The pycnogonids they are arthropods that belong to the class Pycnogonida. They are also called sea spiders, due to the great resemblance they have with arachnids. They were first described in 1810 by the French entomologist Pierre André Latreille.
It is a very particular group of animals, since they have very long limbs and a very small body. Due to this, the organs that make up its various internal systems have had to be relocated, making use of the interior space available in the animal's legs..
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Pycnogonids are animals that are made up of highly specialized cells in various functions. Thanks to this they are known as multicellular organisms.
Likewise, they are tripoblastic animals, since during their embryonic development the presence of the three germ layers has been evidenced: ectoderm, mesoderm and endoderm. These are of vital importance, since it is from them that all the organs and tissues that make up the animal are generated..
Pycnogonids are almost sessile animals, as they have quite reduced mobility and locomotion.
They are animals whose size varies, since there are specimens of a few millimeters, even others that can measure more than 50 cm with extended legs..
These are animals that have a high capacity to camouflage themselves with the elements of the external environment. With this, they manage to go unnoticed by predators.
Similarly, pycnogonids belong to the group of animals with bilateral symmetry. This implies that each one is made up of two exactly equal halves.
Finally, pycnogonids are dioecious animals, that is, they have separate sexes, with female individuals and male individuals..
The taxonomic classification of pycnogonids is as follows:
Domain: Eukarya
Animalia Kingdom
Phylum: Arthropoda
Subphylum: Chelicerata
Class: Pycnogonida.
This type of animal has a body of small dimensions, from which several appendages are detached, which are of great length..
As with all arthropods, pycnogonids have a segmented body in several areas or zones. In general, the body of these animals is divided into two segments: prosoma (cephalothorax) and opistosoma (abdomen).
Likewise, they present a series of articulated appendages. There are a total of 12 appendages, distributed as follows: 1 pair of chelicerae, 1 pair of pedipalps and 4 pairs of legs.
This part of the body in turn is made up of two areas: the cephalon and the thorax. First, the cephalon has an anterior and a posterior end.
At the anterior end there is an extension known as the proboscis, which houses the mouth opening. The latter is surrounded by three lips with a chitinous texture..
At the posterior end of the cephalon there is a protrusion known as the ocular tubercle, in which the organs of sight are housed..
Seven pairs of appendages emerge from the prosome: the chelicerae or cheiphores, the pedipalps or palps, the ovigers and the legs..
They constitute the first pair of appendages of these animals. They are small in size and are made up of a chela (mobile finger + fixed finger) and a joint called an escapement. The main function of these appendages is related to the feeding process and the immobilization of prey..
They are the second pair of appendages of the pycnogonidae and are poorly developed. They are located on both sides of the proboscis. They are not present in all species, since some have been described that lack these appendages. The functions performed by pedipalps are varied, depending on their degree of development.
They constitute the third pair of appendages. Their function is directly related to the reproduction process and consists of keeping the eggs fixed during their development until their subsequent hatching. In addition, males use ovigers to stimulate the release of eggs in the female..
As with all arthropod appendages, ovigers are also made up of artifacts. The number of these varies according to various factors, among which sex and gender can be mentioned. In general, ovigers can be made up of up to 10 pieces.
They are four pairs and they are also very long compared to the length of the body. They are made up of a total of eight joints, being from the distal to the proximal: propod, tarsus, two tibiae, femur and three coxae.
The last joint (propode) has a nail at its distal end, as well as auxiliary nails. In addition, it is common to get species in which the legs are covered by some protrusions such as thorns..
It is the smallest segment of the body of pycnogonids. At its rear end is an opening that corresponds to the anus, the termination of the digestive system.
The nervous system of pycnogonids is made up of groups of neurons that make up ganglia. As with the rest of arthropods, the nervous system is closely related to the digestive system. In this sense, there are per-esophageal and sub-esophageal nodes.
All these ganglia emit nerve fibers to various structures of the animal.
Pycnogonids have a rather particular open circulatory system, since they do not have a main organ such as the heart, but rather use the structures of the digestive system, specifically the stomach, to pump circulating fluid.
In this regard, the circulating fluid is not blood, but hemolymph.
Despite this, there are specialists who state that pycnogonids do have a tubular-like heart, with a few ostioli (2 or 3).
The digestive system of these animals is complete, with the mouth as the entrance opening, and the anus as the exit opening..
It begins at the proboscis, which presents the mouth opening that opens into a pharynx that continues with a small esophagus. Next is the stomach, which performs functions both within the digestive and circulatory systems. Nutrient absorption begins in the stomach.
The stomach communicates with the intestine. The disposition of the intestine is quite particular. This branches and is distributed on the legs of the animal. The intestine also has a cecum of lateral position.
Finally, the digestive system empties into the anal opening, which is where digestion waste is released..
Pycnogonids are purely marine animals, so they are only found in saltwater habitats.
These types of animals are ubiquitous, since they are widely distributed in all the oceans of the planet. The water temperature is not a limitation for them, since species have been found both in warm waters and in waters with low temperatures..
Because they have reduced mobility, they must be adequately camouflaged to protect themselves from potential predators. This is why they are sometimes found hidden among the algae, buried or under some rocks that may be there..
The members of the Pycnogonida class can be of two types, according to their diet: predators and saprophages. These animals are known predators of others that, like them, are found at the bottom of the sea. Among these, we can mention sea anemones.
On the other hand, pycnogonids feed on debris floating near them. These residues are mainly represented by organic remains of algae.
Their way of feeding when they are predators is as follows: when they catch their prey, they project their proboscis and attack it, also using it to absorb the liquids coming from the prey..
It is important to remember that most species have a small digestive tract, so they cannot ingest large amounts of food and, therefore, feed on substances of a softer consistency such as mush or liquids..
The reproduction of pycnogonids is sexual. This means that there must be the fusion of a female gamete with a male gamete. In addition, they present external fertilization, are oviparous and present an indirect development.
Regarding the type of fertilization, this is external, because in the different known species of pycnogonids it has not been observed that there is a copulation process. On the contrary, the fertilization of the eggs occurs outside the body of the female.
Specialists have reported that these animals show a courtship prior to fertilization. This process is initiated by the male, who, when he meets the female, proceeds to rub his ovigero on her body, stimulating her. As a consequence of this stimulation, the female releases the eggs.
The eggs are then transferred to the male's ovigerous legs, where the fertilization process finally occurs. At this moment, the mucus secreted by the glands that are at the level of the male's femur (cementum) hold the eggs together, forming a large shapeless mass..
In a somewhat atypical case of the animal kingdom, the male is the one who carries the eggs until they hatch. When this happens, a larva emerges from the eggs known as the protonymph, which is exclusive to this type of animal..
These larvae are characterized by having three pairs of appendages and one proboscis. Later, to complete their development, they are known to adhere to various marine invertebrate animals. However, the post-hatching maturation process of the eggs still remains largely unknown..
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