Sea spiders. Sea spider - not a spider at all, but a crab Sea spiders
Sea spiders are often referred to as multi-legged animals. They belong to the class Cheliceraceae, the type of these creatures is Arthropods. Also acceptable is the classification by which the term "Chelicerate" is defined as the subtype from which sea spiders are separated into a class of their own. There are several more scientific names for this class - Pantopods, Pycnogonids and others.
Some general information
The concept of "sea spider" includes more than 1300 different species from a dozen families. They live in the seas all over the world. You can meet marine arthropods at different depths. Some species prefer the lower littoral (tidal section of the coast), others descend to the abyssal (deep zone). In saline and slightly saline waters, the Multi-elbows are much more common than in desalinated inland seas. In coastal areas, spiders settle in thickets of algae and on the ground.
Deep-sea and littoral spider species have differences in both body structure and size. In deeper layers of water, the sea spider will be larger, it will have significantly longer and thinner legs, which may have long hairs. These devices allow you to reduce the rate of immersion. The spider does not just swim, but seems to soar in the water. To sink to the bottom, it is enough for him to compactly fold his long limbs under the body.
Coastal forms are more compact. Their legs are thicker and shorter, but they have developed tubercles and spikes necessary for hunting and protection.
Structural features
Any sea spider, both deep-sea and coastal species, has a typical structure. The body is divided into two tagmas (sections). Their names are segmented prosoma and non-segmented opisoma. The prosoma is cylindrical or disc-shaped.
The torso of sea spiders is smaller than the limbs and is covered with a chitinous cuticle. There is a division into the cephalothorax and abdomen (it is rudimentary). There are from 7 to 9 segments in the cephalothorax, 4 of them are fused together. The fused part of the cephalothorax is called the head segment. The remaining segments can be either fused or dissected. In front of the head segment is a cylindrical or ovoid trunk. On the lateral parts of the trunk, 2 pairs of limbs are fixed: heliphores and palps. The third pair of limbs (ten-segmented egg-bearing legs) is fixed on the ventral side of the head section. One of the structural features of sea spiders is that 3 front pairs of legs do not reach the ground and do not participate in walking.
The walking legs of the sea spider are attached to the lateral processes of the head segment of the body. Most often there are 4 pairs, but some representatives have 5-6 pairs.
Digestive system
The sea spider has a digestive system in the form of a poorly differentiated through tube with diverticula. The diverticulum in this case is a process of the intestine that goes into each leg. The digestion of these arthropods is combined. Both cavity and intracellular form are used in common.
diet
It is not difficult to guess what sea spiders eat. Most of them are predators. Their diet consists of sessile and inactive invertebrates. These can be polychaetes, bryozoans, ciliates, anemones, intestinal and cephalobranch mollusks, small echinoderm starfish. Prey is held by claws on heliphors. They also break off pieces of food and go into the mouth.
gigantomania
Not so long ago, a giant sea spider was found in the waters of Antarctica. Studying the individual, scientists drew attention to a mysterious phenomenon, which they called polar gigantism. For some as yet unknown reason, the icy waters of Antarctica are transforming common sea spider species into giants. Perhaps the increased growth is due to the amount of oxygen, which is more in cold water than in warm water.
It has been established that not only spiders, but also some mollusks, crustaceans and echinoderms suffer from gigantomania in Arctic waters. Research is ongoing.
"Starfish and Spider"
Do you think we will continue to discuss the structure and life of marine animals? But you are wrong! In this section, we will talk about a fascinating book that explains the principle of success for various companies and organizations. Some of them are traditional, like spiders: they have legs growing from the body, they have a head and eyes. They can function with part of a leg or an eye missing, but without a head they will die.
The starfish is another matter, although its body parts look ordinary, they have completely different functions: the animal has no head and brain at all, and the main organs are repeated in each limb. Moreover, if you cut off a limb of a star, it will be restored. Even if you cut the sea beauty into several parts, it will not die, and after a while the halves will become independent animals. In fact, using this unique animal as an example, we can consider companies that function like decentralized networks.
