The Pycnogonida are a group of marine arthropods known as "sea spiders" for their resemblance to true, terrestrial spiders. Although there has been much debate during the past century regarding which arthropod groups are their closest relatives, growing consensus places the pycnogonids as an early branching lineage that is the sister group to the chelicerates (arachnids plus horseshoe crabs) (Giribet and Edgecombe 2012).
There are around a thousand described species of pycnogonids (and probably hundreds more not yet described). They are found worldwide from the intertidal zone to depths of nearly 7,000 m. Most pycnogonids are small, with leg spans of less than a centimeter (in some species, just a few millimeters), but some deep-sea species have leg spans of up to 60 cm. Many species are errant (i.e., they actively move about), but others live on seaweeds or on other invertebrates, such as sea anemones, hydroids, ectoprocts, and tunicates (at least one or two species live on the bells of pelagic medusae and pycnogonids have also been observed on the huge vestimentiferan worms living in the hydothermal vent community of the Galapagos Rift).
The walking legs of pycnogonids are 9-segmented. Most of the commonly encountered intertidal species have short, thick legs and are quite sedentary, moving very slowly. Deep-water bottom-dwelling species tend to have longer, thinner legs and to be more active, walking on the tips of their legs. Many pycnogonids are also known to swim periodically.
Most species are generalist predators (and, in many cases, scavengers), although a few feed on algae. Food is obtained by sucking it through the proboscis (often just body fluids and tissue fragments from their prey.).
Pycnogonids are dioecious (i.e., with separate males and females) and embryos are brooded by the male. In many species, distinctive 6-legged larvae (immature forms) are released by the brooding male that live in some sort of symbiotic relationship with cnidarians, molluscs, or echinoderms.
(Brusca and Brusca 2003)
Bamber and El Nagar provide an online database on pycnogonid names, taxonomy, museum specimens, and literature.
The Pycnogonida, also known as the Sea Spider, is a class of unidentified aquatic Chelicerata’s in the Pantopoda order and has many genera including the Pseudopallene circularis.
The Sea Spiders are mainly found in the Mediterranean Sea but can be seen in the Atlantic Ocean, Artic Ocean, and Caribbean Sea. They have a similar look to a common spider but can be larger; their size varies on their environment and has longer legs than most Arachnids. These aquatic animals are best known for its long legs and similar look to Arachnids though they are in the Chelicerata subphylum.
An adult male can be as large as 76 centimeters (Around 2.8 feet) in length from leg to leg and as small as 5 centimeters. It is a very light weight class as heavy as .7kg (1 pound) and as light as .01kg (.1 pounds). Sea Spiders have a proboscis out of its mouth that sucks out the nutrients of prey.They have a small center and 8 legs but can have 10 or 12 legs. They craw or swim with its legs pushing down the water. The color greatly varies from yellow to even blue.
They have no found origins yet because only two complete fossils have been found. It seems they are most often found in the Atlantic Ocean near Europe and in the Mediterranean Sea. The fossils were found in these areas. These animals have many connections and seem to have originated from crabs and a form of early spider. They have only one common ancestor that we know as and that is to the prehistoric monkey-spider hybrid.
The Pycnogonida are predators of many soft tissued invertebrates. They mainly feast on sponges. They keep to themselves and stay hidden most of the time. They have no one predator that lives off of them. They live in small communities and are rarely found. Their only defense is their legs with contain a poison of sorts that disables the attacker or prey.
There are no recorded discoverers of the Pycnogonida. Many biologists are trying to discover the connections of the Sea Spiders and discover more genus and species. There are many genus recorded but no intense studying of just one.
The Pycnogonida Sea Spiders are a very elusive and new class. They are magnificent in their ways or live and should have more in-depth studies in my view.
Sea spiders are marine arthropods of the order Pantopoda[1] (lit. ‘all feet’[2]), belonging to the class Pycnogonida,[3] hence they are also called pycnogonids (/pɪkˈnɒɡənədz/;[4] named after Pycnogonum, the type genus;[5] with the suffix -id). They are cosmopolitan, found in oceans around the world. The over 1,300 known species have legs ranging from 1 mm (0.04 in) to over 70 cm (2.3 ft).[6] Most are toward the smaller end of this range in relatively shallow depths; however, they can grow to be quite large in Antarctic and deep waters.
