such as the Black or Baltic. Most sea spiders are free-living bottom animals, some are symbionts of bottom invertebrates: coelenterates, echinoderms or mollusks, and sometimes planktonic organisms (jellyfish). Individual dwarf forms live in the capillary spaces between particles of sea soil. Some species have colonized areas of underwater volcanism - hydrothermal zones.
Male sea spider Nymphon longitarse, caught in the White Sea. Photo by S.A. Belorustseva
The sizes of sea spiders vary greatly: from 4 mm to 70 cm in leg span. The body is tiny compared to the legs - from 1 mm to several centimeters, so sea spiders look very strange: it seems that the animal’s body consists only of legs. Thanks to the protective, camouflaging coloration, many pycnogonids - animals with a small body and long thin legs - turn into “ghosts” that are difficult to notice among algae, in thickets of hydroids or corals. In addition, sea spiders are very leisurely. Some of them - with a massive disc-shaped body and relatively short legs - sit motionless (for example, on the body of echinoderms or sea anemones) or slowly crawl along the bottom. Others - slender with long limbs - are able to walk along the bottom and even swim, moving their legs, as when walking, or pushing - folding and straightening their legs. For only a few species, swimming is a normal form of activity. As a rule, sea spiders find themselves in the water column by accident and tend to sink to the bottom faster, taking a characteristic pose - gathering their legs together and placing their legs behind their back, which reduces hydrodynamic resistance.
Structure
The body of the sea spider is divided into four segments, from which seven pairs of limbs usually extend. Four of them belong to a complex head segment consisting of four fused parts: heliphores armed with claws (with their help pycnogonids hold, tear into pieces, and sometimes catch prey), palps covered with sensitive bristles, egg-bearing legs and one pair of walking legs. The remaining three pairs of walking legs are each attached to its own segment. The leg, consisting of eight segments, extends from a long lateral process of the body segment and ends with a main claw and usually two more appendages. With them, sea spiders cling so tightly to the substrate that it is difficult to remove them from the mass of fouling where they feed. In nature, sea spiders often break off their long legs. There are often individuals in which some legs are lighter and smaller than others - apparently, this is what regenerating limbs look like.
Often the set of limbs in pycnogonids differs from the typical one, which is what their classification is based on. Firstly, all or some of the first three pairs of limbs may be missing. Many species are characterized by sexual dimorphism: females have no egg-bearing legs or are shorter than those of males. Secondly, the number of body segments, and therefore walking legs, may also differ from the usual: seven species are known with five pairs of walking legs and two with six. So multi-legged and, as a rule, large forms are found in different families and are strikingly similar to some genus of typical eight-legged sea spiders, from which they probably originated.
Diagram of the structure of sea spiders using the example of a male Nymphon brevirostre
and a micrograph of its head segment (ventral view).
Here and below, microphotographs of E.V. Bogomolova
The body cavity in the trunk and legs is divided by a horizontal partition (septa) into dorsal and abdominal sections, in which the hemolymph moves in opposite directions. The cross-section of the heart tube is triangular: the dorsal side is simply a body wall, and the lateral ones converge and attach to the intestine from the dorsal side. The heart of pycnogonids is reduced, with thin walls without a continuous layer of contractile elements and, apparently, does not play an important role in the circulation of hemolymph. Perhaps the peristalsis of the intestine, entwined with a network of striated muscle fibers, and the vibrations of the horizontal septum are of much greater importance for its movement.
It is generally accepted that sea spiders do not have specialized respiratory and excretory systems. However, recently Nymphopsis spinosissima organs are described that are similar in structure to the excretory glands of other arthropods; they are located in the basal segments of the heliphora. The cuticle, which in pycnogonids is relatively thin and non-calcified, is penetrated by the ducts of numerous skin glands, which facilitates the transport of gases through the integument. Sea spiders “breathe” with the entire surface of their body - with thin legs and a small body, this is enough.