The book "The Starfish and the Spider" is a vivid example of the fact that everything in nature is reasonable, and it is useful to apply many laws of development in other areas of human activity.
© Bogomolova E.V., Malakhov V.V.
sea spidersE.V. Bogomolova, V.V. Malakhov
Vladimir Vasilievich Malakhov, corresponding member RAS, prof., head. cafe zoology of invertebrates of the Biological Faculty of Moscow State University. M.V. Lomonosov.
Ekaterina Valerievna Bogomolova, cand. biol. sciences, scientific collaborator the same department.
In order not to mislead anyone, we will immediately make a reservation - there are no spiders in the sea. They are generally extremely reluctant to leave the land; only one species leads an aquatic lifestyle - the silver spider living in fresh waters ( Argyroneta aquatica). Sea spiders are a special group of invertebrates, which, along with all the familiar arachnids, crustaceans and insects, is included in the type of arthropods - the most numerous and diverse multicellular animals in the modern biosphere that have mastered all environments on Earth.
In zoology, sea spiders are called Pantopoda(from Greek panioV - whole and podi - leg), i.e. "consisting of one leg", or Pycnogonida(from the Greek pucnoV - frequent, dense and gwnic - angle), i.e. "polygonal" or "multi-elbow". Although sea spiders have been known to zoologists since the middle of the 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 fauna of pycnogonids in 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. Usually they live in conditions of normal oceanic salinity, only a few species are able to exist in the desalinated waters of such seas as the Black or Baltic. Most sea spiders are free-living benthic animals, some are symbionts of benthic invertebrates: coelenterates, echinoderms or mollusks, and sometimes planktonic organisms (jellyfish). Separate dwarf forms live in capillary spaces between particles of sea soil. Some species have mastered areas of underwater volcanism - hydrothermal zones.
male sea spider Nymphon longitarse, caught in the White Sea. Photo by S.A. Belorustseva
The size of sea spiders varies greatly: from 4 mm to 70 cm in leg span. The torso is tiny compared to the legs - from 1 mm to several centimeters, so sea spiders look very strange: it seems that the body of the animal consists only of legs. Thanks to the protective, masking 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 spreading 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 together and winding their legs behind their backs, which reduces hydrodynamic resistance.
Structure
The body of a sea spider is divided into four segments, from which seven pairs of limbs usually depart. Four of them belong to a complex head segment consisting of four merged parts: heliphorae armed with claws (with their help, pycnogonids hold, tear apart, and sometimes catch prey), palps covered with sensitive bristles, oviparous 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, departs from the long lateral process of the trunk segment and ends with the main claw and usually two more accessory claws. 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. Often there are individuals in which some legs are lighter and smaller than others - apparently, this is how regenerating limbs look.
Often, the set of limbs in pycnogonids differs from the typical one, on which their classification is based. First, all three first pairs of limbs or some of them may be absent. Many species are characterized by sexual dimorphism: in females, egg-bearing legs are absent or shorter than in males. Secondly, the number of body segments, and hence walking legs, can also differ from the usual: seven species are known with five pairs of walking legs and two with six. Such many-legged and generally large forms occur in various families and are strikingly similar to some genus of typical eight-legged sea spiders from which they probably originated.
Scheme of the structure of sea spiders on the example of a male Nymphon brevirostre
and a micrograph of its head segment (view from the ventral side).
Hereinafter, microphotographs by E.V. Bogomolova
The body cavity in the trunk and legs is divided by a horizontal septum (septa) into the dorsal and abdominal sections, in which the hemolymph moves in opposite directions. The heart tube is triangular in cross-section: the dorsal side is simply the wall of the body, 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 hemolymph circulation. Perhaps much more important for its movement is the peristalsis of the intestine, braided with a network of striated muscle fibers, and the fluctuations of the horizontal septum.