Although "sea spiders" are not true spiders, nor even arachnids, their traditional classification as chelicerates would place them closer to true spiders than to other well-known arthropod groups, such as insects or crustaceans, if correct. This is disputed, however, as genetic evidence suggests they may be a sister group to all other living arthropods.[7][8]
Sea spiders have long legs in contrast to a small body size. The number of walking legs is usually eight (four pairs), but the family Pycnogonidae includes species with five pairs, and the families Colossendeidae and Nymphonidae include species with both five and six pairs.[9] There are nine polymerous (i.e., extra-legged) species: seven species distributed among four genera (Decolopoda, Pentacolossendeis, Pentapycnon, and Pentanymphon) with five leg pairs and two species in two genera (Dodecolopoda and Sexanymphon) with six leg pairs.[10] Pycnogonids do not require a traditional respiratory system. Instead, gasses are absorbed by the legs and transferred through the body by diffusion. A proboscis allows them to suck nutrients from soft-bodied invertebrates, and their digestive tract has diverticula extending into the legs.
Certain pycnogonids are so small that each of their very tiny muscles consists of a single cell, surrounded by connective tissue. The anterior region (cephalon) consists of the proboscis, which has fairly limited dorsoventral and lateral movement, the ocular tubercle with eyes, and up to four pairs of appendages. The first of these are the chelifores, followed by the palps, the ovigers, which are used for cleaning themselves and caring for eggs and young as well as courtship, and the first pair of walking legs. Nymphonidae is the only family where both the chelifores and the palps are functional. In the others the chelifores or palps, or both, are reduced or absent. In some families, also the ovigers can be reduced or missing in females, but are always present in males.[11] In those species that lack chelifores and palps, the proboscis is well developed and flexible, often equipped with numerous sensory bristles and strong rasping ridges around the mouth. The last segment includes the anus and tubercle, which projects dorsally.
In total, pycnogonids have four to six pairs of legs for walking. A cephalothorax and a much smaller unsegmented abdomen make up the extremely reduced body of the pycnogonid, which has up to two pairs of dorsally located simple eyes on its non-calcareous exoskeleton, though sometimes the eyes can be missing, especially among species living in the deep oceans. The abdomen does not have any appendages, and in most species it is reduced and almost vestigial. The organs of this chelicerate extend throughout many appendages because its body is too small to accommodate all of them alone.
The morphology of the sea spider creates an efficient surface area-to-volume ratio for respiration to occur through direct diffusion. Oxygen is absorbed by the legs and is transported via the hemolymph to the rest of the body.[12] The most recent research seems to indicate that waste leaves the body through the digestive tract or is lost during a moult. The small, long, thin pycnogonid heart beats vigorously at 90 to 180 beats per minute, creating substantial blood pressure. The beating of the sea spider heart drives circulation in the trunk and in the part of the legs closest to the trunk, but is not important for the circulation in the rest of the legs.[12] Hemolymph circulation in the legs is mostly driven by the peristaltic movement in the part of the gut that extends into every leg, a process called gut peristalsis.[12][13] These creatures possess an open circulatory system as well as a nervous system consisting of a brain which is connected to two ventral nerve cords, which in turn connect to specific nerves.
All pycnogonid species have separate sexes, except for one species that is hermaphroditic. Females possess a pair of ovaries, while males possess a pair of testes located dorsally in relation to the digestive tract. Reproduction involves external fertilisation after "a brief courtship". Only males care for laid eggs and young.
The larva has a blind gut and the body consists of a head and its three pairs of cephalic appendages only: the chelifores, palps and ovigers. The abdomen and the thorax with its thoracic appendages develop later. One theory is that this reflects how a common ancestor of all arthropods evolved; starting its life as a small animal with a pair of appendages used for feeding and two pairs used for locomotion, while new segments and segmental appendages were gradually added as it was growing.
At least four types of larvae have been described: the typical protonymphon larva, the encysted larva, the atypical protonymphon larva, and the attaching larva. The typical protonymphon larva is most common, is free living and gradually turns into an adult. The encysted larva is a parasite that hatches from the egg and finds a host in the shape of a polyp colony where it burrows into and turns into a cyst, and will not leave the host before it has turned into a young juvenile.[14]
Little is known about the development of the atypical protonymphon larva. The adults are free living, while the larvae and the juveniles are living on or inside temporary hosts such as polychaetes and clams. When the attaching larva hatches it still looks like an embryo, and immediately attaches itself to the ovigerous legs of the father, where it will stay until it has turned into a small and young juvenile with two or three pairs of walking legs ready for a free-living existence.
These animals live in many different parts of the world, from Australia, New Zealand, and the Pacific coast of the United States, to the Mediterranean Sea and the Caribbean Sea, to the north and south poles. They are most common in shallow waters, but can be found as deep as 7,000 metres (23,000 ft), and live in both marine and estuarine habitats. Pycnogonids are well camouflaged beneath the rocks and among the algae that are found along shorelines.