Sea spiders do not have complex compound eyes, such as those found in crustaceans and insects. On the dorsal side of the head segment there is an eye tubercle with two pairs of ocelli, which are capable of determining only the direction and intensity of light, and another pair of “lateral organs” with an as yet unclear function. In deep-sea forms that live in complete darkness, the eyes, and even the ocular tubercle itself, are usually reduced. Among other sensory organs, pycnogonids have setae and small sensilla. There are many of them on all parts of the body, especially on the legs.
Nutrition
If sea spiders resemble terrestrial spiders in one way, it is in the way they feed. Both of them have few structures suitable for collecting and grinding food: their mouthparts do not contain either mandibles or maxillae, which crustaceans and insects process food with. Real spiders inject enzymes into the victim's body and then consume liquid, semi-digested tissue (external digestion). Sea spiders, with their trunks with a Y-shaped mouth, simply suck up the soft tissues of invertebrates and digest them in the processes of the midgut located in the limbs (!). Real spiders also have intestines with lateral processes, but they are never as long as in pycnogonids and do not extend into the limbs.
Primary processing of food occurs in the pharynx (it is triangular in cross section), which penetrates the entire trunk. During feeding, the radial and circular muscles contract, causing a rhythmic narrowing and expansion of the lumen of the pharynx. In its posterior half, the cuticular lining forms a filtering apparatus, which is designed for very fine grinding of food. It consists of numerous spines arranged in rows and directed forward towards the mouth. The spines are pinnate: thin lateral “barbs” extend from the “trunk”, between which there are gaps less than 1 µm wide. The combination of spines and barbs forms a sieve with a very fine mesh, so a gruel that does not contain not only whole cells of the victim, but even organelles (!) enters the esophagus. Such thorough grinding of food is necessary for subsequent intracellular digestion inside the processes of the midgut, which reach almost to the end of the heliphora and walking legs. The digestive system of pycnogonids ends with a short hindgut.
Micrograph of the trunk N.brevirostre in a longitudinal section.
Sea spiders usually feed on bottom-attached or sedentary soft-bodied invertebrates, most often coelenterates. Pycnogonids are able to sense their presence from a distance; for this they have special receptors located on the torso, walking legs and trunk. Many subtidal species of sea spiders feed on colonies of hydroid polyps: holding the leg of the hydroid with its claws, the predator plunges the end of the trunk into the calyx surrounding the polyp and sucks it out. In a large individual Nymphon it takes about a minute. Of course, hydroids, like all cnidarians, know how to defend themselves: their stinging cells shoot a thread rolled up in a capsule, the contents of which are toxic to many invertebrates, but, apparently, not to sea spiders. Pycnogonids with a large trunk often feed on the tissues of sea anemones (such pycnogonids usually lack heliphores) and can completely absorb scyphists - individuals of the polypoid generation of scyphoids (for example, Aurelia jellyfish). Sometimes sea spiders use heliphora to tear off pieces of food, bring them to their mouths and suck them in with their trunks. Many pycnogonids specialize in feeding on bryozoans; some can catch benthic crustaceans and polychaetes. Some sea spiders eat algae and detritus, but these are the exception. Pycnogonids can tolerate long-term (up to 18 months!) fasting; the physiological mechanisms providing this ability have not yet been studied.
Pycnogonids themselves rarely serve as food for other animals. Only sometimes is their share in the stomach contents of fish, crabs and shrimp so large that we can talk about selective eating of sea spiders.
Epibionts
The large surface area of the body with a sedentary lifestyle contributes to the fact that the integument of sea spiders in the periods between moults is populated by a variety of epibionts. So, when studying sea spiders White Sea on their covers, in addition to various bacteria and algae (red, green, diatoms), a rich fauna was found, including representatives of eleven classes of invertebrates. The most common are foraminifera, hydroid polyps, bryozoans, juveniles bivalves. In addition, ciliates, camptozoans, and ascidians settle on the cuticle of sea spiders. On the body of large pycnogonids you can even find barnacles - balanus. For most organisms whose life cycle includes a free-swimming dispersal stage, pycnogonid integument is simply a solid substrate suitable for settling larvae from the water column.