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 heliphorae. The cuticle, which in pycnogonids is relatively thin and non-calcified, is pierced by ducts of numerous skin glands, which facilitates the transport of gases through the integument. Sea spiders "breathe" the entire surface of the body - with thin legs and a small body, this is enough.
Complex compound eyes, such as those of crustaceans and insects, are not present in sea spiders. On the dorsal side of the head segment there is an ophthalmic tubercle with two pairs of ocelli, which are able to determine only the direction and intensity of light, and another pair of “lateral organs” with an unexplained function. In deep-sea forms living in complete darkness, the eyes, and indeed the eye tubercle itself, are usually reduced. Of the other sense organs, pycnogonids have setae, as well as small sensilla. There are many of them in all parts of the body, especially on the legs.
Nutrition
If anything resembles terrestrial sea spiders, it is the way they feed. Both of them have structures that are not very suitable for collecting and grinding food: their mouthparts do not contain either mandibles or maxillae, which are used by crustaceans and insects to process food. Real spiders inject enzymes into the victim's body and then absorb liquid, semi-digested tissues (external digestion). Sea spiders, on the other hand, with a trunk with a Y-shaped mouth, simply suck in the soft tissues of invertebrates, and digest them in the processes of the midgut located in the limbs (!). True spiders also have lateral processes in their intestines, but they are never as long as those of pycnogonids, and do not go into the limbs.
Primary food processing occurs in the pharynx (it is triangular in cross section), which permeates the entire trunk. During feeding, the radial and annular muscles contract, causing a rhythmic contraction and expansion of the pharyngeal lumen. In its posterior half, the cuticular lining forms a filtering apparatus, which is designed for very fine grinding of food. It consists of numerous spikes arranged in rows and directed forward towards the mouth. The spines are pinnate: thin lateral “beards” depart from the “trunk”, between which there are gaps less than 1 μm wide. The combination of spines and beards forms a sieve with a very fine mesh, so a slurry enters the esophagus that does not contain not only whole cells of the victim, but even organelles (!). Such a thorough grinding of food is necessary for subsequent intracellular digestion within the processes of the midgut, which reach almost to the end of the heliphorae and walking legs. The digestive system of pycnogonids ends with a short hindgut.
Micrograph of a trunk N.brevirostre in longitudinal section.
Sea spiders usually feed on bottom-attached or sedentary soft-bodied invertebrates, most often coelenterates. Pycnogonids are able to feel their presence at a distance, for this they have special receptors located on the body, walking legs and trunk. Many sublittoral species of sea spiders feed on colonies of hydroid polyps: holding the leg of the hydroid with a claw, the predator plunges the end of the trunk into the cup surrounding the polyp and sucks it out. In a large individual Nymphon it takes about a minute. Of course, hydroids, like all cnidarians, are able to defend themselves: their stinging cells shoot out 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 anemones (such pycnogonids usually lack heliphores), they can completely absorb scyphists - individuals of the polypoid generation of scyphoids (for example, Aurelia jellyfish). Sometimes sea spiders tear off pieces of food with heliphors, bring them to their mouths and suck them up with their trunks. Many pycnogonids specialize in feeding on bryozoans, while some may catch benthic crustaceans and polychaetes. Some sea spiders eat algae and detritus, but this is an exception. Pycnogonids can tolerate long-term (up to 18 months!) starvation; the physiological mechanisms that provide this ability have not yet been studied.
Pycnogonids themselves rarely serve as food for other animals. Only sometimes their share in the contents of the stomachs of fish, crabs and shrimps is so large that one can talk about selective eating of sea spiders.
epibionts
A large surface area of the body with a sedentary lifestyle contributes to the fact that the integument of sea spiders in the periods between molts is populated by a variety of epibionts. Thus, when studying sea spiders of the White Sea, in addition to various bacteria and algae (red, green, diatoms), a rich fauna was found on their covers, including representatives of eleven classes of invertebrates. The most common are foraminifera, hydroid polyps, bryozoans, and juvenile bivalves. In addition, ciliates, camptozoa, and sea squirts 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-floating dispersal stage, the integuments of pycnogonids are simply a solid substrate suitable for settling larvae from the water column.