Sea spiders either walk along the bottom with their stilt-like legs or swim just above it using an umbrella pulsing motion.[15] Sea spiders are mostly carnivorous predators or scavengers that feed on cnidarians, sponges, polychaetes, and bryozoans. Although they can feed by inserting their proboscis into sea anemones, which are much larger, most sea anemones survive this ordeal, making the sea spider a parasite rather than a predator of anemones.
The class Pycnogonida comprises over 1,300 species, which are normally split into eighty-six genera. The correct taxonomy within the group is uncertain, and it appears that no agreed list of orders exists. All families are considered part of the single order Pantopoda.
Sea spiders have long been considered to belong to the Chelicerata, together with horseshoe crabs, and the Arachnida, which includes spiders, mites, ticks, scorpions, and harvestmen, among other lesser-known orders.[16]
A competing hypothesis proposes that Pycnogonida belong to their own lineage, distinct from chelicerates, crustaceans, myriapods, or insects. This Cormogonida hypothesis contended that the sea spider's chelifores, which are unique among extant arthropods, are not positionally homologous to the chelicerae of Chelicerata, as was previously supposed. Instead of developing from the deutocerebrum like the chelicerae, the sea spider chelifores were thought to be innervated by the protocerebrum, the anterior part of the arthropod brain and found in the first head segment that in all other arthropods give rise to the eyes and the labrum. This condition of having protocerebral appendages is not found anywhere else among arthropods, except in fossil forms like Anomalocaris, which was taken as evidence that Pycnogonida may be a sister group to all other living arthropods, the latter having evolved from some ancestor that had lost the protocerebral appendages. If this result had been confirmed, it would have meant the sea spiders are the last surviving (and highly modified) members of an ancient stem group of arthropods that lived in Cambrian oceans.[17] However, a subsequent study using Hox gene expression patterns demonstrated the developmental homology between chelicerates and chelifores, with chelifores innervated by a deuterocerebrum that has been rotated forwards; thus, the protocerebral Great Appendage clade does not include the Pycnogonida.[18][19][20]
Phylogenomic studies place the Pycnogonida as the sister group to the remaining Chelicerata (consistent with the chelifore-chelicera putative homology[21]), though few other relationships at the base of the chelicerates are well resolved.[22][23][24][25] The first phylogenomic study of sea spiders was able to establish a backbone tree for the group and showed that Austrodecidae is the sister group to the remaining families.[26]
According to the World Register of Marine Species, the order Pantopoda is subdivided as follows:[27]
This taxonomic classification replaces the older version in which Pantopoda is subdivided into families as follows:
The fossil record of pycnogonids is scant. The earliest fossils are known from the Cambrian 'Orsten' of Sweden (Cambropycnogon), though some researchers have argued that this putative larval sea spider is not a Pycnogonida at all.[26] Unambiguous sea spider body fossils include the Silurian Coalbrookdale Formation of England (Haliestes) and the Devonian Hunsrück Slate of Germany (Flagellopantopus, Palaeopantopus, Palaeoisopus, Palaeothea and Pentapantopus). Some of these specimens are significant in that they possess a longer 'trunk' behind the abdomen and in two fossils the body ends in a tail, something never seen in living sea spiders.
In 2013, the first fossil pycnogonid found within an Ordovician deposit was reported from William Lake in Manitoba.[31]
In 2007, remarkably well preserved fossils were exposed in fossil beds at La Voulte-sur-Rhône, south of Lyon in south-eastern France. Researchers from the University of Lyon discovered about 70 fossils from three distinct species in the 160-million-year-old Jurassic La Voulte Lagerstätte. The find will help fill in an enormous gap in the history of these creatures.[32]
Sea spiders are marine arthropods of the order Pantopoda (lit. ‘all feet’), belonging to the class Pycnogonida, hence they are also called pycnogonids (/pɪkˈnɒɡənədz/; named after Pycnogonum, the type genus; with the suffix -id). They are cosmopolitan, found in oceans around the world. The over 1,300 known species have legs ranging from 1 mm (0.04 in) to over 70 cm (2.3 ft). Most are toward the smaller end of this range in relatively shallow depths; however, they can grow to be quite large in Antarctic and deep waters.
Although "sea spiders" are not true spiders, nor even arachnids, their traditional classification as chelicerates would place them closer to true spiders than to other well-known arthropod groups, such as insects or crustaceans, if correct. This is disputed, however, as genetic evidence suggests they may be a sister group to all other living arthropods.
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