Sea spiders are able to clear themselves of adhering particles and uninvited settlers by alternately dragging their limbs through an egg-bearing leg rolled into a ring, on the last segments of which there is a “brush” of large feathery spines. By strongly bending these legs, pycnogonids can reach the lateral processes and even the eye tubercle. In addition, sea spiders may be protected by the secretion of numerous skin glands. However, they can completely free themselves from epibionts only during molting.
Microphotograph of the last segments of the egg stalk N.brevirostre.
Reproduction
In addition to cleaning the surface of the body (apparently this is their original function), the egg-bearing legs of pycnogonids play another role vital role: Males bear offspring on these limbs.
Sea spiders are usually dioecious (only one hermaphroditic species is known - Ascorhynchus borderoi). The gonads are adjacent to the intestine from the dorsal side and form processes that extend into the walking legs in males to the end of the second segment, and in females - to the end of the fourth, which is usually expanded, since it is there that the eggs mature. Unlike other arthropods, pycnogonids have several pairs of genital openings, and they are located not on the body, but on walking legs (on the second segments).
The female lays eggs, the sizes of which vary from 20 microns ( Halosoma) and 30 µm ( Anoplodactylus) up to 200-300 microns ( Callipallenidae) and 500-600 µm ( Chaetonymphon spinosum And Ammothea tuberculata), and transfers them to the male. He, in turn, fertilizes the eggs (in sea spiders, fertilization is external) and forms “muffs” (cocoons) from them on his egg-bearing legs, or immerses the legs in a shapeless mass of eggs.
The eggs in the clutch are held together by a gelatinous substance secreted by cement glands located on the femoral segments of the male’s walking legs. Mating takes from half an hour to several hours, and in some species (for example, Pycnogonum litorale) up to five weeks. During the breeding season, a male can mate several times, with different females. In this case, there may be several cocoons on its egg-bearing legs, each of which contains eggs from one of the females. Further care for the new generation falls literally on the father’s shoulders - the male carries clutches until the very latest stages of embryonic development, and often until hatching and even full development of the larvae, which are very diverse in size and lifestyle [,].
Most often, a larva (protonymphon) 100-250 microns in size emerges from the egg with an underdeveloped intestine (no hindgut and anus) and three pairs of limbs - heliphores armed with claws and two pairs of attachment legs with a claw-like last segment. But not only these limbs allow the larva to stay on the egg cocoon: sea spiders, like their terrestrial namesakes, can make webs, but only at the larval stage. To do this, they have a spinning apparatus - glands in the heliphores and spinning spines [,].
Larvae N.brevirostre. They are held on the egg cocoon by spider threads,
as well as claws and special attachment legs.
Right- larva-protonymphon Nymphon micronyx(from the abdominal side).
The proboscis, limbs, spinning spine and spider thread are visible.
In many sea spiders, the eggs and the protonymphons emerging from them are very large, with a large supply of yolk, and their spinneret apparatus is especially well developed. In this case, the juveniles remain on the male’s egg-bearing legs for a very long time - until all the legs and abdomen appear, while the body length of juveniles can be only three times less than the size of the parents.
With the most specialized variant of lecithotrophic development, characteristic of representatives of the family Callipallenidae, it is not the protonymphon that emerges from the egg, but a later stage with the rudiments of two pairs of walking legs. The juveniles leave their parents with cheliphores, two pairs of developed legs and an abdomen with an anus. In such larvae the spinning apparatus is highly developed, and the larval attachment legs are completely absent [,].
Some families of pycnogonids are characterized by a certain type of development; in other families there are different options. For several families, mostly deep-sea, the larvae have not been described, and how their development proceeds is still unknown.