Sea spiders are able to cleanse themselves of adhering particles and uninvited settlers, alternately dragging their limbs through an egg-bearing leg folded into a ring, on the last segments of which there is a “brush” of large feathery spikes. 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 get rid of epibionts only when they molt.
Micrograph of the last segments of the oviparous leg N.brevirostre.
reproduction
In addition to cleaning the surface of the body (apparently, this is their original function), the oviparous legs of pycnogonids play another important role: males bear offspring on these limbs.
Sea spiders, as a rule, have separate sexes (only one hermaphroditic species is known - Ascorhynchus corderoi). The gonads are adjacent to the intestine from the dorsal side and form processes that go 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 ranging in size from 20 µm ( Halosoma) and 30 µm ( Anoplodactylus) up to 200-300 µm ( Callipallenidae) and 500-600 µm ( Chaetonymphon spinosum And Ammothea tuberculata), and passes them on to the male. He, in turn, fertilizes the eggs (in sea spiders, fertilization is external) and forms “couplings” (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, the male can mate several times, and 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 lies literally on the father's shoulders - the male carries clutches until the very late stages of embryonic development, and often until hatching and even full development of larvae, which are very diverse in size and lifestyle [ , ].
Most often, a larva (protonymphon) 100-250 microns in size with an underdeveloped intestine (there is no hindgut and anus) and three pairs of limbs - armed with helifor claws and two pairs of attachment legs with a claw-like last segment comes out of the egg. 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 chelifores and spinning spikes [,].
Larvae N.brevirostre. On the egg cocoon, they are held with the help of spider webs,
as well as claws and special attachment legs.
On right- larva-protonymphone Nymphon micronyx(from the ventral side).
The proboscis, limbs, spinning spike and gossamer thread are visible.
In many sea spiders, the eggs and protonymphons emerging from them are very large, with a large supply of yolk, and their spinning apparatus is especially well developed. In this case, the juveniles remain on the egg-bearing legs of the male for a very long time - until all the legs and abdomen appear, while the body length of juvenile individuals 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 a protonymphon that emerges from the egg, but a later stage with the rudiments of two pairs of walking legs. Juveniles leave their parents with heliphores, two pairs of developed legs, and an abdomen with an anus. In such larvae, the spinning apparatus is highly developed, and the attachment larval legs are completely absent [ , ].
Some families of pycnogonids are characterized by a certain type of development, in other families there are different variants. For several families, mainly deep-sea, larvae are not described, and how their development proceeds is still unknown.
Many species of sea spiders have a breeding season of several months, while others have a relatively short breeding season. Apparently, many forms living near the lower boundary of the littoral migrate deeper into the sublittoral for the winter. The life cycles and seasonal migrations of pycnogonids are very poorly understood. 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 ties
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 been 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 chelicerae (this group includes horseshoe crabs, scorpions, spiders and mites) than to mandibulates (these include crustaceans, centipedes and insects). The chelicerae and palps of sea spiders can be regarded as homologues of the chelicerae and pedipalps of the chelicerae, and specialists relying on this homology include the pycnogonid subphylum chelicerae at class rank. This view is not accepted by all zoologists. It is difficult to compare the body parts of pycnogonids and chelicerae, since the anatomy and embryology of sea spiders are not well understood, in addition, they have unique structural features. Only sea spiders have egg-bearing legs and such a complex trunk, which provides a kind of mechanism for absorbing and processing food. A large number of genital openings and their localization on the second segments of the legs are unusual. Only sea spiders are characterized by such a small number of segments, and, apparently, their oligomerization was not associated with a decrease in body size. The abdomen of modern pycnogonids is also shortened, strongly reduced, but this was not the case in fossil species.