For many species of sea spiders, the breeding season extends over several months, while for others it is relatively short. Apparently, many forms living at the lower boundary of the littoral zone migrate deeper into the sublittoral zone for the winter. Life cycles and seasonal migrations in pycnogonids have been studied very poorly. The same can be said in general about the biology of sea spiders, their functional morphology, physiology, phylogeny, and paleontology. Many of these problems began to be developed only in the second half of the 20th century.
Family connections
The phylogenetic relationships of pycnogonids are unclear; even their place in the arthropod system has not been finally determined. More recently, methods of molecular systematics have begun to be used to solve this problem, but the possibilities of the comparative anatomical method are far from being exhausted. Early hypotheses about the possible relationship of sea spiders with crustaceans have now been abandoned. Undoubtedly, these animals are closer to chelicerates (this group includes horseshoe crabs, scorpions, spiders and mites) than to mandibulates (which include crustaceans, millipedes and insects). The chelyphores and palps of sea spiders can be considered homologues of the chelicerae and pedipalps of the chelicerates, and specialists relying on this homology include pycnogonids in the subphylum chelicerates at class rank. This idea is not accepted by all zoologists. It is difficult to compare the body parts of pycnogonids and chelicerates, since the anatomy and embryology of sea spiders has not been sufficiently studied, in addition, they have unique structural features. Only sea spiders have egg-bearing legs and such a complex trunk, which provides a unique mechanism for absorbing and processing food. Unusual large number genital openings and their localization on the second segments of the legs. Only sea spiders are characterized by such a small number of segments, and, apparently, oligomerization in them was not associated with a decrease in body size. The abdomen of modern pycnogonids is also shortened, greatly reduced, but this was not the case in fossil species.
There are three known fossil species of sea spiders. Best reconstructed morphology Palaeoisopus problematicus. These were large animals (up to 20 cm long) with four pairs of legs adapted for swimming. Abdomen Palaeoisopus, divided into five segments, it was thin and long. At the anterior end of the body there was a proboscis and heliphores. It is assumed that P. problematicus lived and ate on sea lilies, among which he was found more than once. It is curious that a number of modern species of sea spiders form symbiotic relationships with echinoderms. Palaeopantopus maucheri known from only three specimens, the head end is absent in the found samples, and the abdomen has three segments [,]. Finally, the third type of fossil pycnogonids is Palaeothea devonica- practically no different from modern forms and has a small, unsegmented abdomen.
All paleontological finds of adult pycnogonids date back to the Devonian. However, it cannot be argued that pycnogonids appeared exactly then (about 400 million years ago), and not earlier. The situation was complicated by the discovery of a fossil arthropod Cambropycnogon klausmuelleri, which has been identified as the larval form of a pycnogonid. This means that the appearance of sea spiders must be attributed to at least the Upper Cambrian - this is the dating of the samples Cambropycnogon. Excellent preservation allowed us to describe in detail external morphology Cambropycnogon. In terms of the set of limbs, this animal is comparable to the second larval stage of pycnogonids, only the presence of an “extra” pair of filaments (limbs?) next to the mouth is confusing. In general, almost no structural details were found in it, characteristic of the larvae of living pycnogonids, but the completely different structure of most of the limbs attracts attention. Maybe, Cambropycnogon- the larva of representatives of some group of arthropods that has not survived to our time and is not closely related to sea spiders.
* * *
It is still difficult to assess the role of pycnogonids in marine ecosystems. Meanwhile, the number of sea spiders in some areas of the world's oceans is impressively high. Thus, in the littoral and sublittoral zones of the White Sea, with its rugged shores and strong tidal currents, lush thickets of hydroids develop. These are very favorable conditions for sea spiders. In some places their numbers are so great that they must play a prominent role in the food chains of subtidal communities, being specialized consumers of hydroids, which in turn feed on plankton. Trawls and dredges lowered in the seas of high and temperate latitudes, in open areas of the oceans, bring numerous pycnogonids. It is known that sea spiders can form clusters of hundreds and thousands of individuals. Unfortunately, zoologists have not yet made a correct assessment of the number of pycnogonids and their role in communities.
Pycnogonids are of great interest as a relict group of arthropods, possibly unrelated to the others and retaining a number of ancient structural features. On the other hand, the organization, the life form of sea spiders with their few-segmented reduced body and very long limbs with processes of the intestines and gonads inside them, is unique. Most likely, pycnogonids are an independent branch of arthropods; they have developed a special, unique way of living in the sea. Unable to escape into other habitats, sea spiders populated the entire oceans and retained their characteristic appearance and peculiar way of feeding virtually unchanged for at least 400 million years.
Literature
1. Arnaund F., Bamber R.N.// Advances in Marine Biology. 1987. V.24. P.1-96. 2. Dogel V.A. Multi-jointed class ( Pantopoda). Guide to Zoology / Ed. L.A.Zenkevich. M., 1951. P.45-106. 3. Fahrenbach W.H.// J. of Morphology. 1994. V.222. P.33-48. 4. Bogomolova E.V., Malakhov V.V.// Zoological journal. 2003. T.82. Issue 11. C.1-17. 5. Bain B.A.// Invertebrate Reproduction and Development. 2003. V.43. No. 3. P.193-222. 6. Jarvis J.H., King P.E.// Marine Biology. 1972. V.13. P.146-154. 7. Jarvis J.H., King P.E.// Zoological J. of the Linnean Society of London. 1978. V.63. P.105-131. 8. Waloszek D., Dunlop J.// Paleontology. 2002. V.45. No. 3. P.421-446.
But no less scary, but sea spiders are representatives of a very strange and little-studied group of arthropods, 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 discovery of a sea spider in the form of a larva dates back to the Cambrian period, and there are also descriptions of finds from Silurian and Devonian deposits.
Pantopoda was first described only in the second half of the 18th century by Brünnich and Strom, researchers of marine fauna. The body sizes of sea spiders vary greatly, their length ranges from 1 to 90 centimeters. The smallest pantopods are Anoplodactylus pygmaeus with a length of 0.8 mm, and the largest are Colossendeis colossea and Dodecalopoda mawsoni.
Photo 3. 
These are very strange animals, morphologically unlike anything else, consisting almost entirely of only legs. Their body is so tiny that even half of the internal organs that normal animals should have can fit in it. 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.
Photo 4. 
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.
Photo 5. 
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. Using these eyes, the male spider finds a female whose slender legs are filled with ripening eggs, sits on top of her and rides on her, waiting for the eggs to mature. Most sea spiders are dioecious, but one hermaphroditic species is also known - Ascorhynchus corderoi.
Photo 6. 
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 literally: It carries cocoons on itself 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.
Photo 7. 
When the larvae hatch, the caring father continues to carry a ball of protonymphon babies, which feed and grow from the supply of yolk from the egg. They are held on to the parent not only with the help of special larval legs with which they are born, but also thanks to the web, which sea spiders also know how to make, but only in the larval stage.
Photo 8. 
In 2009, scientists from the Monterey Bay Aquarium Research Institute (MBARI) first obtained images of how deep-sea spiders of the order Pantopoda feed in their natural habitat and found out their preferences.
The filming, carried out at a depth of three kilometers, revealed that the spiders’ favorite treat is sea anemones.
The remains of whale carcasses and sunken wood formed several unique organic oases at the bottom, where deep-sea spiders of the species Colossendeis gigas and C. japonica settled. During each of the twelve dives, scientists observed a similar spectacle: arthropods enthusiastically caught up with and devoured the sea anemones that are usually found there.
Before this work appeared, biologists only guessed about the food preferences and feeding strategy of deep-sea spiders, but now for the first time this process has been captured on film.
Photo 9. 
Photo 10. 
Photo 11. 
Photo 12. 
Photo 13. 
sources