Three fossil species of sea spiders are known. Best reconstructed morphology Palaeoisopus problematicus. They were large animals (up to 20 cm long) with four pairs of legs adapted for swimming. Abdomen Palaeoisopus, subdivided into five segments, was thin and long. At the front 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 repeatedly. It is curious that a number of modern species of sea spiders form symbiotic relationships with echinoderms. Palaeopantopus maucheri is known from only three specimens, the head end is absent in the found specimens, and the abdomen has three segments [ , ]. Finally, the third type of fossil pycnogonids - Palaeothea devonica- practically does not differ from modern forms and has a small non-segmented abdomen.
All paleontological finds of adult pycnogonids date back to the Devonian. However, it cannot be argued that pycnogonids appeared just 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 a pycnogonid larval form. This means that the emergence of sea spiders must be attributed to at least the Upper Cambrian - such is the dating of samples. Cambropycnogon. Excellent preservation allowed a detailed description of the external morphology Cambropycnogon. In terms of the set of limbs, this animal is comparable to the second larval stage of pycnogonids, the only thing that confuses is the presence of an “extra” pair of filaments (limbs?) near the mouth. In general, almost no structural details were found in it, characteristic of the larvae of living pycnogonids, but a completely different structure of most limbs attracts attention. Maybe, Cambropycnogon- the larva of representatives of some group of arthropods that has not survived to our time and has no close relationship with 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 oceans is impressively high. Thus, lush thickets of hydroids develop in the littoral and sublittoral zones of the White Sea with its rugged coasts and strong tidal currents. For sea spiders, these are very favorable conditions. In some places, their abundance is so great that they must play a significant role in the food chains of subtidal communities, being specialized consumers of hydroids, which, in turn, feed on plankton. Trawls and bottom grabs 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 dealt with a correct assessment of the abundance of pycnogonids and their role in communities.
Pycnogonids are of great interest as a relic group of arthropods, possibly not related to the rest, and retaining a number of ancient structural features. On the other hand, the organization, the life form of sea spiders with their reduced torso of few segments and very long limbs with processes of intestines and gonads inside them, is unique. Most likely, pycnogonids are an independent branch of arthropods; they developed a special way of life in the sea that no one else has. Unable to enter other habitats, sea spiders populated the entire World Ocean and retained their characteristic appearance and peculiar way of feeding almost 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-Crank Class ( Pantopoda). Guide to Zoology / Ed. L.A. Zenkevich. M., 1951. S.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.
Sea spiders, they are spider crabs, they are also marble crabs, 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 coast of the Caucasus, at shallow depths with a rocky or rocky bottom.
Sea spiders are members of the Grapsidae family. These crabs are called "spiders" because of their long, dark legs, and they got the name "marble" because of the characteristic pattern on the shell.
Description of the sea spider
The spider crab is small and agile, measuring only 38 millimeters long and 43 millimeters wide. The shell is square and flat. The anterior margin between the eyes is especially wide and straight, with 3 sharp teeth on each side. The upper part of the shell can be overgrown with small crustaceans called balanus, as well as algae.
The skeleton is external, breathing is carried out with the help of gills. On the left claw there are small teeth that are tightly closed together. The right claw is larger than the left, the teeth are bent, and a gap is obtained between them. Outwardly, the right claw resembles tongs. The marble crab belongs to the ten-legged crustaceans, has 10 long, strong paws, covered with hairs. The color of the shell is from brown-greenish to brown-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 a depth of up to 10 meters.
Marble crabs tolerate drying well and love to bask on rocks in the sun. Spider crabs build their own homes. The crab chooses a stone and begins to climb under it, throwing sand grains out from under the stone with its claws, and the crab hides in the resulting niche. Having accumulated supplies, and having eaten well, the sea spider hides in a safe haven.
Sea spiders feed on plant and animal remains, plankton, mollusks and polychaetes. They climb onto the stones 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 throws itself into the water.
At night, he carefully crawls out of the old shell. At night, they can climb 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 a claw, then 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 takes place 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 occurs at 2 years.
Population of marble crabs
Like other Black Sea crabs, sea spiders are used to make souvenirs, but they are not a commercial species.
Spider crabs are included in the Red Book of Ukraine, as their number has sharply decreased recently. These crabs are protected in the natural reserves of Karadagsky and Cape Martyan.
The 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 largest crabs in the Black Sea. The width of the shell of a stone crab 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 has a more modest size, and its shell is covered with numerous yellowish bristles-hairs. They live closer to the shore, under rocks;
Mediterranean or grass crabs have a green shell, which is why they are called "grass". Grass crabs are shallow water dwellers;
Aquatic crab or lilac crab. It is slower and prefers to live exclusively in shallow water;
The swimming crab is a lover of digging into the ground. Its small hind legs look like shoulder 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 water of ships. But the water of the Black Sea 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, lean sea creatures are excellent at camouflage;
Pea crab usually lives among mussels, and sometimes can even climb inside the shell. It is extremely difficult to see this crab, since an adult has a size of no more than a ten-kopeck coin;
Freshwater crab is an unusual Crimean crab. It differs not in size, but in origin and way of life. From the name it is clear that it lives in fresh water: in mountain rivers and ponds.
Freshwater crabs cannot be dispersed by the current, so they have to travel over land at night. In this pedestrian way, they once crossed the entire mainland, 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, while at the bottom there should be a variety of shelters, for example, snags, stones, ceramics. To make the aquaterrarium look more beautiful, it can be revived with the help of plants.
Sea spiders breathe through their legs, or rather through their tips. In the course of evolution, their limbs began to function as gills.
Sea spiders or pantopods are a class of marine arthropods. More than 1300 species have been described. Such spiders live in all seas and at different depths. Pantopods are especially widespread in the oceans. In seas with fresh water, they are almost never found. Sea spiders do not spin webs.
Deep-sea spider species are larger than coastal ones. The body length of sea spiders can reach from 1 mm to 90 cm. Coastal species have a compact body, short legs and developed tubercles and spines. Deep-sea species have longer and thinner limbs, a smooth body and correspondingly poorly developed spines and tubercles. Pantopods are capable of swimming or hovering in the water column. With sharp movements of the legs, they push off the ground and hang in the water, where they can soar almost without movement. They sink to the bottom, raising their legs up and folding them under the body.
Sea spiders are predators. Pantopods feed on sea anemones and sponges. Their mouth is located at the end of the proboscis and works like a pump, drawing in soft food. In the back half of the pharynx there is a kind of filter, which serves to crush and strain food. The sense organs of sea spiders are poorly developed. For touch, they use sensitive hairs and bristles located throughout the body. The organs of vision in spiders are 2 pairs of eyes located on the dorsal side of the head. Deep-sea forms may lack eyes. With the help of the eyes, animals distinguish the direction of light and some movement.
Sea spiders are strange, unlike animals, consisting almost entirely of legs alone. The limbs are rather weak due to poorly developed muscles. Creatures lead a leisurely lifestyle and can spend 40 minutes without any movement at all. Their body is so tiny that some of the internal organs are located just in the legs. For example, these are the organs of the reproductive and digestive systems. Sea spiders are dioecious creatures. The reproductive system is represented by several pairs of genital openings located along the entire length of the walking legs.
The intestines of pantopods are distributed evenly throughout the body, going into each leg until its very end. In addition to digestion, it performs a kind of function of the circulatory system. With its help, hemolymph flows through the body - an analogue of blood in mammals. The movement of the hemolymph is created by contraction of the intestine. But that is not all. Sea spiders also breathe through their legs, or rather through their tips. In the course of evolution, their limbs began to function as gills. Based on this feature, sea spiders do not have the usual gas exchange organs. A leisurely lifestyle does not require a large amount of oxygen.